[filmscanners] RE: Density vs Dynamic range - was: RE: opinions? Reviews? of Primefilm 1800 ?
Hi Laurie, In so far as my use of the two terms in the mistatement, dynamic range and density range tend to be used in the literature and manufacturer's specs synonymously as denoting the same thing (ie. the contrast range), Yes, I know...(heavy sigh). Marketing people tend not to really understand the difference, and neither do most lay people...or even quite a few technical people. They have also been used interchangeably in photography for many many years...unfortunately... Having said that,let me see if I got it by putting it into my own words. Density range refers to the contrast range from white point to black point covered by the scanner and is measured in terms of 3.0 to 4.2 Dmax or D Coorect! Whereas bit depth refers the degree of quality information or detail that can be obtained within the given density range Bit depth is actually nothing more than the limiting factor FOR the detail...but does not assure you that you will get the detail. Thus, in your terms since most of the literature does not use any term to describe what is being measured except to refer to it as bit depth or x number of bits, bit depth measures the dynamic range that a device can capture within its given density range, as meeasured by the Dmax specification. Again, bit depth is the limiting factor OF dynamic range, but does not assure you that the dynamic range of the scanner is equal to the bit depth...as in the 24 bit, w/ 12 bits of noise, example. Is my understanding close enough for government work or do I need to go back to the contemplation place and rethink it? It's fine, if you understand the caveat WRT bit depth... Regards, Austin Unsubscribe by mail to [EMAIL PROTECTED], with 'unsubscribe filmscanners' or 'unsubscribe filmscanners_digest' (as appropriate) in the message title or body
[filmscanners] RE: Density vs Dynamic range - was: RE: opinions? Reviews? of Primefilm 1800 ?
On Sat, Jun 08, 2002 at 09:43:00AM +0100, dickbo wrote: Bits equals available grey levels per pixel - Original Message - From: Laurie Solomon [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Friday, June 07, 2002 4:22 PM Subject: [filmscanners] RE: opinions? Reviews? of Primefilm 1800 ? Another misconception...though equally as common...the number of bits the converter has, have nothing to do with the scanners ability to capture any particular density range. Just out of curiousity and in simple layman's terms, what do the number of bits that the converter has have to do with if not the density range? How does it impact on what is captured? Austin, I am asking a serious question here out of my lack of knowledge and sure would appreciate a good discussion in layman's terms so that I can understand what is being said without having to hire an engineer to interpret. It has always been my understanding, rightly or wrongly, that the higher the number of bits the more detailed or refined the informational date captured from the original that is transmitted as data in the digital file with respect to highlight and/or shadow detail with the density range figure representing the range of contrast that can be captured. In other words, dynamic range representing the contrast range of the capture's capabilities, while the bit depth represented the quality of the data captured within that range particularly the extremes. If this is wrong, please explain where and how it is and provide me with a more accurrate description (but once again, I urge you to try and do it in non-engineering terms if possible). The number of scanner bits is a necessary but not sufficient requirement for seeing densities at a scanner's theoretical dmax (i.e. log 2^bit-depth). Number of bits have two main roles. They do indeed represent the theoretical maximum density that a scanner could have if the electrical components were up to scratch and could use those bits to their full. More importantly though the number of bits determine with what resolution the scanner can see the density levels that it is capable of scanning. Reread that last sentence again! Lets take a sample scanner which can only record film density up to a 2.4 dmax. This should be within the realms of even the cheapest modern film scanner hardware. This density limit is imposed by (let's say) the cheap design of the scanner and the manufacturer's requirements to be cost effective. Now lets take this scanner and give it an 8-bit CCD to start with. In a minute we'll give it a 14-bit CCD and see the difference. The number of bits represents the POTENTIAL density range that the scanner is capable of. Scanner CCDs are linear devices - it is important you understand this concept because it determines how well scanners can potentially see into the shadows. With a linear device each doubling or halving of a scanned linear data value represents a doubling or halving of light, or 1 stop, or 0.3 density units. Let us take our 8-bit scanner then - 8-bits gives us a total of 256 values that the scanner can use to scan the image. Let us look at the numbers to see how these data values can be spread out across a density range. I'm going to assume that the operator has placed the scanner's white point (value 255) exactly at the lightest portion of the film/paper. 