Double Exposure Calculations

[Kirks518]

Now that I have an LF camera, I want to try some double exposure images that have been bouncing around in my head for years. I haven't tried it with smaller formats simply because with smaller formats (ie roll film), you have to shoot the shots consecutively (ideally). With LF, you take shot one, and just hold onto that image until you're ready to take the second image.

So my question is; do you have to make exposure compensations for doubles? Ie, do you underexpose them, or do you just expose each shot as metered? Or is there some oddball calculation that needs to be done to get it the way you want? Any advice appreciated.

[StoneNYC]

I believe the "normal" rule is 2/3 and 2/3 for a double exposure.

I know it seems odd, but that's what I follow for even exposure between them. Obviously it depends on what you're going for.

[Miguel Coquis]

Try with separate negs. Expose each as it should.
Once dev. and dry, check highlight, middle, low densities on each neg.
Merge them or put one on top of the other and observe quietly with much attention how different densities go.
This can give an idea when pre-visualizing "double exposures".

[Rick A]

I've always divided the total exposure equally by how many sub-exposures. This usually only works when you place the subjects in different places on the negative, overlaying exposures will need experiments. One double exposure I did many years ago, I placed my subject against a black background, illuminating only the subject, and gave full exposure for a closeup bust shot, then pulled back and placed the subject to appear as tho she was sitting on her own shoulder and gave another full exposure. I wish I still had the finished portrait to show, it worked beautifully. My ex-wife kept the portrait.

[StoneNYC]

I've always divided the total exposure equally by how many sub-exposures. This usually only works when you place the subjects in different places on the negative, overlaying exposures will need experiments. One double exposure I did many years ago, I placed my subject against a black background, illuminating only the subject, and gave full exposure for a closeup bust shot, then pulled back and placed the subject to appear as tho she was sitting on her own shoulder and gave another full exposure. I wish I still had the finished portrait to show, it worked beautifully. My ex-wife kept the portrait.

You don't have the negative?

[Rick A]

No Stone, I shot that portrait in the early 70's and it's long since been gone. I closed my studio and packed everything away, lost most in a flood many years ago. My ex still has the 16x20 on her wall though.

[Doremus Scudder]

Multiple exposures are pretty easy, just divide the total exposure time by the number of exposures you are making (after adjusting a bit for intermittency, see below). If your total exposure is 1/4 sec, two 1/8-sec exposures will give you what you need, etc.

I often make long exposures of several seconds/minutes in many increments to make up the total time. For example, when people or traffic is passing by your scene, you can just open the shutter when nothing's in the way and close it when people/cars/etc. are passing by, then reopen it later, building up the entire exposure slowly. This is also useful for capturing the quiet periods between wind gusts, etc.

There is a phenomenon known as the "intermittency effect," which basically says that several small exposures don't quite equal up to one long exposure of the total time. This effect is not very large, but is there. Therefore, it is a good idea to give add a bit more exposure to your metered exposure as a safety factor (for negative films I use 20% or so, which is probably too much, but a good safety factor).

Stone's 2/3 plus 2/3 would work just fine for negative film, but likely result in a negligible bit of overexposure that one would just print through (4/3 is just 33% overexposed, not much at all with neg materials and even less when you subtract the intermittency).

Best,

Doremus

[StoneNYC]

Multiple exposures are pretty easy, just divide the total exposure time by the number of exposures you are making (after adjusting a bit for intermittency, see below). If your total exposure is 1/4 sec, two 1/8-sec exposures will give you what you need, etc.

I often make long exposures of several seconds/minutes in many increments to make up the total time. For example, when people or traffic is passing by your scene, you can just open the shutter when nothing's in the way and close it when people/cars/etc. are passing by, then reopen it later, building up the entire exposure slowly. This is also useful for capturing the quiet periods between wind gusts, etc.

There is a phenomenon known as the "intermittency effect," which basically says that several small exposures don't quite equal up to one long exposure of the total time. This effect is not very large, but is there. Therefore, it is a good idea to give add a bit more exposure to your metered exposure as a safety factor (for negative films I use 20% or so, which is probably too much, but a good safety factor).

Stone's 2/3 plus 2/3 would work just fine for negative film, but likely result in a negligible bit of overexposure that one would just print through (4/3 is just 33% overexposed, not much at all with neg materials and even less when you subtract the intermittency).

Best,

Doremus

Thanks Dormeus,

I knew there was some "loss" but didn't know the term or why.

Intermittency, got it! I like your calculations.

Here's an example (censored) of an image I did using my 2/3 calculation, it is a bit over-exposed.

It was done with Profoto strobes so the exposures were fairly accurate, I would pull it back some if I were shooting daylight, the strobes pop at exact light amounts and essentially ignore the shutter speed, but most LF shutters are a tad bit slow, for a single shot that's probably ok but for multiple exposures it could cause issues.

As always, the best thing to do is start with a baseline and test yourself as your metering etc might be different than someone else's, and Dormeus is more experienced than me so start with his calculations and adjust accordingly.

Good luck!

[DG 3313]

FWIW......I multiply the film speed by the numbers of exposures (that overlap) and set my meter at that new film speed. ISO 100 with 4 overlapping exposures is now ISO 400 for each of them. If they are shot against a black backdrop and don't overlap.......it's IOS 100 for each of them.