Re: OT - Why preflash? (was:Re: DSLR slide duplicator)
I wrote: > > Gamma (also written as the infinity > symbol, IIRC) . > WRONG, WRONG, WRONG! Shame on me for not checking, and shame on list-members who should have known and didn't spot my mistake. Gamma is written as the Greek letter gamma, of course. Sort of like a handwritten "y" but with a loop in the tail. I wonder how many other goofups I'll need to correct. >> In reply, John Francis wrote: > And what about those of us who spotted it, but didn't consider it > worth correcting a minor error in a parenthetical aside? It's not > as if this was particularly germane to the central discussion. > John, Considering other factual errors I have made that have been corrected promptly, I was surprised that nobody mentioned this one. I did assume that some of us would be familiar with the terms, but it never entered my head that anyone recognising the error would let it pass uncommented. I should be grateful that I got to make my own correction, but I'm sorry if it seemed I thought nobody else ~knew~ the correct symbol for gamma. A mix of sleep deprivation and overmedication for an autumn flu, led to my omission of the all forgiving smiley. That's one more goofup to add to my tally. regards, Anthony Farr
Re: OT - Why preflash? (was:Re: DSLR slide duplicator)
> > - Original Message - > From: "Anthony Farr" <[EMAIL PROTECTED]> > > > Gamma (also written as the infinity > > symbol, IIRC) . > > > > WRONG, WRONG, WRONG! Shame on me for not checking, and shame on > list-members who should have known and didn't spot my mistake. And what about those of us who spotted it, but didn't consider it worth correcting a minor error in a parenthetical aside? It's not as if this was particularly germane to the central discussion.
Re: OT - Why preflash? (was:Re: DSLR slide duplicator)
This is a response to an off-list question from Mike Wilson. However, I thought it addressed a few undeveloped lines of reasoning from my earlier posts in the thread, and raised some questions that I can't answer myself. In which case I hope Mike doesn't mind that I post my response to him, to the list as well. Thanks for the question, Mike. - Mike, In truth I don't know how a digi sensor would handle preflashing, especially overflashing. I think I got a bit muddled there saying that it would clip into the blacks. The nature of digital is that there's no result until it's adjusted for display or printing, in which case everything would get moved out to the black and white limits, and all you'd be doing by fogging the blacks is robbing yourself of bit-depth. OTOH preflashing, if carefully controlled to avoid exceeding the inertia point (the threshhold between no record of exposure and the smallest trace of dark tones being captured) should in theory work just the same as it does with film. I also suspect that preflashing would actually need to be simultaneous to the main exposure, because after each capture the sensor would fall back to its unexposed state. It wouldn't be possible to accumulate any latency of fogging exposure for the next exposure to piggyback upon, unless it was within a single capture. I also don't know if digi-sensors are able to accumulate intermittent exposures. The exposure may need to be all in one dose. Ask an owner :-) Multi-layering may work, but begins to negate the appeal of getting a good quick and easy alternative to using a film scanner. For all that, it may not be necesary to use any contrast control in digital copying, especially if shooting to RAW files which capture 12 bits per colour (on the *istD) rather than 8 bits per colour that JPG or TIF files capture ( I don't think the *istD writes 48bit TIF, don't know really). How many stops is 12 bits equal to? From memory a slide has a 5 stop straight line. Pad that out a bit for the toe and shoulder so let's say 7 stops of image. But remember that it's recorded at a gamma of 1.8, or 1.8 times the contrast of real life, that makes it a density range of 12.6 stops. (I wasn't sure if my reasoning was correct here, or whether gamma 1.8 would only add just under 1 extra stop, so I metered some pro 120 tranny on a lightbox and got close to 10 stops range. In that case 12.6 stops for consumer slide film seems reasonable) The big question is:- Can a digi sensor hold over 12 stops in a single capture? The answer to that will be the opposite answer to:- Is contrast control required to copy a slide with a DSLR? Tell me if you know, please. regards, Anthony Farr - Original Message - From: "mike wilson" <[EMAIL PROTECTED]> To: "Anthony Farr" <[EMAIL PROTECTED]> Sent: Thursday, April 22, 2004 3:51 AM Subject: Re: OT - Why preflash? (was:Re: DSLR slide duplicator) (snip) > > I understand about preflashing - done it a few times myself. I am just > curious as to why (or if) it is neccessary on a DSLR. I would have > thought that there are other techniques (for example, multiple exposure > overlay) that could be used. I am also unsure of the similarity to film > in this respect of the sensor. > > mike > >
Re: OT - Why preflash? (was:Re: DSLR slide duplicator)
Got it in one. I use a piece of white perspex as a diffuser. Andre Langevin wrote: Thanks for the nice theory behind the applications. Now, in practical terms, for non-professional work, if I use KR (64 asa) to copy slides, should I expose all my film at 6400 asa (100:1 ratio), go back to take 1 (easy with a LX) and take my slides normally at 64 asa? Andre
