[rollei_list] Re: Contrast and Resolution
----- Original Message ----- From: "Neil Gould" <neil@xxxxxxxxxxx> 
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Thursday, November 08, 2007 7:28 AM
Subject: [rollei_list] Re: Contrast and Resolution



Hi all,
The discussion regarding contrast vs. resolution is interesting and raises a point that I would like explained. How was resolution determined independent of contrast? Today, we see resolution specifications that include a contrast figure, and it is "common knowledge" (a.k.a. layman's understanding) that lower contrast will accompany lower resolution performance. Yet, the comments in this thread state the opposite! Can one of you provide a definition and/or a method of determining resolution that 
excludes contrast?

Regards,

Neil
The most common measurement for resolution is a modulation transfer graph arranged to show resolution vs: contrast. The contrast being the that between the dark and light lines. The curve will show the effect being discussed. It is a misunderstanding to think the "contrast" being discussed here is overall image contrast, rather it is the contrast between the light and dark lines. This is essentially the same as the MTF curves shown for film. For a "high contrast" lens the MTF chart will show the contrast staying at a high level into the mid range of lines per unit measure and then falling off fairly rapidly. For a high resolution lens the contrast at mid values will usually be lower but the curve will fall off slowly and extend further than the high contrast condition. At very high values of resolution the high resolution lens will have _higher_ contrast than the "high contrast" lens. The difference in the two conditions depends on how the designer has balanced the higher order aberrations and on what I will call the illumination of the entrance pupil. If one charts the energy distribution of a beam of light focused by the lens one finds that the high contrast lens concentrates the light in a small beam or spot but has many smaller peaks or beams surrounding the main beam. These cause a sort of flare around the main beam lowering the contrast. By suitable adjustment these secondary beams can be much reduced at the cost of making the main beam broader. The second condition reduces the resolution but increases the contrast. The process is known as apodisation or removal of the feet. For those familiar with radio antennas or acoustic radiators an exactly analagous condition exists there, the suppression of minor lobes with the concurrent broadening of the main lobe. Since the eye tends to interpret high contrast at the edges of objects in an image as sharpness a lens with moderately high resolution but high edge contrast will be interpreted as being sharper than one with very high resolution but only moderate edge contrast. In some applications resolution is more important and lenses are optimised for maximum resolution rather than edge contrast. Resolution of a lens must be measured using the aerial image rather than on film because the magnitude of resolution of the two is similar. Modern pictorial films have resolutions as high as around 100 line pairs per millimeter from a high contrast target (1000:1). Many lenses will exceed this but a lens having over 100 lp/mm is considered outstanding. There are a couple of formulas for calculating the approximate value of combined resolution but they do not take into account the fact that there is not actually a single value. A true combination requires the convolution of the two curves but the approximate value is (1/film * 1/lens)/(1/film + 1/lens). Sometimes the root mean square is given as the right approximate value. Note that on film the effect of edge contrast is called "acutance", a term invented by Kodak. On film the value of contrast of mid resolution values can actually be higher than the contrast of the original target. This is the effect of so called acutance developers. I am not sure if a lens can be made to produce the same effect. On film, the increase in acutance is often accompanied by a loss of the highest values of resolution. In that respect the effect is the same as for lenses but the mechanism is different. 

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx 
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