View Full Version : Division of Long exp by short exp

2-Oct-2016, 09:12
Can anyone assist please.

If have a metered exposure of f32 at 1 second, but want to make up the 1 second period out of multiple shorter exposures - say 250 second for example. How do I calculate how many times I must expose at 250 sec to achieve the 1 second period?

I assume there is a simple math process for determining this, but will the mathematical correct number of exposures, give more "actual" exposure to the negative, than the one single exposure of the total time.


2-Oct-2016, 09:26
You need to give extra exposures, 1/250 would probably need 300-400 individual wexposure not the mathematical 250, you might actually need more than that.

It's an idea to make the total 1 second with a few exposures but I wouldn't go higher than 6-10. I tend to use a nixture say 2 of 1/8, 2 of 1/15 maybe 3 of 1/4, a lot depends on how steady the camera and tripod are. It's probably easier to give 6 1/4 exposures as changing the shutter speed could disturb the camera on the tripod.

This image is multiple exposures on a windy day using a dark green filter.



2-Oct-2016, 10:30
Thanks for the picture, Ian. It is inspiring.

...and that would be a good opportunity to have a Copal Press Shutter.

Drew Bedo
3-Oct-2016, 05:45
I understand the technical aspects of this discussion, but the creative vision that drives it eludes me . . .its not you, its me.

Please clarify the what and why.

Doremus Scudder
3-Oct-2016, 06:03

As Ian points out, it usually takes more exposure than merely dividing the longer time into parts. There is something called the intermittency effect, which says that the total exposure is less than the sum of its parts. The more parts you break an exposure into, the more exposure you'll need to add.

That said, I've had good luck with long exposures (30 seconds or longer) broken down into 4 or 5 parts and with adding 1/3 to 2/3 stop plus the additional time for reciprocity failure. Given that black-and-white film has a lot of latitude for overexposure, I'd err on the side of too much exposure. If you have a project that really needs a lot of 1/250th-second exposures to make up one second, you may want to test first, say starting at 300 exposures and then one at 500 exposures (you can always extrapolate a middle value).


Using intermittent exposures can be helpful in a number of situations. I've used it to ensure that things were still when there was intermittent wind or other subject movement (e.g., people/cars passing in front of the camera during a long exposure). I open the shutter and keep an eye on things; whenever there was movement, I close the shutter and wait for things to die down, then open the shutter again. This works surprisingly well. A similar use is to split exposure with and without brightly lit areas. A forest scene exposed without sunlight and then again with, balancing the exposures so that you don't have to deal with the contrast issues of a dark forest with dappled sunlight. Or, a shot of a cityscape or building with lights at dusk: one exposure for the scene with sky and without lights, the next for the scene with lights. A third use is to make a montage of a moving object similar to stop-motion photography on one frame.



Mark Sampson
3-Oct-2016, 08:46
Paul Strand used this technique when shooting forest still-life in the 1920s- one particular photograph of his, of a toadstool, comes to mind. He knew that grasses and leaves would return to their original positions after the wind had died down. I've never tried it, but it must take a great deal of patience.

Drew Wiley
3-Oct-2016, 09:27
Simple math simply won't work. You'll need more time than the sum of the fragments. While the theory can be explained, there is really no substitute for basic
testing in advance using your specific film and filter.

3-Oct-2016, 09:36
Thank you all for your input and information.

Drew - The late Barry Thornton made an image of water flowing over a wall with many high lights, and the repeated short time exposures gave an amazing effect. Not sure I will reach Paul Strand's efforts but nothing like having something to aim for..


Bill Burk
8-Oct-2016, 08:06
You could probably make tourists vanish from monuments using this technique.

8-Oct-2016, 16:32
P...A similar use is to split exposure with and without brightly lit areas. A forest scene exposed without sunlight and then again with, balancing the exposures so that you don't have to deal with the contrast issues of a dark forest with dappled sunlight...

Genius. You have made me a very happy printer.

Drew Wiley
10-Oct-2016, 13:30
Tourists in the monuments? Oh, that's an easy problem to fix. Bring along a pet skunk.

10-Oct-2016, 15:53
Bonjour Peter,

Two years ago I've made a series of divided exposures in my house studio, alternating exposures only with speedlights, SL + painting light and candles lights + SL.
My calc ended up giving me 1/2 or 2/3 more light then the simple math. At that time, I remember had used my digital camera to chech exposures read by the flash meter, and adjusting it accordingly. Now I have a flash meter that makes the calc for me with cumulative metering readings, but still have the option to use my "electronic polaroid" check just to make sure the film will not come under or over-exposed.

Ex.: of shot, that I made:

https://c5.staticflickr.com/8/7438/14074377452_04068201e2_b.jpg (https://flic.kr/p/nrGS5d)4x5_2014-3886-89 (https://flic.kr/p/nrGS5d) by Renato Salles (https://www.flickr.com/photos/sallesrenato/), no Flickr

with candles

Sinar F2, Rodenstock Sinaron 5.6/210 at f22, 4x5 Shanghai GP3 ISO80, Rodinal 1:150 in stand 2hs..
Two exposures with the shutter open: first with (3) speedlights and the candles lights off, and the second only with the candles lights for 10s.
Negative scanned with a FF DSLR in 4 passes, stitched in PS.