128-255: 1 stop (0.3 density units) 64-127: 1 stop (0.3 density units) 32-63: 1 stop (0.3 density units) 16-31: 1 stop (0.3 density units) 8-15: 1 stop (0.3 density units) 4-7: 1 stop (0.3 density units) 2-3: 1 stop (0.3 density units) 0-1: 1 stop (0.3 density units) Note that I cannot go any further than this because I need at least two distinct values to represent a change in density. The 8-bit scanner then can scan up to 8 potential stops. It is impossible to see anything denser because it would need more bits. Even now you have so few bits at the shadow end that you would almost certainly have posturization. Let us move now to the same scanner with 14-bits to play with. How do those 8-stops map to the scanner data values? Well 14-bits gives us 16,384 values: 8192-16383: 1 stop (0.3 density units) 4096-8191: 1 stop (0.3 density units) 2048-4095: 1 stop (0.3 density units) 1024-2047: 1 stop (0.3 density units) 512-1023: 1 stop (0.3 density units) 256-511: 1 stop (0.3 density units) 128-255: 1 stop (0.3 density units) 64-127: 1 stop (0.3 density units) Heck we've reached 8-stops here and there's loads of bits to spare! Better still that last stop which, on the 8-bit scanner, could only be represented by either a 0 or 1 can now be represented by 64 distinct values. Undoubtedly you could get better tonal resolution in that last stop of shadow area with 14-bits. What about the values from 0 to 63 I hear you ask? Well as agreed at the start our scanner is only capable of recording up to 2.4 dmax from the film. This is a hardware limitation. What if they came along and created a better
[filmscanners] RE: Density vs Dynamic range - was: RE: opinions? Reviews? of Primefilm 1800 ?
Hi Tony, Number of bits have two main roles. They do indeed represent the theoretical maximum density that a scanner could have if the electrical components were up to scratch and could use those bits to their full. Not true. You can represent ANY density by any number of bits. I can represent an entire density range with two bits: 00 - at or below dMin 10 or 01 - between dMin and dMax 10 - at or below dMax BTW, that should be at or ABOVE dMax...sorry. This is a completely valid encoding of density range. I have to run now, so I'll comment on the rest of your post later. Regards, Austin And...to add further to this, as I have a few moments... Density values are absolute values, just like one foot is an absolute value. They have meaning in and of them selves. Someone decided what the exact length of one foot is (within a tolerance of course), as well as density values. They mean the same thing everywhere...a density value of 1.6 means exactly that in La Jolla CA, as well as in Pascagoula, MS. Same with one foot. But, the tonal values out of a scanner are not the same everywhere, nor are they the same even between scans! The value 136 in La Jolla, CA...does not have the same tonality as the value 136 in Pascagoula, MS. Scanners are not calibrated TO anything, except themselves. That is why the data values you get from the scanner are not the same density values. There is no direct correlation between them, unless you were to calibrate your scanner in the same way densitometers are calibrated (even that is insufficient, as the sensors used in film scanners have different characteristics than the sensors used in densitometers). Now, WHY would anyone in the first place say that number of bits has anything to do with density range...because they made some erroneous assumptions. Density range is stated in a ratio of to 1 (:1), and a density of 3.6 is 10**3.6:1 or 3981:1 (and someone, at some time, decided what the density value of 1 physically is), which the number 3981 requires 12 bits to represent in the binary system, if you are going to represent every integer value from 1 to 3981...BUT...that's the rub...a value of 3981 from the scanner is NOT the same as a density value of 3.6 (3981:1), for the reasons explained above. And, no, they are not close enough for government work ;-) Regards, Austin Unsubscribe by mail to [EMAIL PROTECTED], with 'unsubscribe filmscanners' or 'unsubscribe filmscanners_digest' (as appropriate) in the message title or body
[filmscanners] RE: Density vs Dynamic range - was: RE: opinions? Reviews? of Primefilm 1800 ?
