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Thread: how to do the math for speed film test

  1. #11

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    Re: how to do the math for speed film test

    Quote Originally Posted by Peter Gomena View Post
    When you photograph the gray card, you first must have your camera focused at infinity. Then fill the ground glass with the gray card. If you merely de-focus the camera, you've changed the focal length and the proper exposure.

    So on a 4x5 view camera that means extending the bellows all the way to it's maximum?
    Alexis

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  2. #12

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    Re: how to do the math for speed film test

    Just go with the (ISO) speed on the box first, and see if it fits in with what I posted before... Remember, the developing time controls the highlights, but the exposure will control the shadow level... Read what I posted again carefully... ;-)

    This will get the proper exposure level on the film... Then comes the developing tests... Later you can try overexposing the film slightly (+ 1/3 to 1/2 stop, and slightly underdeveloping the film to find a "sweet spot" for nice highlights/shadows...

    Steve K

  3. #13

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    Re: how to do the math for speed film test

    OK, thanks Steve, but as you will soon discover I am as dense as some negatives So let me clarify a bit. I should still do the grey card test, right? Then I should do another test with a grey card, a white sheet and a post it note n the scene, right? That should likewise be focused at infinity, or should it be focused normally? Do I do this highlight test after I have completed the grey card test to determine speed? Should it be shot at the newly determined speed test, or do I shoot several shots at varying apertures 1/3 stop apart?

    I'm sorry that I don't completely understand.

    I need a cheat sheet that lists all the steps I need to take. Does that exist somewhere?

    Thanks again,

    Alexis



    Quote Originally Posted by LabRat View Post
    In addition the the grey card, you also need to see what is happening to the extremes... For the highlight test, put a white sheet with a yellow post-em note next to the grey card... The white should be close to D-max with the density just enough to barely see through it, and the post-em note should just separate into another step if exposed/developed correctly...

    For a D-76 type developer/normal exposed film in daylight, expect a range something like this:

    If you spotmeter a scene in sunlight, place a bright white object (you want to hold detail in) and meter that... One stop under that reading should be middle grey, and two stops under that will be the safe shadow region that will hold detail well, but the 3rd stop under middle grey will record form, but no detail... Under that will be no exposure... This is about the range for a standard developer + film... If you meter for these, you will safely be within range...

    For the developing, just long enough before the D-max blocks up... This is the simplest "zone" type system you can use for normal (N) negs, and will allow you to fit the range of exposure onto most films (with standard film developers), and print easily, before other processes/steps are tried... And will work with the vast majority of normal scenes you will encounter...

    Good Luck!!!!

    Steve K
    Alexis

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  4. #14

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    Re: how to do the math for speed film test

    Yes, the reason to focus for infinity (or at least > 6 or 8 ft), is so that there is no bellows factors that will influence the exposure... Yes, you can include the white sheet with the grey card exposure, but you would need the grey card exposure for the print calibration step where you would be matching the grey card to the print grey patch... But for film, the white/yellow step is important for exposure as overexposure would make this step invisible... The next test would be to shoot a daylight scene with highlights you want to hold detail in, to shadows you want to keep... (Use the measuring scale I mentioned in other post...) If your highlights are too dense, it might be overdeveloped, or you would have to underexpose, but use the posted info to tell the difference... (If you can see the wh/yel step and see through the dense part of the neg slightly, you are there... Then look at the shadows on the neg and see if they fit within what I posted when metering...

    This should get you started...

    Steve K

  5. #15

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    Re: how to do the math for speed film test

    As a question, how do you know that the 1/60th film speed that you used is accurate? I bought a shutter speed tester from Calumet decades ago. I find the actual shutter speed at a setting and use that value on my meter.

  6. #16

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    Re: how to do the math for speed film test

    Quote Originally Posted by LabRat View Post
    In addition the the grey card, you also need to see what is happening to the extremes... For the highlight test, put a white sheet with a yellow post-em note next to the grey card...
    Alexis, Please be aware that this is not part of finding personal film speed.

  7. #17

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    Re: how to do the math for speed film test

    What kind of person are you? Do you enjoy testing things or do you enjoy taking photographs and making prints? I do not mean to pejoratively imply that a desire to test things is lesser than, or opposed to, a desire to make photographs/prints - I do both and I do own a densitometer that I use on a regular basis. However, if you do not enjoy the idea of testing things, I would venture that you might be better off finding a simpler method of establishing your own exposure/development regimen.

    The basic idea here is that you are trying to control the characteristic curve/H-D curve/transfer function - think of this as the rule that governs how a brightness/luminance value out there in the world is changed to density values on the print. Now there are actually two stages with B&W film - so there are two transfer functions you need to get to grips - one which translates luminance values in the world into a density on the negative and one that governs how a negative density (actually the luminance of a printing light that has been changed by a negative density but we assume that the intensity of the printing light does not change) translates into a print density. The basic principle here is that exposure mainly affects the shadows and development mainly affects the highlights. Paper is usually developed to completion and so we try to do whatever adjustments we can at the negative exposure/development stage.

    As you can imagine, there are dozens of ways of varying accuracy/ease to get these down. The big problem is that we need to know what a properly exposed negative should look like and what a properly developed negative looks like, the problem being that we don't know what these look like. Experienced workers know what these look like and can usually home into very reasonable solutions for new material within a few exposures. If you don't know what to look for, some lab learning can help but so can some field learning. Like all learning, it tends to be initially difficult - and the point to keep in mind is that lab learning is a different process from field learning.

