Raimo sez: "it (lpm) is effectively MTF" Bob sez: "No, it is not effectively MTF."
Raimo insists: "Yes, it is. And Raimo continues: "Where do you find your points and lines made of points to measure your lpm?" Bob's Answer: For measurement, we create the lines. And Raimo continues: "In real world the edges of lines are not sharply defined if you look close enough." Bob's Answer: "Bullpucky! Consider a photo of an automobile (or an aerial photo of a tank). The outline of the automobile (or an aerial photo of a tank) against the background is a sharp edge and its reality is sharply defined, unless you want to get into some silly notion about fuzzy molecules on the surface of the object. The world is full of edges. Edges are how we recognize objects. In fact, the first layer of human visual processing is edge detection performed by neurons in the retina just behind the photo sensitive cells. Go to http://www.photodo.com/art/articles.shtml# The graph shows contrast vs. spatial frequency. This is a graph of the modulation transfer function (MTF). Lines per millimeter is a single point on this graph. It is the point where the MTF crosses 0 contrast. Anywhere just to the left of this crossing, items can be distinguished from one another. It may not look natural due to severe lack of contrast, but they can be distinguished. In some forms of photography, it is this ability to distinguish separation that is all important. Note that the example, while sufficient for photographers such as yourself and most on this list for evaluation of lenses is insufficient for others because it does not show where contrast becomes 0. To them, this is the all important single point on the graph. If they can get better contrast at the lower spatial frequencies, that's icing on the cake. In other words, lpm is not "effectively the MTF". It is just a single point solution to the modulation transfer function where contrast = 0. Regards, Bob... -------------------- "Let us contemplate our forefathers, and posterity, and resolve to maintain the rights bequeathed to us from the former, for the sake of the latter. The necessity of the times, more than ever, calls for our utmost circumspection, deliberation, fortitude, and perseverance. Let us remember that 'if we suffer tamely a lawless attack upon our liberty, we encourage it, and involve others in our doom.' It is a very serious consideration that millions yet unborn may be the miserable sharers of the event." - Samuel Adams, 1771 ----- Original Message ----- From: "Raimo Korhonen" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Thursday, January 17, 2002 8:42 AM Subject: Vs: And did I mention wrong > Yes, it is. Where do you find your points and lines made of points to measure your >lpm? In real world the edges of lines are not sharply defined if you look close enough - but I see you point: if you set limits you can measure lpm - but if you set a different limit, the result will be different. > All the best! > Raimo > Personal photography homepage at http://personal.inet.fi/private/raimo.korhonen > > -----Alkuperäinen viesti----- > Lähettäjä: Bob Blakely <[EMAIL PROTECTED]> > Vastaanottaja: [EMAIL PROTECTED] <[EMAIL PROTECTED]> > Päivä: 16. tammikuuta 2002 22:51 > Aihe: Re: And did I mention wrong > > > >No, it is not effectively MTF. In fact, lpm is not a transfer function. In > >fact, being a single point, it does not meet the mathematical definition of > >a function! > > > >Further, absolute black is available in nature. I did *NOT* claim that > >absolute black exists or is necessary, but it exists. Stop into an Afghan > >terrorist cave, have an F-14 seal it up with a missile, and you will > >"discover" absolute black in the visual spectrum. > > > >Further, no mythical absolute white is necessary to measure lpm *NOR* did I > >claim that it exists. I have no idea where you came up with this nonsense. > > > >All that is necessary for the measurement is that the power in the white be > >significantly higher that the power in the "black". This is why there is a > >reference to 3 dB or half power in what I said. This is a _relative_ > >_power_, relative to the peak power in the white. How high is significantly > >higher? well, that depends on how accurate you want the measurement. Just 15 > >dB or 5 stops is certainly enough. > > > >Measurement of MTF is subject to analogous constraints. > > > >Regards, > >Bob... > > > >From: "Raimo Korhonen" <[EMAIL PROTECTED]> > > > > > >> No - I am not wrong - when it is done the way you say, it is effectively > >MTF. There is no absolute black or white in the nature, or in the world if > >you will. > > > >> Lähettäjä: Bob Blakely <[EMAIL PROTECTED]> > >> > >> >No. You are wrong. Lines Per Millimeter is objective. It's often read > >with > >> >the human eye, but can be read by instruments. Lines Per Millimeter can > >also > >> >be calculated mathematically from the rise from black to white of a > >single > >> >edge (or fall from white to black). It is identical to the concept of > >> >measuring the resolution of a radar (in this case the measure is > >degrees). > >> >Generally, the figure is not measured but calculated from the parameters > >of > >> >the antenna of which gain is the greatest determining factor. The edge of > >> >resolution is where (for example) the light (from white to black) falls > >off > >> >by 3 dB or half power. It's at this point that two separate objects > >cannot > >> >be separated mathematically, let alone visually. For photo surveyors and > >> >aircraft/spacecraft spies, this is the ONLY measure that counts. FYI, lpm > >> >for these lenses are not measured by eye but by instruments. Stay tuned > >for > >> >an installment on how contrast affects this number. > >> > > >> >I do err, but not often and not in areas where I have worked. > >> > > >> >Regards, > >> >Bob... > >> > > >> >From: "Raimo Korhonen" <[EMAIL PROTECTED]> > >> > > >> > > >> >> Nope - lpm is measured by human eye and it is quite subjective. I think > >> >that MTF is the most objective method, it takes into account both > >contrast > >> >and sharpness objectively. > >> >> All the best! > >> >> Raimo > >> >> Personal photography homepage at > >> >http://personal.inet.fi/private/raimo.korhonen > >> >> > >> >> Lähettäjä: Bob Blakely <[EMAIL PROTECTED]> > >> >> > >> >> >"Sharpness" and resolution are closely tied together, so I will > >discuss > >> >them both. > >> >> > > >> >> >Resolution is objective and quantifiable. It is a measure of the > >ability > >> >to separate two > >> >> >identical objects as they are brought close together. In photography, > >> >this is measured in > >> >> >lines per millimeter where the lines are black the spaces between the > >> >lines are white. > >> >> <snip> > >> >> >Regards, > >> >> >Bob... > >> - > >> This message is from the Pentax-Discuss Mail List. To unsubscribe, > >> go to http://www.pdml.net and follow the directions. Don't forget to > >> visit the Pentax Users' Gallery at http://pug.komkon.org . > >- > >This message is from the Pentax-Discuss Mail List. To unsubscribe, > >go to http://www.pdml.net and follow the directions. Don't forget to > >visit the Pentax Users' Gallery at http://pug.komkon.org . > - > This message is from the Pentax-Discuss Mail List. To unsubscribe, > go to http://www.pdml.net and follow the directions. Don't forget to > visit the Pentax Users' Gallery at http://pug.komkon.org . - This message is from the Pentax-Discuss Mail List. 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