Hi Austin, Yes I am fully cognizant of the fact that we are talking about optimum conditions and limits under usually ideal conditions when we talk about capabilities or capacities and that we are not talking about certainties in practice under practical concrete empirical conditions. Thanks. I think I am clear on this now; at least, I am until some engineer comes around throwing techical talk at me about the subtlties involved which will only muddy up the water for me. :-) -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Austin Franklin Sent: Saturday, June 08, 2002 6:42 AM To: [EMAIL PROTECTED] Subject: [filmscanners] RE: Density vs Dynamic range - was: RE: opinions? Reviews? of Primefilm 1800 ? Hi Laurie, In so far as my use of the two terms in the mistatement, dynamic range and density range tend to be used in the literature and manufacturer's specs synonymously as denoting the same thing (ie. the contrast range), Yes, I know...(heavy sigh). Marketing people tend not to really understand the difference, and neither do most lay people...or even quite a few technical people. They have also been used interchangeably in photography for many many years...unfortunately... Having said that,let me see if I got it by putting it into my own words. Density range refers to the contrast range from white point to black point covered by the scanner and is measured in terms of 3.0 to 4.2 Dmax or D Coorect! Whereas bit depth refers the degree of quality information or detail that can be obtained within the given density range Bit depth is actually nothing more than the limiting factor FOR the detail...but does not assure you that you will get the detail. Thus, in your terms since most of the literature does not use any term to describe what is being measured except to refer to it as bit depth or x number of bits, bit depth measures the dynamic range that a device can capture within its given density range, as meeasured by the Dmax specification. Again, bit depth is the limiting factor OF dynamic range, but does not assure you that the dynamic range of the scanner is equal to the bit depth...as in the 24 bit, w/ 12 bits of noise, example. Is my understanding close enough for government work or do I need to go back to the contemplation place and rethink it? It's fine, if you understand the caveat WRT bit depth... Regards, Austin Unsubscribe by mail to [EMAIL PROTECTED], with 'unsubscribe filmscanners' or 'unsubscribe filmscanners_digest' (as appropriate) in the message title or body Unsubscribe by mail to [EMAIL PROTECTED], with 'unsubscribe filmscanners' or 'unsubscribe filmscanners_digest' (as appropriate) in the message title or body
[filmscanners] RE: Density vs Dynamic range - was: RE: opinions? Reviews? of Primefilm 1800 ?
Tony, Thank you for your lengthy and detailed response. I am posting this just to let you know that I have not had time to give it the reading and thought it desearves yet; but I will get back to you when I have had an opportunity to read and digest it. I did notice in the headers that under the second original message header it says that the message was from me to [EMAIL PROTECTED] when in actuality the original message of mine was to Austin not Dickbo. In fact, I did not see Dickbo's post until well after I posted the quoted message of mine. Not that this means anything in terms of the content of the discussion; but it does help keep clear who was posting to whom and when. :-) -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Tony Terlecki Sent: Saturday, June 08, 2002 7:46 AM To: [EMAIL PROTECTED] Subject: [filmscanners] RE: Density vs Dynamic range - was: RE: opinions? Reviews? of Primefilm 1800 ? On Sat, Jun 08, 2002 at 09:43:00AM +0100, dickbo wrote: Bits equals available grey levels per pixel - Original Message - From: Laurie Solomon [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Friday, June 07, 2002 4:22 PM Subject: [filmscanners] RE: opinions? Reviews? of Primefilm 1800 ? Another misconception...though equally as common...the number of bits the converter has, have nothing to do with the scanners ability to capture any particular density range. Just out of curiousity and in simple layman's terms, what do the number of bits that the converter has have to do with if not the density range? How does it impact on what is captured? Austin, I am asking a serious question here out of my lack of knowledge and sure would appreciate a good discussion in layman's terms so that I can understand what is being said without having to hire an engineer to interpret. It has always been my understanding, rightly or wrongly, that the higher the number of bits the more detailed or refined the informational date captured from the original that is transmitted as data in the digital file with respect to highlight and/or shadow detail with the density range figure representing the range of contrast that can be captured. In other words, dynamic range representing the contrast range of the capture's capabilities, while the bit depth represented the quality of the data captured within that range particularly the extremes. If this is wrong, please explain where and how it is and provide me with a more accurrate description (but once again, I urge you to try and do it in non-engineering terms if possible). The number of scanner bits is a necessary but not sufficient requirement for seeing densities at a scanner's theoretical dmax (i.e. log 2^bit-depth). Number of bits have two main roles. They do indeed represent the theoretical maximum density that a scanner could have if the electrical components were up to scratch and could use those bits to their full. More importantly though the number of bits determine with what resolution the scanner can see the density levels that it is capable of scanning. Reread that last sentence again! Lets take a sample scanner which can only record film density up to a 2.4 dmax. This should be within the realms of even the cheapest modern film scanner hardware. This density limit is imposed by (let's say) the cheap design of the scanner and the manufacturer's requirements to be cost effective. Now lets take this scanner and give it an 8-bit CCD to start with. In a minute we'll give it a 14-bit CCD and see the difference. The number of bits represents the POTENTIAL density range that the scanner is capable of. Scanner CCDs are linear devices - it is important you understand this concept because it determines how well scanners can potentially see into the shadows. With a linear device each doubling or halving of a scanned linear data value represents a doubling or halving of light, or 1 stop, or 0.3 density units. Let us take our 8-bit scanner then - 8-bits gives us a total of 256 values that the scanner can use to scan the image. Let us look at the numbers to see how these data values can be spread out across a density range. I'm going to assume that the operator has placed the scanner's white point (value 255) exactly at the lightest portion of the film/paper. 128-255: 1 stop (0.3 density units) 64-127: 1 stop (0.3 density units) 32-63: 1 stop (0.3 density units) 16-31: 1 stop (0.3 density units) 8-15: 1 stop (0.3 density units) 4-7: 1 stop (0.3 density units) 2-3: 1 stop (0.3 density units) 0-1: 1 stop (0.3 density units) Note that I cannot go any further than this because I need at least two distinct values to represent a change in density. The 8-bit scanner then can scan up to 8 potential stops. It is impossible to see anything denser because it would need more bits. Even now you have so few bits at the shadow end that you would almost certainly have posturization. Let us move now to the same
[filmscanners] RE: Density vs Dynamic range - was: RE: opinions? Reviews? of Primefilm 1800 ?