    At one end, you could approach this very technically. You could start by exposing a piece of your photo paper to a step wedge in your enlarger/whatever your print making process entails (for e.g., if you are working in 4x5, Stouffer makes 4x5 step wedges that fit into enlarger stages). The step wedge has bars of various densities, usually varying in 1/2 stop or 1/3 stop values (0.15 or 0.1 density units - density values of 0.3 correspond to 1 stop i.e., a density of 0.3 units more will allow half the light to pass through). Adjust exposure till you get a range from black to unexposed paper base white (usually you will look for an exposure that gives you a series of bars of white at one end and a series of bars of black at the other end. Look for the first off-white and the first black bar and count up the number of bars in between, multiply by the step wedge factor (i.e., 0.15 or 0.1 density units). That is the negative density range your paper wants to see to give you a full range from the best white to the best black the paper is capable of giving. In other words, this is telling you how long you should develop the negative. Next expose some film in a manner similar to what you have done. Develop for the recommended time with fresh developer. Measure these densities with a densitometer or compare then visually with the step wedge to assess the density - your eye is actually very good at comparing densities. Many of the negatives will be underexposed i.e., the film did not receive enough exposure and so the development does not produce any density beyond that contributed by the film base and general fog (often abbreviated to fb+f). The first value that shows you an increase in density is your film speed. The next stage can involve sandwiching the step wedge to your negative and making an exposure (actually expose a bunch of film this way - this can be done under an enlarger). Now develop these for different times. From your paper tests, you found the density range your paper wants to see. Find the development time that gives you what you need.

    Needless to say, many find this kind of testing regimen stifling. And with good reason. If you want to get on with the making of photographs and prints, this kind of testing can be trying to the soul. It is like learning equations within a stuffy classroom while you want to be out in the real world digging up fossils of dinosaurs.

    So, at the other extreme, you have systems that do not break down things into the individual components. Instead, you go out into the world and dig up fossils and try to learn on the job - just relax and accept the fact that sometimes, there will be screw ups. Screw ups happen in the lab too - its just that they happen in a controlled situation with little to lose. The usual advice that is given is to start with about half the ISO stated on the box and about 2/3 the recommended development time. Then make photographs and print them - lots of them. Sit down with the prints and the negatives and look at the shadow regions. If you consistently find that your shadows do not have enough detail, drop your film speed some more. If not, try increasing film speed in 1/3 stop values till you get to that point where your shadows do not have enough detail. People usually recommend some margin here and to give somewhat more exposure than this point. This is your film speed. Now use this film speed and make some more photographs and print them. If your highlights are always washed out/difficult to print/require lots of dodging and burning, reduce development time. If your highlights are consistently flat and grey/muddy, increase development time. So, this is a trial and error way to find out what a properly exposed and developed negative should look like and you can expect to lose some pictures along the way. Some of us are comfortable with that, some of us are not. Those of us who are not tend to pooh-pooh this approach but it is actually surprisingly good. And it is what you will use constantly because you will unconsciously be doing something like this kind of assessment with each subsequent negative you make and that provides a constant feedback cycle to help you keep your process under control - a bit like tasting the dish as you cook it, except the dish here is not an individual picture but a process.

    As someone who owns a densitometer and uses it on a regular basis, I will argue that you should not underestimate the utility of the second approach. There is a fair bit of margin in this system. For e.g., consider that many will establish a film speed with step wedges and densitometers and then put in a margin and overexpose slightly. This actually makes sense since slight overexposure is preferable to underexposure but it highlights the issue that we are not aiming for some scientific level of precision but are trying to establish the useful boundaries for our work to ensure we stay within those boundaries or are at least aware of when we are sailing too close to the edge of the world....

    I enjoy making photographs and printing and use something very much like the second method in practical terms. But I do enjoy also learning about and testing photographic materials and over a large amount of time, that learning and information becomes second nature. More importantly, the two ways of work actually inform each other and become a creative tool in their own way. Your testing might give you ideas about photographs you could try to make and the photographs you make will start to give you ideas about different kinds of controls that might be worth testing. I think the important thing is to make sure you are enjoying yourself at some level - that's what keeps you at the game.

    Cheers, DJ

  8. #18

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    Re: how to do the math for speed film test

    Quote Originally Posted by neil poulsen View Post
    Alexis, Please be aware that this is not part of finding personal film speed.
    But it should be done prior to the testing; using an inaccurate shutter speed in the testing is a waste of time/film/chemicals. I made my own shutter speed tester and have checked all my shutters at all settings, then taped the results on my lens boards so I can make the necessary corrections in the field.

  9. #19
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    Re: how to do the math for speed film test

    Quote Originally Posted by ic-racer View Post
    Put the negatives over you meter. Look for the negative that drops the meter reading by 1/3 stop over base. That is your 0.1 log d zone one.
    The math to find ISO is not difficult if you know the intensity of your sensitometer and your step wedge densities. (Speed = (0.8 / Hm))
    However, when doing your exposure index, no math is needed. Just look back at your notes to see the ISO to which your meter was set when it exposed the 0.1 log d negative.

  10. #20
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    Re: how to do the math for speed film test

    Quote Originally Posted by Jerry Bodine View Post
    But it should be done prior to the testing; using an inaccurate shutter speed in the testing is a waste of time/film/chemicals. I made my own shutter speed tester and have checked all my shutters at all settings, then taped the results on my lens boards so I can make the necessary corrections in the field.
    IMHO, that is the most important point made so far. I've written this before: large format shutters in general are highly susceptible to inaccuracies. Some of us find the speeds that actually correspond to a standard and work from there. For example, the only shutter speed I can always trust is B (or T). If we are lucky we might fine other speeds that correspond with our other lenses. By chance, I find 1/60th more consistent among the lot. Your experience is likely to vary. It is a crap shoot.

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