Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site onfcanim.UUCP Path: utzoo!watmath!watnot!watcgl!onfcanim!dave From: dave@onfcanim.UUCP (Dave Martindale) Newsgroups: net.rec.photo Subject: Re: Product Quality: Color prints from slides (contrast buildup) Message-ID: <14708@onfcanim.UUCP> Date: Fri, 25-Oct-85 02:58:30 EDT Article-I.D.: onfcanim.14708 Posted: Fri Oct 25 02:58:30 1985 Date-Received: Sat, 26-Oct-85 06:20:15 EDT References: <298@tekig4.UUCP> <349@vaxwaller.UUCP> <5746@tekecs.UUCP> <1505@utcsri.UUCP> <334@tekig4.UUCP> <2141@amdahl.UUCP> Reply-To: dave@onfcanim.UUCP (Dave Martindale) Distribution: net Organization: ONF, Montreal Lines: 55 In article <2141@amdahl.UUCP> ems@amdahl.UUCP (ems) writes: >Something which has always bothered me, when you copy a print onto >a slide (an perhaps other copies as well) you get 'contrast buildup'. >It seems to me that if the contrast range of the print is *less* >than that of the original subject, then the contrast range of the >slide should also be *less* than the original. So why does the >slide show *higher* contrast? There are two things to consider here - the contrast range that can be *represented* by a particular medium, and the contrast ratios of "good- looking" images in that medium compared with the contrast ratios of the original. A good print material is only capable of about a 30:1 range of reflectances. Thus, when printing, you set your exposure so that some area of the print comes out white, and anything in the image that is darker than 1/30 of this is mostly lost in the shadows. Transparency material, on the other hand, is capable of 100:1 contrast range or more. But representable contrast range has nothing to do with contrast buildup. For a particular photographic material, if you plot the log of reflectance (or transmittance) against the log of the illumination level, you will get an S-shaped curve with a centre section that is roughly straight. The slope of this curve is called "gamma". If it is 1.0, then a given change of luminance between two parts of the image will produce the same change in the photograph - there is no change in contrast (provided both luminances fall in the linear part of the curve, of course). If the gamma is 2.0, for example, then a 2:1 luminance change in the scene will produce a 4:1 change in the final image. Why would you want a gamma of other than 1? Well, it seems that a gamma of 1 is about right for prints, that will be looked at by reflected light. But for images that are projected (slides, movies) or self-luminous (TV), they appear subjectively "flat" or low in contrast if they have a gamma of 1, reproducing luminance ratios correctly. I believe that this has to do with darker tones appearing subjectively lighter than they are because the image is seen in a dark environment. Anyway, an overall gamma of about 1.5 seems to give these types of images enough additional contrast so that they look correct. So direct-positive films, television systems, and negative-positive movie film systems are all designed to produce a final gamma of about 1.5. And they work well if you photograph real images. But if you photograph a slide to duplicate it, you have now increased the total gamma to 2.25 (gammas multiply) and it looks too contrasty. To avoid this problem, you need to do duplication with a film that has a gamma somewhere down near 1.0. In real life, things are a bit more complicated than the description above, since the low-light end of the response curves of many films have a greater gamma than at higher light levels in order to compensate for the loss of contrast due to lens and camera (or enlarger or printer) flare. Dave Martindale