Tony, The number of scanner bits is a necessary but not sufficient requirement for seeing densities at a scanner's theoretical dmax (i.e. log 2^bit-depth). Number of bits have two main roles. They do indeed represent the theoretical maximum density that a scanner could have if the electrical components were up to scratch and could use those bits to their full. I've discussed my disagreement with this in another post... More importantly though the number of bits determine with what resolution the scanner can see the density levels that it is capable of scanning. That is correct, and the only thing number of bits is. It's called dynamic range. The number of bits represents the POTENTIAL density range that the scanner is capable of. And that's not right... Scanner CCDs are linear devices - it is important you understand this concept because it determines how well scanners can potentially see into the shadows. What is done in some scanners, is the analog output from the CCD to the A/D is adjusted (like a tonal curve) such that it boosts the ability of the scanner to see into the shadows, or any other parts for that matter. With a linear device each doubling or halving of a scanned linear data value represents a doubling or halving of light, or 1 stop, or 0.3 density units. But, as a note...don't confuse relative density units with absolute density values. Yes, two values from the scanner can be 2:1 relatively...but that does not say what the absolute density value of either of them is. Also, because the CCD is linear, does not mean that that the system doesn't have gain and offset... Let us take our 8-bit scanner then - 8-bits gives us a total of 256 values that the scanner can use to scan the image. Let us look at the numbers to see how these data values can be spread out across a density range. As I've said, there is no direct calibration/correlation between density range and data values. I can take the same density range, and spread it across ANY number of bits, or position it anywhere within any number of bits...it solely depends on the output characteristics of the CCD, as well as the analog front end between the CCD and the A/D. It is up to the CCDs sensitivity that determines what density range is presented to the analog front end, and then what the analog front end does with it before it gets to the A/D that determines what density range the A/D sees. I can take the exact same CCD that is used in a 14 bit scanner and put an 8 bit A/D in, and still get the exact same density range (less the lower 6 bits). I could also take your 8 bit scanner example, and simply put a 4 bit A/D, yet get the exact same density range out of it... I'm not speculating on this. I design digital imaging systems, and have been for 20 years. I've designed quite a few film scanners, and I know how they are designed, or at least how I've designed them ;-) One easy example to show that what you say is inaccurate, is that the density range of a 16 bit file, converted to an 8 bit file, is the same (except minus the lower 8 bits)...and the converse is true, you can take 8 bit data and convert it to 16 bit data, and it still represents the same density range. Regards, Austin Unsubscribe by mail to [EMAIL PROTECTED], with 'unsubscribe filmscanners' or 'unsubscribe filmscanners_digest' (as appropriate) in the message title or body
[filmscanners] Re: Density vs Dynamic range - was: RE: opinions? Reviews? of Primefilm 1800 ?
