How to calculate long exposure times?

JoshC. · Mar 27, 2011

Hey guys,
I'm curious as too how one would go about calculating the proper exposure time after dark once the camera is set to bulb while shooting manual. Obviously it will be a fairly long time, but what needs to be taken into account and is there some kind of formula?
If it makes a difference, I'm planning to photograph a stream and I'm looking for a very long exposure to get a special look. I know I can get a similar look with just a one second exposure but I want it to be exaggerated.
Any help would be much appreciated

480sparky · Mar 27, 2011

If it's so dark you need an exposure longer than 30 seconds, you're probably going to have to just go by trial and error.

JoshC. · Mar 27, 2011

480sparky said:
If it's so dark you need an exposure longer than 30 seconds, you're probably going to have to just go by trial and error.

Ok thank you. So when you see exposure times that exceed 5 minutes its still just guessing?

ann · Mar 27, 2011

Try an exposure and then check your histogram.

Adjust the time until the histogram shows a range you want. I.E. don't block out the shadows or clip the highlights.

Derrel · Mar 27, 2011

One method is to jack the ISO level of the meter up pretty high, to 1600 or 3200, and use a wide-aperture lens like an f/1.4 or f/1.8, and meter a white sheet of typing paper in the dim,dim light. Get "some kind" of a baseline, and then use an exposure of 20 to 40 times as much for a night scene, AFTER lowering the ISO back down to something that will be less-noisy.

KmH · Mar 27, 2011

In other words, a more stuctured way of doing trial and error?

JoshC. · Mar 27, 2011

Thanks for the replies all!

And Keith... yes

TheBiles · Mar 27, 2011

I've been curious about this myself in regards to shooting star trails. Are there any kind of guidelines specifically for star trails? I'd imagine that it depends heavily on the light pollution in your area, but I'd hopefully find an isolated location outside of the city. Is it even possible to do with a single exposure and a normal DSLR, or do you just need to use multiple exposures and additional software?

molested_cow · Mar 27, 2011

It's really hard to gauge. There are many occasions when I captured lights in long exposure shots that are not visible to the naked eye. So even if you try to project a metering reading, it probably will have quite a bit of inaccuracy.

It's really about trial and error. The bad thing about long exposure shots is you cannot just keep trying. Each shot takes a long time, and so there are limited tries one can make. Furthermore, as the night goes on, the light condition actually changes without you noticing. For an example, 2 hours after dark vs 4 hours after dark may not be different to human eye, but will require quite different exposure time. It also depends on which direction you are shooting at. If you are shooting east, it will remain dark. If you shoot towards west, there will be chance that you will catch the stray light from sunset.

I'd say try to aim for a range of possibilities and plan to photoshop them later.

David Dvir · Mar 27, 2011

Well, again this want always work, but I always use a simple method. I take a picture of what I'd like the long expose of any way I can. f/1.4... super high ISO whatever is necessary, and then once I figure out what settings generate an adequately lit shot, I frame up the long exposure..

Let's say you shot a night scene at 1.4, ISO 3200 and shutter 1/60 and you got a shot that was lit the way you want, just not long exposure.. well.... THEN you can just play with the numbers.. if you want to shoot at any f-stop or ISO you can adjust the shutter speed accordingly. Say you wanted to shoot at f/8 and ISO 100, well, that's 5 stops for f-stop and another 5 in ISO.. That means you have to give 10 stops to your shutter speed. So you'd be shooting at 15"!! You can always do the math and figure out what you're going to need roughly, and then you're most likely going to have to tweak it a bit from there! Goodluck!

PS first post in a long while and I love the new look of the forum!

Dave

480sparky · Mar 27, 2011

TheBiles said:
I've been curious about this myself in regards to shooting star trails. Are there any kind of guidelines specifically for star trails? I'd imagine that it depends heavily on the light pollution in your area, but I'd hopefully find an isolated location outside of the city. Is it even possible to do with a single exposure and a normal DSLR, or do you just need to use multiple exposures and additional software?

Click here.