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Re: [OM] Film latitude

Subject: Re: [OM] Film latitude
From: dreammoose <dreammoose@xxxxxxxxx>
Date: Wed, 22 May 2002 14:45:10 -0700
Further comments and a different perspective:

John A. Lind wrote:

At 11:10 5/22/02, you wrote:

This is probably a dumb question, but when people say "negative film has
more latitude than slide film", which stage(s) are they referring to where
you gain/lose latitude?


Latitude is the difference between how much light creates "pure white" and how little creates "pure black" on the film. It is the ability to capture detail in both highlight and shadow. The wider the separation between the two (higher contrast), the wider the latitude required from the film to maintain detail.

Light from the subject is 'mapped' onto a range of densities on the film. There are 4 important characteristics of this map that bear on 'latitude': 1. Absolute latitude, the range between the brightest incoming light which the film can distinguish from the next less bright step (see #3) of light. and the equivalent lowest brightness it can record.
2. The range of densities on the film to which the latitude is mapped.
3. The resolution of latitude, the smallest difference in incoming light level which can be measured as a difference in density in the processed film.
4. The linearity of the mapping.

Comparing Color negative and transparency films:
1. Color negative film has wider absolute latitude than color transparency film. 2. It maps this wider latitude to a slightly smaller range of densities than does transparency film. 3. While there is a theoretical question about loss of information through mapping a greater range of brightness into a narrower range of densities, it seems it isn't a practical problem, probably, at least in part, because prints have even poorer latitude and resolution. Anyway, I've never heard complaints about color negative film that sound like this would be the cause. (I have heard complaints on this list about the lack of smooth density mapping in some B&W negative films.) 4. Linearity shows up in comments such as those about the way shadows in one film 'go green' while those in another 'go magenta' etc. I'm sure film engineers study it. I suspect is is a significant factor in what we users see as subjective differences between films.

These differences have practical consequences for photographers. For someone like John who primarily works with 'conventional' printing to a medium that has a lower latitude than film through projection printing and chemical processing, you can tell a lot about the image with light table and loupe or a projector, whereas color negatives have a very high 'gobbledygook' factor when viewed directly.

For a photographer who primarily electronically 'processes' film images by scanning them, negative film has some advantages: 1. It's wider latitude means it contains more total information about the original range of light that came through the lens. This gives more control over the contrast/brightness characteristics of the final print. 2. It's narrower latitude mapping occurs in such a way that it's highest density (brightest light input) is less than the highest density in the darkest areas of transparency film. Since current technology in film scanners tends to produce noise in the form of random light pixels in the densest areas of the film being scanned, it is easier to get a noise free scan of negative film. 3. From the lack of 'noise' about it, I assume that the film has equal or better latitude resolution than the scanners, so it is a non-issue with current technology?
4. ????

Having the widest possible latitude is not always a Good Thing. In low contrast scenes it can make the image look "flat" and often "muddy." Contrast between highlight and shadow is one of the visual cues that gives two-dimensional graphic images a feeling of depth and dimension (there are several others). It also enhances object/subject shape and surface texture.

Since I can control brightness and contrast and decide what part of the latitude to retain and what extremes of the brightness range to discard (in Photoshop or whatever), I prefer wide latitude in the film. The more information I bring back on the film, the better off I am.

<big snip> A print is a photograph of the negative (or transparency). Just as the photographer can make errors in focus and exposure, the "printer" can also make similar errors with print density (same as exposure), enlarger focusing and color balancing.

In the case of a transparency (technically 'reversal' film) the transparency is a chemical 'print' of a negative image. The color chemistry first develops a negative image very similar to color negative film (but without the orange mask). It then chemically reverses the image to a positive. I believe limitations in this process are responsible for the lower latitude of reversal film.

Printing from transparency is much easier; the transparency itself is a "witness" for print density and color balance; the print either looks like it or it doesn't.

This is true, it is relatively easy to see if the print matches the transparency, and one can do the same when using digital scan/process/print from transparencies. In digital scanning of negative film, I do something similar. I scan without any color or contrast adjustment in the scan, to produce a relatively neutral digital reference image to start with. (I also use Vuescan so the removal of the color mask is specific to the film type to minimize color balance effect from unmasking.)

It is important to distinguish this matching process from matching the print to the original scene. There is no process, theoretical or practical, which can reproduce in the eye and brain of someone viewing a photographic image the 'equivalent originals' in the eye and brain of the photographer. Read all the endless discussions about the objective and subjective qualities of various films and you will see that this is true. Take a whole bunch of shots of the same scene with different transparency films, show them to a bunch of people who saw the original and you will have an endless discussion about which one(s) more accurately reproduce the original. They will, of course, be comparing two types of imperfect chemical memory of an event which no longer exists, Given this truth, I am quite happy to attune my prints from color negative film to my fallible memory of the original scene. I think this is no more nor less 'accurate' than the choice of transparency film with which to take the picture and choice of filters to use in taking it.

I used to take slides for 'serious' pics and negative for the rest, often carrying an OM with slide film and an XA with print film. Now I use negative for everything. This is just one person's perspective and your's will likely vary. I agree with everything John says, from one perspective, and see shading of difference from others.

Moose


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