Austin, I find this conversation fascinating and hope you will stay the course with all my (possibly naiive responses.) I'm no expert at all but I have ideas that I thought were true and I'd like to work them through with you whatever the outcome... On Sat, Jun 08, 2002 at 01:18:17PM -0400, Austin Franklin wrote: Hi Tony, Number of bits have two main roles. They do indeed represent the theoretical maximum density that a scanner could have if the electrical components were up to scratch and could use those bits to their full. Not true. You can represent ANY density by any number of bits. I can represent an entire density range with two bits: 00 - at or below dMin 10 or 01 - between dMin and dMax 10 - at or below dMax BTW, that should be at or ABOVE dMax...sorry. This is a completely valid encoding of density range. Sure it is. Is that what happens in a scanner though? I thought that the CCD behaved in a linear manner? Does that not dictate the encoding method somewhat? I have to run now, so I'll comment on the rest of your post later. Regards, Austin And...to add further to this, as I have a few moments... Density values are absolute values, just like one foot is an absolute value. They have meaning in and of them selves. Someone decided what the exact length of one foot is (within a tolerance of course), as well as density values. They mean the same thing everywhere...a density value of 1.6 means exactly that in La Jolla CA, as well as in Pascagoula, MS. Same with one foot. But, the tonal values out of a scanner are not the same everywhere, nor are they the same even between scans! The value 136 in La Jolla, CA...does not have the same tonality as the value 136 in Pascagoula, MS. Yep I agree with you here. That density values are absolute values. But relative density is also commonly used in film sensitometry is it not? Scanners are not calibrated TO anything, except themselves. That is why the data values you get from the scanner are not the same density values. There is no direct correlation between them, unless you were to calibrate your scanner in the same way densitometers are calibrated (even that is insufficient, as the sensors used in film scanners have different characteristics than the sensors used in densitometers). OK I have just bought a densitometer which lets me calibrate so that a 0 reading can be set to film base, then readings taken from there. Cannot the same be done for a scanner? If I scan a piece of film and the film base gives me a value of 248 (assume 8-bit here just for argument's sake) cannot I set that as my point from which all my other density values can be measured? Of course I am making an assumption that the data is behaving linearly (see http://www.bobwheeler.com/photo/Documents/ZoneDigital.pdf for an interesting article on this. I'd certainly like your opinion on whether the information presented seems feasible.) I may not know the absolute density reading but I would know the density range of the film I am scanning. If I were scanning a very dense piece of film (say Velvia) then I would need more than 8 bits to see well into those shadows would I not? Now, WHY would anyone in the first place say that number of bits has anything to do with density range...because they made some erroneous assumptions. Density range is stated in a ratio of to 1 (:1), and a density of 3.6 is 10**3.6:1 or 3981:1 (and someone, at some time, decided what the density value of 1 physically is), which the number 3981 requires 12 bits to represent in the binary system, if you are going to represent every integer value from 1 to 3981...BUT...that's the rub...a value of 3981 from the scanner is NOT the same as a density value of 3.6 (3981:1), for the reasons explained above. And, no, they are not close enough for government work ;-) Yes I agree with you here also but please see my comments above about relative density readings. Surely one is capable of determining relative density with one's own scanner? I've made some film charateristic curves using this method that seem reasonably good. -- Tony Terlecki [EMAIL PROTECTED] Running Debian/GNU 2.2 Linux Unsubscribe by mail to [EMAIL PROTECTED], with 'unsubscribe filmscanners' or 'unsubscribe filmscanners_digest' (as appropriate) in the message title or body
[filmscanners] RE: Density vs Dynamic range - was: RE: opinions? Reviews? of Primefilm 1800 ?
