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Thread: contrast vs focal lenght?

  1. #1

    contrast vs focal lenght?

    I recently got the project of producing large format negatives from pinehole camera with the (long term?) objective of doing direct contact printings. So I built a rather large pinehole camera from a wooden wine bottle box (côtes du Marmandais, caves de Beaupuy, cuvée ‘Confidentiel’, 1996, to be precise… :P ) : focal length, 150mm; hole diameter, 0.50 mm; film holder 7x9.5inch. From the tests I ran it doesn’t seem to be faulty in any obvious way; no light leak, black painted etc…. However, all the films I developed so far from it systematically exhibited EXTREMELY LOW CONTRASTS…. I over- and under-exposed, under- and over-developed at different temperatures with always the same under-contrasted result!.... By comparison, the negatives I produced in exactly identical conditions (same light, same day, same film, same chemistry, same development procedures and so on) with a 40 mm focal length pinehole camera showed very satisfying contrasts to my amazement!..... Same think on my 6x6 Mamiya, so I conclude that low contrast does not originate from film/chemistry/development problem but from something else.

    So the QUESTION is: is there any direct correlation between focal length and contrast of the negative, meaning that contrast would decrease with increasing focal length? Because, as it is, this is the only reasonable explanation I can imagine so far…. (the other one being that the year of the wine box influences the result, but 1996 is acknowledged as a good one, so it does not really fit!) Did anyone ever encounter such a problem and could share his experience with me?

    For information, I use orthochromatic X-Ray film (very easy because you can cut it any size you want under inactinic light) that surprisingly produces excellent negative (I mean with anything else than my wine box of course…), developed in high contrast Kodak D19 developer.

    I apology for the naïve question, and thank you in advance for your enlightment,

    yann

  2. #2

    contrast vs focal lenght?

    Hmm. If you're following some sort of optimal pinhole size for each focal length, then the longer focal length will have a larger F/#; your 7x9 is F/300, you didn't say what the 40mm camera's focal ratio is.

    The higher f/# camera should require longer exposure times, as compared to the shorter camera, given all other variables unchanged (i.e. same film type, same lighting, same processing, etc.) I usually find the problem with my photography has been underexposing larger F/# cameras, until I get the calibrated times right for my process.

    My experience has been that should you under-expose the film, it should look like too much contrast; the highlights develop first, and there's not enough exposure to get detail in shadows, so they don't develop, resulting in excess contrast.

    Of course eliminate all the variables. Cut a negative for each camera (short and long) from the same sheet of ortho film. Expose both cameras simultaneously in the same light. Develop both negatives in the same tray simultaneously; you should be able to inspect them as the images come up.

    Assuming your short camera's focal ratio is about half of the longer's, then you should ensure the longer camera gets double the exposure time of the shorter. Since you already know exposures details with the shorter camera to get good negatives, just double that figure for the longer camera.

    Good luck and keep us posted.

    ~Joe

  3. #3

    contrast vs focal lenght?

    thank's a lot for your helpful answer!

    yes indeed, from the 7x9 camera, I computed a F/300 and from the smaller camera a F/80 (0.5 mm diameter for a focal lenght of 40 mm, not optimal ratio, but still gave me very nicely contrasted negatives....) and adjusted the exposure accordingly (I use the very nice pineholeDesigner from pinhole.cz to do all calculation).

    I did the exact experiment you recommended : exposing the two films from the same sheet of ortho film in the two cameras, one exposed for F/80 and the other for F/300.... and the result was stricking: one had excellent contrast (F/80) the other bad one (F/300).....

    OK then, I will give that more thinking I guess...

  4. #4

    contrast vs focal lenght?

    I wonder if with the film you use, you have to correct the longer exposure times needed by your larger camera for heavy reciprocity failure. In this case, what would be the effect of the same failure correction (same exposure time) on differently illuminated points of your negative, which will develop different densities : could it cause the range of densities be somehow squeezed, yielding lower contrast ? I don't know, it's just an idea.

    To be sure, use the large camera with a very large pinhole giving f/80, you will get a more blurred picture, but maybe with the same contrast as your little pictures. I mean, with this comparison, you will have exactly the same reciprocity conditions, as you will use exactly the same exposure time.

    Paul

  5. #5

    contrast vs focal lenght?

    dear Polka,

    thank you very much for the tip. I just changed pinehole on my big camera to a larger one in order to decrease to focal from f260 to f130, and obtained 7x9.5 X-Ray neg with a very good contrast (and a more blurred picture of course, but then I do not mind at all with pinhole). I do not really understand the bottom line of this story: why the f number modifies the contrast, but now I can play with that to obtain optimum results. I will post the first decent neg I obtain this way...

    thanks again Polka and Joe for your precious help!!!

  6. #6

    contrast vs focal lenght?

    I'm glad it's starting to work for you. As Paul alluded to reciprocity failure, should you decide to go back to using the F300 pinhole, try doubling the exposure time from what you used before, and see if you get better contrast.

    ~Joe

  7. #7

    contrast vs focal lenght?

    For the "tinkerers" :

    If this hypothesis of differential low intensity reciprocity failure were confirmed, it would beg for extension of the "zone system" methodologies WITH differential reciprocity failure corrections (that is to say : to find out how to adapt exposure and development for desired contrast (as did AA), but taking also into account differential LIRF).

    Paul

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