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  1. #21
    PeterB's Avatar
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    Quote Originally Posted by Bill Burk View Post
    There's meter spectral response, human eye spectral response, film spectral response and the spectral distribution of light and the "color" of the test target. Lots of variables.

    Many people inadvertently obtain a "Tungsten" speed rating by their tests without realizing it.

    Others put an 80b filter over the light or lens to (at least partially) simulate daylight.

    More important in my mind is to stick with a test plan where you understand it's limitations, and strive for consistency. Yes it's significant in your case, but if your test-to-test result stayed within 2/3 stop (or 2/3 N step), then you can use your test to control your processes.
    Thanks again Bill. I agree with what you wrote above. I might perform one more test before deciding on what process control test I will regularly perform. That test I'll do is to repeat my testing of photographing a transmission step tablet under particular controlled conditions. That is to stick the tablet onto a piece of white translucent perspex, masking off all areas outside the tablet as seen by the camera then affixing that assembly to an outside facing window, and finally draping a black cloth between the mask and the camera to ensure no additional light can enter. This is about as controlled as I can get the conditions when including the camera in the testing process.

    regards
    Peter

  2. #22
    Stephen Benskin's Avatar
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    Quote Originally Posted by Michael R 1974 View Post
    I'm not defending OPs particular test method, but I think Phil Davis overstates the case slightly. After all he had to sell books too.
    Michael, you've said it yourself how a lot of people get by with bad testing techniques because of the tolerances in the photographic process. In order for a black box approach to work properly, you need to know what goes in as well as what comes out. The higher the confidence with the input, the higher confidence in the results, and the more likely testing errors will be identified.
    Last edited by Stephen Benskin; 04-24-2013 at 07:07 PM. Click to view previous post history.

  3. #23

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    I don't disagree. I just think Davis is a little "over the top". There's bad in-camera testing and then there's bad in-camera testing. Knowing the variables and pitfalls can help one design a much better test - although I agree it will never be as good as a contact test. I credit you with helping me to see that. But even when it comes to contacting (not contacting in the camera), I still have some issues to sort through in my own tests. I still find it difficult to do properly because I don't have a sensitometer. Using an enlarger is the closest most people can get. But even though the illumination is exceptionally even (at least in my case), timing an exposure precisely without a shutter, and knowing the actual illuminance at the film plane/baseboard is problematic. So basically I'm saying it's hard to figure out the exposure.

  4. #24
    Stephen Benskin's Avatar
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    Good analogies are hard.

    I was fortunate enough to have my sensitometer calibrated at one time. I also used a calibrated step tablet, and my old boss paid. Whether it's still calibrated is anyone's guess. I took it to that level because once done, there was less worry, and someone else paid (I paid for the sensitometer though).
    Last edited by Stephen Benskin; 04-25-2013 at 12:11 AM. Click to view previous post history.

  5. #25
    Bill Burk's Avatar
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    Quote Originally Posted by PeterB View Post
    This is about as controlled as I can get the conditions when including the camera in the testing process.
    I'd recommend an artificial light source. I think actual daylight is too hard to control, it may vary too much to give you consistent results (so while it is the correct light source to match your shooting planned conditions, it's not as good for testing since it is different each time you use it).

    Your test sounds simple enough that you could make a "light box" to hold behind the white translucent perspex that turns it into a "light table"

  6. #26
    Bill Burk's Avatar
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    PeterB,

    I don't know if you saw this thread where we explored a test strip maker. I found it remarkably easy to do.

    Enlarger --> Sensitometer (test strip maker)

  7. #27
    PeterB's Avatar
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    Quote Originally Posted by Bill Burk View Post
    I'd recommend an artificial light source. I think actual daylight is too hard to control, it may vary too much to give you consistent results (so while it is the correct light source to match your shooting planned conditions, it's not as good for testing since it is different each time you use it).Your test sounds simple enough that you could make a "light box" to hold behind the white translucent perspex that turns it into a "light table"
    Thanks Bill. After my interesting results with two considerably different colour temperatures giving noticeably different gammas, it makes sense to control for that by using an artificial source to provide a more repeatable colour temperature.

    rgds
    Peter

  8. #28
    PeterB's Avatar
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    Quote Originally Posted by Bill Burk View Post
    PeterB,

    I don't know if you saw this thread where we explored a test strip maker. I found it remarkably easy to do.

    Enlarger --> Sensitometer (test strip maker)
    Not until I just spent the last 30 minutes digesting it ! You are more geeky than me Bill , and I have 2 t-shirts and a cap with the word GΣΣK written on them (given to me by my lovely wife).

    The only eBay search I've had running for about the last year is one for a sensitometer, and after reading that thread I have even less of an idea which brand/model to look out for. I hope your EG&G sensitometer is proving useful, you certainly spent some time getting to work it out. Surely there have been ones made since, that are at least as accurate (and feature-full) but not so old fashioned.

    I'd prefer not to use my enlarger to expose a step tablet into my film because I'd then want to test my enlarger lamp response/warm up time at different filament temperatures and then try to control for that which doesn't sound too appealing. I have already tested my camera's shutter speeds (and had the CLA guy do it too) so I'm more comfortable with those being repeatable and accurate to within 5-10% (my intended accuracy of this Process test).

    Perhaps if I get a sensitometer my quick Process Test will change to using that, but for now I will use my camera, and either a Tx step tablet in the manner I describe above or a reflection test target illuminated with either the sun at a similar time of the day (or an artificial light source) and vary the amount of light by adjusting the aperture over its entire range (the original idea I posed with this thread and the 'easiest' to do if one doesn't have access to a step tablet or sensitometer.)

    rgds
    Peter

  9. #29
    PeterB's Avatar
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    Quote Originally Posted by Bill Burk View Post
    First thing that comes to mind is... Color separation negatives through the blue filter have to be developed longer than the other separations because blue light results in lower gamma. (That's an explanation I've read, others may corroborate or dispute). Maybe the light on the white towel had more blue than the cream wall.
    Bill, I just found this effect you thought of reported here by Stephen and is called the Gamma-Lamba effect . The magnitude of my results correlate closely to those reported by Kodak for Tri-X.

    In fact UV photography suffers from this effect and when I photographed the white towel it would have had optical brighteners from my washing liquid (that fluoresce UV) and possibly further amplified the effect

    What a cool serendipitous discovery I stumbled upon in my testing.

    rgds
    Peter

  10. #30
    PeterB's Avatar
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    Quote Originally Posted by PeterB View Post
    when I photographed the white towel it would have had optical brighteners from my washing liquid (that fluoresce UV) and possibly further amplified the effect
    On second thoughts the optical brighteners wouldn't have amplified this gamma-lambda effect I observed as it wouldn't have created proportionally more blue light. The blue cast would have come from the fact that the white towel was in shade and only exposed to blue sky while the cream wall (already 'warmer' than white) was exposed to more direct sunlight which has a warmer colour temperature (see ct chart here)

    rgds
    Peter

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