I find this conversation fascinating and hope you will stay the course with all my (possibly naiive responses.) I'm no expert at all but I have ideas that I thought were true and I'd like to work them through with you whatever the outcome... Hi Tony, I'll do my best ;-) On Sat, Jun 08, 2002 at 01:18:17PM -0400, Austin Franklin wrote: Hi Tony, Number of bits have two main roles. They do indeed represent the theoretical maximum density that a scanner could have if the electrical components were up to scratch and could use those bits to their full. Not true. You can represent ANY density by any number of bits. I can represent an entire density range with two bits: 00 - at or below dMin 10 or 01 - between dMin and dMax 10 - at or below dMax BTW, that should be at or ABOVE dMax...sorry. This is a completely valid encoding of density range. Sure it is. Great start ;-) Is that what happens in a scanner though? The major point is, though they MAY be somewhat similar, they are not the same. They they is density values and the numbers you get out of the scanner. IF the scanner were calibrated, as densitometers are (but more complexly), one could make them close enough to be considered the same, but no commercially available scanner I am aware of does this. I thought that the CCD behaved in a linear manner? Yes, but relative only unto it self! Does that not dictate the encoding method somewhat? Well, the linearity doesn't, but, the sensitivity does. If the CCD outputs +-1V and has a noise level of .001V, then you would have 2/.001 or 2000 discrete values you needed to discern, and would need 10 bits...but that's dynamic range, NOT density range. The density range of the sensor is what ever it is...again, part of the sensor's spec. I have to run now, so I'll comment on the rest of your post later. Regards, Austin And...to add further to this, as I have a few moments... Density values are absolute values, just like one foot is an absolute value. They have meaning in and of them selves. Someone decided what the exact length of one foot is (within a tolerance of course), as well as density values. They mean the same thing everywhere...a density value of 1.6 means exactly that in La Jolla CA, as well as in Pascagoula, MS. Same with one foot. But, the tonal values out of a scanner are not the same everywhere, nor are they the same even between scans! The value 136 in La Jolla, CA...does not have the same tonality as the value 136 in Pascagoula, MS. Yep I agree with you here. That density values are absolute values. But relative density is also commonly used in film sensitometry is it not? Yes...but I don't see how that ties into us here. Again, the scanner isn't calibrated, and dMax and dMin values ARE calibrated/absolute values... Scanners are not calibrated TO anything, except themselves. That is why the data values you get from the scanner are not the same density values. There is no direct correlation between them, unless you were to calibrate your scanner in the same way densitometers are calibrated (even that is insufficient, as the sensors used in film scanners have different characteristics than the sensors used in densitometers). OK I have just bought a densitometer which lets me calibrate so that a 0 reading can be set to film base, then readings taken from there. Cannot the same be done for a scanner? Yes, but that has nothing to do with dMax and dMin...with respect to any other scanner, and comparing scanners. You can't say that film has a dMax of X when measuring in that mode, you can only say it has a relative density value of Y compared to the film base. If I scan a piece of film and the film base gives me a value of 248 (assume 8-bit here just for argument's sake) cannot I set that as my point from which all my other density values can be measured? Yes, but again, that doesn't relate back to absolute density values... I may not know the absolute density reading but I would know the density range of the film I am scanning. But comparing that to another scanner would be meaningless... You would have a density range, sort of...simply because a density range of 1 to 2 is the same relatively as 2 to 4, but obviously, 4 is one heck of a lot denser than 2. If I were scanning a very dense piece of film (say Velvia) then I would need more than 8 bits to see well into those shadows would I not? No. I can design a scanner (and have) that only uses 4 bits to encode the entire range from 0 to 4. It just has no resolution (dynamic range). Think about line art. It has high density range, but no tonality...and you don't need more than 1 bit to encode it ;-) Surely one is capable of determining relative density with one's own scanner? Not accurately, unless, as I said, you calibrated it. You need to calibrate for offset, gain
[filmscanners] RE: Density vs Dynamic range - was: RE: opinions? Reviews? of Primefilm 1800 ?
Density values are absolute values, just like one foot is an absolute value. They have meaning in and of them selves. Someone decided what the exact length of one foot is (within a tolerance of course), as well as density values. Just as a point of levity did not Einstein's theory of relativity call the notion that a foot would be the same under all circumstances in all locations (i.e. a universal absolute)? If so, why would it not apply to the lowly density measure as well as the foot as a unit of measure? :-) -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Austin Franklin Sent: Saturday, June 08, 2002 12:18 PM To: [EMAIL PROTECTED] Subject: [filmscanners] RE: Density vs Dynamic range - was: RE: opinions? Reviews? of Primefilm 1800 ? Hi Tony, Number of bits have two main roles. They do indeed represent the theoretical maximum density that a scanner could have if the electrical components were up to scratch and could use those bits to their full. Not true. You can represent ANY density by any number of bits. I can represent an entire density range with two bits: 00 - at or below dMin 10 or 01 - between dMin and dMax 10 - at or below dMax BTW, that should be at or ABOVE dMax...sorry. This is a completely valid encoding of density range. I have to run now, so I'll comment on the rest of your post later. Regards, Austin And...to add further to this, as I have a few moments... Density values are absolute values, just like one foot is an absolute value. They have meaning in and of them selves. Someone decided what the exact length of one foot is (within a tolerance of course), as well as density values. They mean the same thing everywhere...a density value of 1.6 means exactly that in La Jolla CA, as well as in Pascagoula, MS. Same with one foot. But, the tonal values out of a scanner are not the same everywhere, nor are they the same even between scans! The value 136 in La Jolla, CA...does not have the same tonality as the value 136 in Pascagoula, MS. Scanners are not calibrated TO anything, except themselves. That is why the data values you get from the scanner are not the same density values. There is no direct correlation between them, unless you were to calibrate your scanner in the same way densitometers are calibrated (even that is insufficient, as the sensors used in film scanners have different characteristics than the sensors used in densitometers). Now, WHY would anyone in the first place say that number of bits has anything to do with density range...because they made some erroneous assumptions. Density range is stated in a ratio of to 1 (:1), and a density of 3.6 is 10**3.6:1 or 3981:1 (and someone, at some time, decided what the density value of 1 physically is), which the number 3981 requires 12 bits to represent in the binary system, if you are going to represent every integer value from 1 to 3981...BUT...that's the rub...a value of 3981 from the scanner is NOT the same as a density value of 3.6 (3981:1), for the reasons explained above. And, no, they are not close enough for government work ;-) Regards, Austin Unsubscribe by mail to [EMAIL PROTECTED], with 'unsubscribe filmscanners' or 'unsubscribe filmscanners_digest' (as appropriate) in the message title or body Unsubscribe by mail to [EMAIL PROTECTED], with 'unsubscribe filmscanners' or 'unsubscribe filmscanners_digest' (as appropriate) in the message title or body
[filmscanners] RE: Density vs Dynamic range - was: RE: opinions? Reviews? of Primefilm 1800 ?
