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  1. #11
    nick mulder's Avatar
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    And jumping the gun here a little does anyone care to let the reciprocity issues of film come into the equation ?

    Remembering it is a factor with very small exposures as well as long ...
    Cleared the bowel problem, working on the consonants...

  2. #12
    RalphLambrecht's Avatar
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    Most of the 'working' speeds are fine, but the 'slow' speeds are causing almost 1/2 stop underexposure. For B&W work, I would correct for that by rating the film 1/3 stop slower.
    Regards

    Ralph W. Lambrecht
    www.darkroomagic.comrorrlambrec@ymail.com[/URL]
    www.waybeyondmonochrome.com

  3. #13
    RalphLambrecht's Avatar
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    Quote Originally Posted by nick mulder View Post
    And jumping the gun here a little does anyone care to let the reciprocity issues of film come into the equation ?

    Remembering it is a factor with very small exposures as well as long ...
    I wouldn't worry too much about reciprocity failure in this range, but you are right, the underexposure at the slow speeds will be worsened through reciprocity failure.
    Regards

    Ralph W. Lambrecht
    www.darkroomagic.comrorrlambrec@ymail.com[/URL]
    www.waybeyondmonochrome.com

  4. #14
    nick mulder's Avatar
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    Yeh, don't get me mistaken - I'm talking completely in terms of mental wankery here - trying to determine where, how and when our thought experiment begins and ends ...

    If all other things are going to be equal, we need to define 'not other'
    Cleared the bowel problem, working on the consonants...

  5. #15

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    Quote Originally Posted by nick mulder View Post
    Yeh, don't get me mistaken - I'm talking completely in terms of mental wankery here - trying to determine where, how and when our thought experiment begins and ends ...

    If all other things are going to be equal, we need to define 'not other'
    Mental what? Sorry - obviously a technical term with which I am unfamiliar....

    Here's a bit more mental w... er, a bit more food for thought:

    Take a completely different shutter... say a leaf shutter.

    As we probably all know they suffer from 'efficiency' problems. When the shutter is fired and starts to open - it acts like an iris - starting as a pinhole and growing larger until it gets to full aperture. It then sits fully open for a while - then closes. During the opening and closing times the shutter only passes part of the light. At slow speeds the open and close times are negligible. At the fastest shutter speed then there may never be a pause at the 'fully open' point - the shutter starts to close as soon as it is opened. At this speed the shutter is 'less efficient' as the film only sees the full light from the lens for an instant. This is why most leaf shutters do not exceed 1/250 or maybe 1/300 of a second. Even the very fastest are restricted to about 1/500th. Usually this is achieved with a (very powerful) supplementary spring that only comes in on the highest speed or two - and the shutter takes a lot of thumb pressure to cock.

    So... Consider another shutter, say the Thornton Picard roller bind shutter. Here a curtain passes the back of the lens with a slot in it just slightly bigger than the aperture it covers. As it opens the light is passed through almost a crescent moon shape - which grows through all the phases of the moon until just for an instant it is full - then it waxes (or is it wanes?) until it is closed. If in 'Instantaneous mode' - i.e: not held open - then this is by design a very inefficient shutter. There is never a period where the full aperture is held open, unlike the later leaf shutters.

    Plot a graph of light intensity against time and you get a triangle - sloping up and then sloping down. With a compur type shutter you get a slope up - a flat top - then a slope down. The faster the shutter speed the shorter the flat top and the lower the efficiency.

    So - stick these two on a shutter tester and what do we have? Well, at some point the tester is triggered 'on' by the rising light intensity. It doesn't matter at which point - as long as it is the same for both (except it might be different for the film!) The point is - if the tester measures the shutter speed - the actual exposure received by the film, especially at lower speeds will be much higher for the compur - since it will have chance to sit fully open for maybe the big majority of this time. For the film to see the same exposure from the TP rollerblind - the actual time from when the shutter actually opens to when it closes would have to be much longer - maybe nearly twice as long? (I have a horrible feeling to be sure would involve... calculus . Let's not go there!).

    Another factor is that with any leaf shutter - the efficiency varies with the aperture (because at f22 the tiny aperture is obviously opened much more quickly than the full aperture of the lens). Another factor to consider.

