Hi Ray!
> Is this really true? In the days before scanning became commonplace, I >always took the advice that "negatives have wider dynamical range" (than >slides) to mean that you can still salvage a good print from an under-exposed >or over-exposed negative - whereas a slide is its own end product, so a slide >which is under-exposed or over-exposed will look wrong and can't be >rescued.
That is true. With negatives you have something called "latitude". Usually it is measured in f/stops, and tells you the range within you still have usable data. Of course, it is measured from the neutral gray. Positives do have some latitude too, but it is smaller... much smaller.
>I never thought that it meant that there was anything much inherently different >between slides and negatives, in terms of the film emulsion response. >Except maybe in the slope and shape of the contrast/gamma curve, which >would potentially restrict the dynamic range.
The main difference is the development process. The "critical" step is the inversion (sorry, I don't know the propper english word). Positives are developed first as negatives, and then there is a second development that turns them into positives. Old positives used a flashing light to do so... I don't know how this is done today.
>But that would require that all negative film has very low contrast, and that >high-contrast printing would be necessary to give a normal looking shot. That >is hardly the case?
This is exactly what happens. Photographic paper emulsions have a much shorter dinamical range than positives. That's why one must carefully select the exposure paramethers while proyecting the negative into them, and why those 1 hour photo labs give you sometimes pics that you have to tell them to develop again, becouse are too dark or bright; they develop all the pics with the same settings as required by the first ones.
Just to put a few quick and dirt numbers. Let's say that a photographic paper emulsion may have a brightness ratio of 30 or 40. A common positive may have a ratio in the 80 - 100 range. A good negative may be in the 300-1000 range. (Just quick numbers... I may be wrong, but gives you an idea).
> So maybe a film expert out there can clarify this...?
Not an expert... but I had years of experience with black and white negatives (development mainly), and later some with color positives (scanning). Hope this helps. =)
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