Tony, Number of bits have two main roles. They do indeed represent the theoretical maximum density that a scanner could have if the electrical components were .up to scratch and could use those bits to their full. More importantly though the number of bits determine with what resolution the scanner can see the density levels that it is capable of scanning. Reread that last sentence again! Neither of the two points that are made, as I understand them, fits with my understandings. Under ideal conditions all things being equal, my understanding is that the bit depth if all those bits are capable of being used by the device at full capacity do not represent the maximum density that a scanner can have but the maximum ability of that scanner to capture in digital terms the details or discriminate between tonality at along the density range continuum (i.e., the dynamic range which the scanner can capture and digitalize within its desnity range) Namely, whether, in a denisty range from 0 to 100, the scanner can digitally discriminate between 10 discrete steps or 100 discrete steps. With respect to point two, it is not my understanding that the bit depth of a scanner has anything to do with the resolution that the scanner can see or read the density levels unless, of course, you mean by resolution the ability to make qualitative descriminations between different density levels or contrast steps within the contrast range or density range capable of being captured and digitalized by the device. The number of bits represents the POTENTIAL density range that the scanner is capable of. Scanner CCDs are linear devices I take issue with the statement that the number of bits represents the potential density range. Dynamic range yes; density range no. Secondly, I am not an engineer so I do not know for certain; but are we really talking about the CCDs or the DC converter when we speak of linear devices in relation to bit depth. Moreover, are either of them really linear devices in actuality? Let us take our 8-bit scanner then - 8-bits gives us a total of 256 values that the scanner can use to scan the image Are we really saying that the 8-bits are defining a density range of 256 equal units length from Dmin to Dmax as if this were a dimensional measure or that 8-bits are capable of breaking a density range of any dimensionality into 256 different, discretely defined sections as opposed to breaking the same density range into say 100 different descretely defined sections. For purposes of argument and because I lack the knowledge to say otherwise, I am willing to assume that the segments are identical equal interval segments and not ratio interval or non-equal interval segments of variying sizes, although that might also be theoretically problematic. It's difficult describing this purely in layman's terms. If you don't understand anything here then please say so and I'm sure we can elaborate/simplify further. I understand the gist of your argument and explanation; but I am having trouble following the details or knowing enough to raise appropriate arguments or questions concerning the details or the conclusions you have drawn from them. I must say that the exchange between you and Austin and Austin and others on the subject has sort of gotten beyond my ability to comment on with respect to technical details and arguments. I am going to sit back and listen to the discussion, asking questions or raising issues in an attemt to make sense out of what is being said and why. I apologize in advance if I sound confused or way off base. I do appreciate everyone's help in my attempt to clarify things. -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Tony Terlecki Sent: Saturday, June 08, 2002 7:46 AM To: [EMAIL PROTECTED] Subject: [filmscanners] RE: Density vs Dynamic range - was: RE: opinions? Reviews? of Primefilm 1800 ? On Sat, Jun 08, 2002 at 09:43:00AM +0100, dickbo wrote: Bits equals available grey levels per pixel - Original Message - From: Laurie Solomon [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Friday, June 07, 2002 4:22 PM Subject: [filmscanners] RE: opinions? Reviews? of Primefilm 1800 ? Another misconception...though equally as common...the number of bits the converter has, have nothing to do with the scanners ability to capture any particular density range. Just out of curiousity and in simple layman's terms, what do the number of bits that the converter has have to do with if not the density range? How does it impact on what is captured? Austin, I am asking a serious question here out of my lack of knowledge and sure would appreciate a good discussion in layman's terms so that I can understand what is being said without having to hire an engineer to interpret. It has always been my understanding, rightly or wrongly, that the higher the number of bits the more detailed or refined the informational date captured from
[filmscanners] RE: Density vs Dynamic range - was: RE: opinions? Reviews? of Primefilm 1800 ?
