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Exposure additivity


Tony Evans

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<p>As a LF newbie, I am wondering if (with a static subject) exposures are strictly additive. i.e. can 2 secs be given as 2 successive 1 secs, 1 & 1/2 secs as successive 1 sec and 1/2 sec etc. Or are there interactions with the shutter opening and closing functions? Would be handy to use the shutter speeds rather than trying to time. Thank you.</p>
Tony Evans
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<p>Exposure time itself is additive. At fast shutter speeds, there's shutter efficiency issues to consider, because the opening and closing of the shutter is not instantaneous. But this is more an issue that at small apertures, the exposure may be longer than you expect.<br>

Now, the relation between exposure time and film density does get non-linear for long exposures. Look up reciprocity failure.</p>

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<p>Hmmm. You've given us quite a difficult question to answer briefly. Yes, exposures are (generally) additive in the linear way that you describe - BUT, things start to get "interesting" once the exposure time gets longer than a couple of seconds.</p>

<p>There's an effect called "reciprocity law failure" where, as the exposure is lengthened beyond a certain point the effect of further exposure becomes weaker. Think of it as trying to fill a bucket with a small hole in it. You can easily fill the bucket with a gush of water, but if you try to fill it using one small drop at a time then the bucket will empty almost as fast as it's being filled.</p>

<p>So if you give the film a short exposure of, say 1/125th second, and then wait an hour or two to give another exposure of 1/125th second, you won't get quite the same effect as giving one exposure of 1/60th second.</p>

<p>Perhaps the above is overcomplicating what you were asking. If you simply give the film five pops of the shutter at 1 second exposure, each in quick succession, then the exposure will be practically identical to giving one exposure of 5 seconds in length.</p>

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<p>It is easier to just set the shutter to T or B and then keep the shutter open while counting or timing 2 seconds. If you are off by a half second it won't make any difference and there is less contact between you and the shutter so there is far less chance of your contributing to vibration by recocking the shutter.</p>
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<p>Recocking the shutter really isn't a big problem. Just think the physics of it and a little back pressure and nothing moves. I have done hundreds of commercial jobs where I did it because I needed multiple pops of the flash--no problem ever with the camera moving--and I used a wooden field camera. But remember that with long exposures, counting is generally fine, you don't have to be perfect.</p>

<p>If you are off 1 second on a 10 second exposure, that is a nominal miss. Reciprocity is a fact, although most newer films have negligible issues until after maybe 20 seconds or so--at least those I use. I had the old film tables memorized and have yet to learn those for new films but never had a failure.</p>

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<p>Yes, exposure is additive. Obviously, you must account for reciprocity failure when the total exposure time exceeds about one second. And you need to be careful about jarring the camera between exposures.</p>

<p>This is a common trick used by architectural photographers. The best architectural images are done after dark when the building's interior lights are on. But that creates a problem with exterior lighting. So architectural photographers routinely use two exposures, one during daylight (or just before dusk), and one just after dusk with the interior lights on.</p>

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<p>I use multiple exposures all the time, especially when I have a moving subject, or a scene with people walking through, etc. and need long exposures. I have lots of shots made this way.</p>

<p>If you figure reciprocity correction first, and arrive at an adjusted exposure, you don't have to do any extra calculations for additive exposure. Just give the total exposure in whatever increments you like.</p>

<p>There is something known as the "intermittency effect" that makes additive exposure somewhat less than the total exposure if given all at once, however, the effect is very small and certainly is not worth considering for black-and-white film, or even color negative film.</p>

<p>As far as cocking the shutter goes, I have had no problems. The commonest problem with long exposures is film popping or shifting during the exposure. This can be minimized by 1) tapping the bottom of the film holder on the palm of your hand before inserting it to seat the film and 2) pulling the dark slide 30 seconds or so before you make the exposure to let the film adjust to the temperature and humidity inside the camera and pop to its final position before you start the exposure (I think John Sexton recommended a couple of minutes).</p>

<p>Best,</p>

<p>Doremus Scudder</p>

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