Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!ncar!gatech!udel!haven.umd.edu!ni.umd.edu!ni.umd.edu!zben From: zben@ni.umd.edu (Ben Cranston) Newsgroups: comp.sys.mac.programmer Subject: Re: what I want to see in future Apple computers Summary: Corrections to my observations on film Gamma Message-ID: <1991Jun26.204153.15322@ni.umd.edu> Date: 26 Jun 91 20:41:53 GMT References: <1991Jun18.135838.3444@potomac.ads.com> <1991Jun20.172609.9795@ni.umd.edu> Sender: usenet@ni.umd.edu (USENET News System) Organization: University of Maryland at College Park Lines: 51 Nntp-Posting-Host: ni.umd.edu In article <1991Jun20.172609.9795@ni.umd.edu> I wrote: > ... Supposedly you want a > linear Gamma function if you're taking a picture of the screen, because > film's response is linear with number of incident photons, ... I received the following correction via email. When I wrote back suggesting it would make an excellent network followup, he gave me permission to repost the item. Posted by permission of author: Date: Fri, 21 Jun 91 12:58:31 PDT From: Theron Trowbridge Uh, is that really what gamma is defined as in the computer world? Gamma for a film stock is MOSTLY linear. The gamma describes, essentally, the contrast of an image. The number assigned to a Gamma describes its slope as drawn on a sensitometry curve for a film stock. The graph describes the relationship (as you say) to the amount of photons hitting the film it takes to cause a certain change in the desnity of exposed silver salts on the film. The higher the gamma, the steeper the slope, and the more exposure it takes to cause a small change in density; this means there is a higher contrast. Lower gamma means, conversly, lower contrast. However, there are curved parts of this graph at the top and bottom of the slope. They are kinda outside the "dynamic range" of the film. But they, too are affected by the film's gamma. Minor thing, tho, and probably parentheical to most computer applications. Video gamma is essentially the same thing. Exposure vs Intensity. The only thing is, it takes twice a change of intensity of two-fold for the human eye to notice a significant change. Each doubling is an apparently equal incrementation. A films gamma is an exponental curve, but its main purpose is not to describe this exponential relationship, but it has this correction built into it so the relationship between exposure and intensity can be better demonstrated... Ugh. it always steams me to find out that the engineers have taken another ages-old term and re-defined it for their own purposes... (I hope this makes some sense to you. I was probably a teacher in a former life and now I feel the urge to explain things to people. This is not a flame) BTW - what photographers consider a "perfect" gamma -- the one that most accurately reproduces its subject as the human eye sees it -- is .5 Gamma theoretically ranges from 0.0 to 1.0 and .5 is dead center. a perfect 45 degree slope on the EvsD curve. -Theron Trowbridge trowbrid@girtab.usc.edu AppleLink: D7029