    The only point I'm trying to make here is that you can't measure 'shutter speed' and expect it to accurately relate to film exposure. It isn't like measuring temperature or even light intensity - you have to consider what is going on.

    The OP said "In the end, a 1/4 of a second is a 1/4 of a second regardless of how the camera is set up".

    Well, no, sorry. I humbly disagree. A 1/4 of a second exposure where the shutter is, on average, only passing half the light from the lens and 1/4 second exposure from a shutter that snaps open and shut at speeds that are negligible compared to the exposure time are certainly NOT the same. With the FP shutter - the design is almost 100% efficient at any aperture - again a different set of factors to a leaf shutter - but here the characteristics of the sensor need to be considered.
    Steve

  6. #16

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    Steve,

    Less well 'advertised', but focal plane shutters too have an efficiency problem.
    It is caused by the distance between the focal plane shutter and the film (the same thing you describe about behind the lens shutters, but a bit less dramatic).
    And it is (part of) the reason why focal plane shutter's fast speeds usually vere towards the boundaries of the tolerance range, i.e. it can quite significant, a long way off your almost 100%.

  7. #17

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    Yes indeed, that's why I include the word 'almost'. Maybe I should have said 'sometimes almost'!

    Another effect I noticed on my Speed Graphic shutter is that the gap between the shutter and film is so big that the slot 'image' has very fuzzy out of focus edges. Quite noticeable - a sort of penumbra, if you like. I suspect this could have the effect of giving the film a slight 'pre-flash' as the shutter transverses the film (and a post flash too, for that matter). Whether it has any real effect on the exposure or not I don't know - but I would have though that with a slow curtain speed and a fast film - it might well do!
    Steve

  8. #18

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    Quote Originally Posted by steven_e007 View Post
    Yes indeed, that's why I include the word 'almost'. Maybe I should have said 'sometimes almost'!

    Another effect I noticed on my Speed Graphic shutter is that the gap between the shutter and film is so big that the slot 'image' has very fuzzy out of focus edges. Quite noticeable - a sort of penumbra, if you like. I suspect this could have the effect of giving the film a slight 'pre-flash' as the shutter transverses the film (and a post flash too, for that matter). Whether it has any real effect on the exposure or not I don't know - but I would have though that with a slow curtain speed and a fast film - it might well do!
    Yes, it has.

    That is exactly the thing that causes the efficiency problems with focal plane shutters.
    They do not produce a sharply defined 'slot' of light on the film, but one with fuzzy edges. The degree of fuzz, of spread, depends (mostly) on the distance between shutter and film.
    That spread means that the exposure is in fact a bit longer than it would have been were the transition from dark to light instantaneous. And with the fuzzy edge being the same width no matter how broad the slit, the effect of it on the total exposure is larger when the slit formed by the shutter curtains gets smaller, i.e. with shorter speeds.
    The spread also depends on the aperture. But the effect is (i believe) less than that of the aperture on leaf shutter's efficiency at short speeds.


    Leaf shutters get (deservedly) 'bad press' when it comes to efficiency. But focal plane shutters do not deserve being spared the same "bad press" the way they are.

  9. #19

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    Hmmmm. Interesting. Thing is though, because the spread is fuzzy and acts like a pre-flash, it doesn't just affect the exposure - it will surely affect the film gradation as well? I wonder if any Speed Graphic users (or any camera where you can use both leaf and FP shutters) have ever noticed a difference in gradation (better shadow detail) between the two shutters on the same lens and film?

    I must try it myself some time.
    Steve

  10. #20

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    Quote Originally Posted by steven_e007 View Post
    Hmmmm. Interesting. Thing is though, because the spread is fuzzy and acts like a pre-flash, it doesn't just affect the exposure - it will surely affect the film gradation as well? I wonder if any Speed Graphic users (or any camera where you can use both leaf and FP shutters) have ever noticed a difference in gradation (better shadow detail) between the two shutters on the same lens and film?

    I must try it myself some time.
    It's just part of the total exposure.
    It will have an uplifting effect on shadow detail, because the shadows (like the rest of the film) get to be exposed a little bit more than they should.

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