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Thread: Help a noob understand the exposure conundrum

  1. #61
    Vaughn's Avatar
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    Re: Help a noob understand the exposure conundrum

    My advice -- take care of the light leaks before trying to determine exposure and development times. Sort of like testing one's car's gas milage with flat tires. Use photopaper if you want to go cheap and easy.
    "Landscapes exist in the material world yet soar in the realms of the spirit..." Tsung Ping, 5th Century China

  2. #62
    Nodda Duma's Avatar
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    Help a noob understand the exposure conundrum

    If you measured 25mm wide open shutter, then your f/# is

    300 / 25 = f/12

    Which is ~2 stops down from what you metered for right there. That's not quite the working f/#. Magnification due to object distance plays a role as well. Additionally, positioning the shutter as-is will cause a change in magnification of the entrance pupil and changing the f/# as well. Going back through, pupil magnification sounds like 2x. Those effects could add up to an additional stop or two.

    Working f/# ~ (1 + m/P) * f/#

    m = d(object) / d(image)
    P = 2

    For object 2m distant, f/# will be about 1.1 x 12 ~ 13.2

    There's also a vignetting effect due to the shutter position so far behind. Sort of like a T-stop effect but in reverse. If I'm calculating correctly, that could be 2-4 stops depending on what the original f/# is.

    So you could be 4-6 stops down from all those effects.

    That negative also looks very dense, as in it looks very overexposed. However, closer inspection of the edges indicate significant fogging. To overcome that fogging requires additional exposure..conversely exposing as you did will not get the image very out of the fog and it will look underexposed. Maybe another stop or two from that effect? Veiling glare due to light leaks could account for another stop. 2-3 stops from those effects.

    Also, developing HP5+ at 200 (N-1) in Rodinal 1:50 at 20C should be more like 7:00 with 30s agitation. N-2 is about 5 minutes, so you're 1 stop underdeveloped as well. Rodinal 1:25 would be about 6:15.

    So I'm estimating 6-9 stops down!

    Conclusion: death by 1000 cuts. I agree with Vaughn..You need to refine your "no budget" camera, verify developing times, and probably buy fresh film!

  3. #63

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    Re: Help a noob understand the exposure conundrum

    Wow, you guys. Just wow. I see all that.

    Don't understand the calcs but get the principles.

    In my tiredness last night I took the white round the image to be a vignette, but probably its light leak fogging. Loads. It's white not dark. The dark slide aperture is pouring light in! Likewise the loading flap joints.

    So, mkII film holder and try again.

    I'll run a thread on the build. Having folk watching makes me up my game and I'm sure I'll get some help.

    Many thanks, I think we got somewhere.

    Photography like nothing else makes me giddy with interest and excitement and crashes me into pits of frustration and despair. I need to attenuate, get more zen. (Man). It's had me hooked for years and just gets more and more of a hold!!

  4. #64

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    Re: Help a noob understand the exposure conundrum

    I have revisited all the advice, especially the light leaks; I concluded that I could fight another 5x7 holder together or, seeing as I was near the end of the pack of film, go to 4x5.

    I had one fidelity elite 4x5 holder so I've made an adapter to the Kodak rear stage.

    Cheaper, less variables and hopefully no light leaks. Some Foma film landed yesterday so I'm good to go.

    Also I can use my de vere enlarger direct too now with no adapters.

    I'll work on nodda's calcs and assumptions and see if 4-6 stops without leaks and accurate processing time gets me anywhere.

    That, and a mental shift of a bit less giddy with enthusiasm and excitement and a bit more calm and deliberate.☺️

  5. #65
    8x10, 5x7, 4x5, et al Leigh's Avatar
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    Re: Help a noob understand the exposure conundrum

    Given that this is a very old shutter, failures are to be expected.

    One that would cause the observed symptoms is if the aperture closed fully during exposure, regardless of the aperture control setting. Such a failure could be caused by a broken part inside the shutter.

    Have you observed the light through the shutter during an exposure (cells removed, obviously)?

    - Leigh
    If you believe you can, or you believe you can't... you're right.

  6. #66

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    Re: Help a noob understand the exposure conundrum

    Fair point, I'll bear it in mind.

    I managed to squeeze some time together today to make two images. I took a meter reading and added four stops for one image and six for another, based on nodda's theorem. Both of the same subject - the bark, lichen and moss of the tree above.

    I processed the neg of the f+4 normal, it looked good. So I took a risky punt and decided that the f+6 could be dense and did a semi stand development. Initial result kind of confirms.

    I'll print them in due course and post for some feedback.

    Suffice to say they are a massive step forward. It gives a datum to now work from.

    One Very Happy Bunny. Thanks team!

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