Re: OT - Why preflash? (was:Re: DSLR slide duplicator)
Thanks for the nice theory behind the applications. Now, in practical terms, for non-professional work, if I use KR (64 asa) to copy slides, should I expose all my film at 6400 asa (100:1 ratio), go back to take 1 (easy with a LX) and take my slides normally at 64 asa? Andre
OT - Why preflash? (was:Re: DSLR slide duplicator)
- Original Message - From: "mike wilson" <[EMAIL PROTECTED]> > Hi, > > Anthony Farr wrote: > > > > > allowed. I don't know, but contrast control by preflashing may not even be > > necessary if the whole range of the slide can be captured. Film to film > > copying needs contrast control, unless duplicating emulsion is used, > > because the gamma of the original slide being about 1.8, and the gamma of > > the camera's slide film also being about 1.8 gets a resulting gamma without > > contrast control of 3.6, or basically unuseable. > > Why would you want to (or be able to - does the *istD have multiple > exposure facility?) preflash with a DSLR? > > mike > > THE SHORT EXPLANATION: Most photography is concerned with recording scenes from real life. A slide film is nearly twice as contrasty as real life. To copy a slide you need either special low contrast emulsions that are sold only in large amounts, or you need to use some method of contrast control if using a normal film. With black & white the development can be modified to alter contrast, but that method raises too many problems with colour. The most common method of contrast control for colour copying is "preflashing", aka "prefogging". THE LONG (LONG) EXPLANATION: Preflashing is an overall brief exposure to light either before or concurrrent with the main image-forming exposure. It's important to know that it's an overall fog being applied, not a pre-exposure to the image being recorded. Two slide duplicators that I can name immediately have this function, the Bowens Illumitran and the Elinchrom Dia-duplicator. The advantage of these units is that ordinary camera film can be used in place of special low-gamma copying emulsions. For the purposes of my (then) employers I could make duplicates with better archival properties with Kodachrome than with Ektachrome duplicating film. First an explanation of terminology. Gamma (also written as the infinity symbol, IIRC) is an antiquated expression of film contrast introduced (I think?) by Edward Weston, but because it's easy to write and say it survives in name, although now it mostly refers to "contrast index" (CI). Another method which I was once familiar with (and was the one my college used) can't be written in plain text without special characters, but is said as "G-bar" and is literally a letter G covered by a horizontal bar. The only significant difference between the three is their definitions of where the straight-line portion of the characteristic curve starts and ends, the slope of which is the measure of an emulsion's contrast. The results of these methods are so close that it matters very little which one is being quoted. I don't know if digi sensors measure their contrast in terms of gamma, I think the bit-depth is the ruling factor, but their contrast/latitude characteristic is close enough to film to consider them as approximate equals. The problem with duplicating is that photographing an original slide is not like photographing real-life. The real world has a gamma of 1, consumer slide films have a gamma of about 1.8 to overcome contrast losses in projection. Professional transparency films have a gamma of about 1.4, being intended not for projection but for print reproduction. Duplicating a slide with a normal slide, with no contrast control, will build contrast to an unacceptable amount. You also need to know about the "inertia point" of a photosensitized material. Essentially, a few photons of light will strike a silver halide grain or a photosite before it will move from a completely unexposed state to a state where it begins to properly record light exposure. Digital performance in this regard is unknown to me, but it is still true that the small amount of light visible to the eye in the shadows of a normal scene nevertheless produces no record of exposure in the corresponding photosites of a digi sensor (apart from a little electronic noise which I think is an artifact of the analogue to digital conversion). Whether or not this point is called the inertia point in digi-speak, that's the term I'll use for the sake of simplicity. The purpose of preflashing is to nudge the film/sensor to the brink of its ability to record exposure, i.e. the "inertia point". If no extra exposure is received the result will be as black as if no light at all was ever recorded. But here's the beauty of the technique - the very least extra exposure will immediately begin to be recorded without inertia. The result is that the film's/sensor's capacity to record shadows and dark tones is dramatically expanded, plus there's a moderate boost in overall sensitivity. Preflashing beyond the inertia point will reduce overall contrast, at the expense of a loss of maximum density in a slide (elevated d-min for negatives), or in digital terms a clip into the darker tones. In duplicating terms this means that the range of densities in the original slide can be captured by the dupli