Austin, That is correctproviding the system can actually take advantage of those bits. If you have a 24 bit converter, and 12 bits of it is useless (noise), then what good are the 24 bits? Yes, that is a given (I understand that and accept that as a limiting condition). As for the general points, I think that my understanding parallels what you are saying for the most part, aside from my terminological mistatement where I used density range instead of dynamic range which you caught and objected to. In so far as my use of the two terms in the mistatement, dynamic range and density range tend to be used in the literature and manufacturer's specs synonymously as denoting the same thing (ie. the contrast range), I, following their lead, slipped into using the two as identical terms in my comments; but in doing so, I did no mean to suggest that bit depth was connected to the contrast range as both dynamic range and density range appear to make reference to in common marketing parlance of some manufaqcturers and magazines. Having said that,let me see if I got it by putting it into my own words. Density range refers to the contrast range from white point to black point covered by the scanner and is measured in terms of 3.0 to 4.2 Dmax or D ( it appears that in the current ads only Canon and Nikon are among the few who refer to the guage as a measure of dynamic range. Whereas bit depth refers the degree of quality information or detail that can be obtained within the given density range and is measured in terms of 8, 12, 16, 24, 36, 48 bits or the like. Thus, in your terms since most of the literature does not use any term to describe what is being measured except to refer to it as bit depth or x number of bits, bit depth measures the dynamic range that a device can capture within its given density range, as meeasured by the Dmax specification. Is my understanding close enough for government work or do I need to go back to the contemplation place and rethink it? -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Austin Franklin Sent: Friday, June 07, 2002 10:37 AM To: [EMAIL PROTECTED] Subject: [filmscanners] Density vs Dynamic range - was: RE: opinions? Reviews? of Primefilm 1800 ? Snip... Hi Laurie, First off, any density range can be represented by any number of bits (2 or more). Because you have 100 bits, doesn't mean you automatically can represent a density range of X...and because you have two bits, doesn't limit the density range these bits can represent. What number of bits give you is DYNAMIC range, NOT DENSITY range. They are NOT the same, no matter how hard people try to claim they are, they are not. They are two entirely different things. Density range is merely the dMax and dMin, it has nothing to do with how finely you can discriminate (resolve tones) within that range. Kind of like the difference between a 3 foot ruler with one foot markings...and a 3 foot ruler with 1/4 markings. They both measure the same RANGE, but not to the same accuracy. Dynamic range is the ability to discern within the overall range, really the resolution. If you want to discern 256 things, you NEED 8 bits...but that has nothing to do with what those 8 bits represent...as they are simply relative unto themselves. It has always been my understanding, rightly or wrongly, that the higher the number of bits the more detailed or refined the informational date captured from the original that is transmitted as data in the digital file with respect to highlight and/or shadow detail That is correctproviding the system can actually take advantage of those bits. If you have a 24 bit converter, and 12 bits of it is useless (noise), then what good are the 24 bits? with the density range figure represetning the range of contrast that can be captured. And...that is correct too. In other words, dynamic range representing the contrast range of the capture's capabilities, No, DENSITY range represents the contrast range...as you said just above... while the bit depth represented the quality of the data captured within that range particularly the extremes. Which is DYNAMIC range. Where apparently the confusion has arisen from, is the fact that density range values are stated in relation :1 as in to 1, so a density of 1.0 is 10**1 or 10, and means 10:1, so people simply assume that the resolution of the system is relative to some 1, which it is not necessarily. The values out of the A/D are not calibrated in any way, shape or form to density ratio values...they are simply a relative measurement in and of themselves. You could, of course, design a film scanner and calibrate it so it reads accurately like a densitometer...but none do. As an example, your BW 256 value (8 bit) file, can represent a density range captured from BW film of say, .1 to 2, which is a density
