Switch to English Language Passer en langue française Omschakelen naar Nederlandse Taal Wechseln Sie zu deutschen Sprache Passa alla lingua italiana
Members: 70,549   Posts: 1,544,631   Online: 682
      
Page 3 of 4 FirstFirst 1234 LastLast
Results 21 to 30 of 31

Thread: Long exposures

  1. #21

    Join Date
    Jan 2005
    Location
    Vermont
    Shooter
    Medium Format
    Posts
    348
    Images
    5
    I've heard that you can somewhat compensate for reciprocity by making incremental exposures of a shorter length, before reciprocity begins to take effect, and adding it up to the total exposure you need. For example, an indicated exposure of 1 minute might yield a reciprociy-adjusted exposure of 3 minutes. So you give 4 or 5 separate exposures of say 20 seconds. I dunno, maybe some genius like PE can weigh in on this.
    Robert Hunt

  2. #22
    CBG
    CBG is offline

    Join Date
    Nov 2004
    Shooter
    Multi Format
    Posts
    894
    PE knows a great deal more than laypeople like myself on reciprocity etc, but I'm pretty sure I can safely state the solution Robert has heard about won't work as intended. I would be glad were PE or other experts to weigh in with real expertise....

    Robert said." I've heard that you can somewhat compensate for reciprocity by making incremental exposures of a shorter length, before reciprocity begins to take effect, and adding it up to the total exposure you need. "

    Unfortunately, there is no such easy cure. Robert's solution will lead to the same level of underexposure as the original poster found problematical, or a tiny bit worse due to a second issue called the intermittency effect. It will not bypass reciprocity.

    First - To get a sense of what goes on with reciprocity failure, the best layman's explanation I've found is at:

    www.camerabooks.com/custom.aspx?id=27

    Having read it, reciprocity failure is more comprehensible. There are a lot of erroneous explanations on the net - so if one wanders the net via search engines - please do so with a sceptical eye.

    Robert's hoped for solution fails, since breaking a long exposure into several shorter ones must still take into account reciprocity effects and will often introduce another issue, admittedly usually minor, the intermittency effect.

    10 two second exposures will generally yield a slightly less exposed image than a single twenty second exposure. I say generally since the intermittency effect is not at all simple to calculate other than by testing for a specific instance.

    The intermittency effect is not the magnitude of destructive monster that reciprocity is, so it often goes by unnoticed. Nonetheless it does diminish the total exposure slightly.

    I have never read an authoritative explanation of intermittency, and assume it stems from the root causes of reciprocity.

    PE??????????????

    Best,

    C

  3. #23

    Join Date
    Feb 2003
    Location
    Hudson, New York
    Shooter
    8x10 Format
    Posts
    171
    maybe you need to approach this the other way. if the lens is going to be open for 6 hours, assume that this is the exposure after compensation for reciprocity failure. then back-track to the original exposure based on gadget's calculation.

    hope this makes sense.
    Paul Hamann

  4. #24

    Join Date
    Sep 2006
    Location
    OH
    Shooter
    35mm
    Posts
    1,789
    Images
    2
    Makes total sense.

  5. #25
    CBG
    CBG is offline

    Join Date
    Nov 2004
    Shooter
    Multi Format
    Posts
    894
    If I understand the lighting situation, i.e. dark as all get out, the light is waaaay below the threshold of sensitivity of any meter I know of.

    Calculation goes out the window at that point. Tests and comparable situations become the way to establish exposure times etc. I'd just open the lens up all the way and try a series of exposures - 1, 2, 4, 8 hours. Then process and see if any show any exposure at all.

    If you get some exposure but not enough - either add a little light or use a much faster film or push your processing really hard or try exotic techniques to boost sensitivity and to boost the latent image.

    Chapter 10 of my copy of Anchell and Troop's "The Film Developing Cookbook" lists some processes for boosting true film speed and latent image. Astronomers use some of those techniques.

    Best,

    C

  6. #26

    Join Date
    Sep 2006
    Location
    OH
    Shooter
    35mm
    Posts
    1,789
    Images
    2
    Quote Originally Posted by CBG View Post
    If I understand the lighting situation, i.e. dark as all get out, the light is waaaay below the threshold of sensitivity of any meter I know of.

    Calculation goes out the window at that point. Tests and comparable situations become the way to establish exposure times etc. I'd just open the lens up all the way and try a series of exposures - 1, 2, 4, 8 hours. Then process and see if any show any exposure at all.
    Yes. I think this is more along the lines of the situation I'm actually dealing with. Sounds like more testing is in order. thanks.

  7. #27

    Join Date
    Jun 2007
    Location
    Austin, TX
    Shooter
    4x5 Format
    Posts
    22
    Quote Originally Posted by Tim Gray View Post
    Yes. I think this is more along the lines of the situation I'm actually dealing with. Sounds like more testing is in order. thanks.
    I hate to say this, but what about testing with digital? Most DSLRs will go up to ISO 1600, even if it is noisy as hell. They don't suffer reciprocity and you get really quick feedback. Why not use one of them to test for the "metered" exposure and then do the reciprocity calculations from that? You're probably looking at a measured exposure that is in minutes, so bumping the ISO to 1600 should let you start with a simple series of 1, 5, 10, 20 minute exposures as test cases. Much quicker than doing a 6 hour test, developing it, doing another with a different f-stop.

    I regularly use a DSLR to test exposure before I burn large format film. My 20d is a very expensive exposure meter. I just have to remember that when the highlight warning flashes my film will probably still have detail.

  8. #28

    Join Date
    Sep 2006
    Location
    OH
    Shooter
    35mm
    Posts
    1,789
    Images
    2
    NOOOO!!!!

    I was actually thinking of trying this. I asked the original question to tap into the vast pool of knowledge here. Great idea!

    One of the problems with digital and really long exposures is the battery can die... My film SLR or my rangefinder uses little or no batteries on bulb.

  9. #29
    gainer's Avatar
    Join Date
    Sep 2002
    Posts
    3,726
    Images
    2
    Quote Originally Posted by pgomena View Post
    I have a tested reciprocity chart that shows a metered reading for Tri-X of 10 minutes requires an exposure of about 9.5 hours. Tri-X ain't gonna work in this situation.

    Peter Gomena
    Srangely, that is very close to what my equation predicts when I use Kodak's value of 1 second for the amount to be added to a calculated exposure of 1 second. The equation gives 600 + (1*600^1.6) = 27864 + 600 seconds = 7.9 hours. Working backwards to find the amount of correction at 1 second that would predict 30000 seconds correction at 600 seconds I find that value to be 1.07 seconds.
    Gadget Gainer

  10. #30
    gainer's Avatar
    Join Date
    Sep 2002
    Posts
    3,726
    Images
    2
    It would have been even closer if I had used 1.62 as in my article instead of 1.6. I would have gotten 9 hours.
    Gadget Gainer

Page 3 of 4 FirstFirst 1234 LastLast


 

APUG PARTNERS EQUALLY FUNDING OUR COMMUNITY:



Contact Us  |  Support Us!  |  Advertise  |  Site Terms  |  Archive  —   Search  |  Mobile Device Access  |  RSS  |  Facebook  |  Linkedin