very long exposures

Discussion in 'Beyond the Basics' started by William Petruzzo, Sep 5, 2008.

  1. William Petruzzo

    William Petruzzo TPF Noob!

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    I've never spent too much time trying to capture long exposures. I've just never really needed to. I understand the principles and I've been satisfied with that. Well, in the past few days I've had the urge to try my hand at it. Nothing too crazy, maybe 10 - 20 minute exposures max... I don't want to waste my whole life looking at a stopwatch!

    Anyways, I set out to give it a shot tonight. Not trying to catch anything spectacular, just seeing if I can get the exposure right. It came kind of close, but it seems that I'm under exposing a bit and I can't quite figure it out.

    My easiest understanding of how the very long exposures work is half using the camera's built in metering by checking for a "correct" exposure at 30-sec, 3200 ISO and a fairly small f/stop. Then, reducing the ISO to 100 and the cranking the f/stop as high as I like and then add the appropriate number of stops to the shutter speed to compensate.

    Now, unless I'm mistaken, if the stop-counting is done correctly, that should yield a correct exposure. Well, it's close, but dark. I know that based on trial and error, I could just boost my exposure time. But I'm not really content with that. I'd like to try out some extremely long exposure, maybe several hours and if that's the plan, I'd like to have some confidence that I'm doing it right. So Is there something I could do to improve this technique or am I missing a vital step here or something?
     
  2. Flash Harry

    Flash Harry TPF Noob!

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    Meter with your chosen lens wide open, this will show you the exposure time, say 1/15 sec, then stop down so that all is in focus and multiply the time by number of stops, this will give you an exposure to work with but as you found out, due to light polution it has come out dark with any bright lights in the scene pretty blown out, probably 2,3 or four stops, double exposure time, check, double again, check, then again and again till you achieve a correct exposure for your shadow area. Its the light meter causing problems, as in a snow scene, if you want white snow, overexpose, and depending on how bright a day more exposure gives better results. H
     
  3. William Petruzzo

    William Petruzzo TPF Noob!

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    Hmm. So it's likely to be trial and error? What about the people up north doing 24 hour exposures of stars? They must have a method for knowing how to judge the length of the exposure. I mean a lot of those pictures feature buildings and trees and stuff. Is it really just educated guess work?
     
  4. Bifurcator

    Bifurcator TPF Noob!

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    I did a whole bunch of long exposures with film back in the dark ages. For me it was all guess work and reading the findings of other photographers that also learned by and prescribed guessing - like pre-exposing (fogging) the film and so on.

    The formula you mentioned above sounds interesting and if the images are coming out "dark" then maybe you're getting it right - I mean the natural scene is "dark" right? So if you want it artificially light then you need to adjust for that.

    In very long exposures of hours in length I was reading here on this site where people are stacking a multiple exposure set in order to achieve the same affect as one very long one. This should reduce noise, problems with batteries, heat, and etc. I say "should" because I haven't actually tried it yet. ;)
     
  5. Garbz

    Garbz No longer a newbie, moving up!

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    More often that not it's a case of get as much light in as possible without sacraficing the image, and then taking the exposure length judging by how long the trails of stars you want.

    The results are spectacular and you need to remember the following. The difference between a 20 minute exposure and a 40 minute exposure is 1 stop. You can push / pull this in photoshop easily. To try and get it as dead on as possible just take a 5 min photo and bear in mind that a 20 min photo is only 2 stops higher.
     
  6. Flash Harry

    Flash Harry TPF Noob!

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    darkness, as we see it, is rarely black, (darkroom excluded) for a correct exposure you would need to meter for the darkest object you wish to appear in a scene preferably using a spot meter, then stop down and go for it however long it takes. Bear in mind if digital, you'll possibly need to scrap the camera afterwards, with colour film you'll get reciprocity failure so bw film is the way to go. I've done nice colour film ones in the past but was hand printing colour at the time so no biggy. H

    Its probably best you tell me whats in the shot, if its the usual traffic trail stuff on a decent night try 8 mins. H
     
  7. josealb

    josealb TPF Noob!

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    How long are the exposures you tried with your DSLRs with acceptable noise? In my D40 using anything longer than 5 - 10 min is spoiling the image...
     
  8. epatsellis

    epatsellis TPF Noob!

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    Long exposures are one area where film makes it almost too easy. The only thing to be aware of is reciprocity error, it depends on the film, and every film datasheet will have reciprocity correction data (as well as filtration suggestions for color films)
     
  9. William Petruzzo

    William Petruzzo TPF Noob!

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    This is all really good information. Thanks everyone.

    I actually didn't have any problem with noise at all. I dropped down to 100ISO and did my best to compensate and the noise was no worse than shorter exposures. Granted, the pictures were under exposed, but hardly looked any different than a similar dark exposure that was much shorter.

    That's a startling thing to say. Harry, you'll have to elaborate on that one some more.
     
  10. reg

    reg TPF Noob!

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    Twenty minute exposure ---> s#!t gets REALLY hot ----> s#!t starts to break.

    SOMETIMES.
     
  11. William Petruzzo

    William Petruzzo TPF Noob!

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    Hmm. Good to know. I wouldn't have thought of that before. Good thing I have insurance. However, for my own edification, how realistic is that scenareo? Is it one of those it could happen type deals, but probably wont?
     
  12. prodigy2k7

    prodigy2k7 No longer a newbie, moving up!

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    The longer the exposure on digital the hotter the sensor will get... If it over heats...bye bye sensor... Dunno how probably it is...I dont think its rare, but not common either...
     

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