Multiple Exposure Compensation

I have a question about doing multiple exposures in a film camera. I would like to be able to give more prominence to one exposure over the others. I have found a way to do it using the tripod but it would be useful to know how to do the calculations and not have to use the tripod. For example, using 100 speed film, to do five exposures I set the ISO to 500, and to give one image more weight I can take two or three identical exposures with the camera on the tripod and the others will be paler. My question is how to calculate the ISO setting to do the same thing, one ISO for the image that should have more weight, another for the rest.

If you use ISO as a mean to do the compensation then in the case of 5 shots and ISO100 film you can either make 5 shots at ISO500 or you can use different ISO for each shots but the average ISO of the 5 shots is 500.

It's going to be something like one shot at 200 and two at 800 or 1600, what I need is the formula....

Sounds like you're going to have to experiment. If it were me, I'd take the five exposure in separate frames and then do a little Photo Shop Edited February 19, 2019 by chuck909

It's going to be something like one shot at 200 and two at 800 or 1600, what I need is the formula....

 

If you do 3 shots with ISO 100 film. If the first shot is ISO 200 then the other 2 would be at 350 each.

This is exactly what I need, thanks! How do you arrive at these numbers, so I can generalize the calculation to different situations, such as different ISOs or more or fewer exposures. Thanks again....

. . .How do you arrive at these numbers, so I can generalize the calculation to different situations, such as different ISOs or more or fewer exposures.

 

If you want to make estimates of the exposures using ISO Ratings then:

1. Simple Multiple Exposure Formula (i.e. all exposure rating the ISO the same for each, attempting to give each exposure the same degree of prominence):

 

n/x = ISO

Where

ISO = Native ISO of the film being used

n = ISO Rating for each exposure

x = number of exposures

 

SAMPLE:

“I want to make a triple exposure using ISO100 Film and all exposures will have equal prominence.”

 

Formula: n/3 = 100, therefore n = 300.

i.e. for each exposure I need to rate the film at ISO300

 

***

 

2. Complex Multiple Exposure Formula (i.e. all exposure rating the ISO different for each, attempting to give each exposure a differing degree of prominence):

 

 

2a. As a general rule, you choose a LARGER ISO value for an exposure that you want to be LESS prominent.

 

 

2b. Example of Complex Multiple Exposures:

 

Use the simple formula first.

 

Employ an ‘averaging formula’ to account for the different degree of prominence that you THINK that want for each exposure. Depending upon the total number of exposures you want to make, you will need to guess at one or more ISO ratings that you think that you want to use.

 

‘Averaging Formula’ is:

 

[A(a) + B(b) + C© . . . ]/x = n

Where A = number of exposures at ISO a

Where B = number of exposures at ISO b

Where C = number of exposures at ISO c

. . . means “and so on”

x = total number of exposures

n = the number derived from the simple formula

 

SAMPLE

 

“I want to make a triple exposure using ISO100 Film and the first exposure will have more prominence and the second and third will have less, but equal prominence. I think that I want the first exposure to be at ISO 200.”

 

Formula:

(use the simple one first) n/3 = 100, therefore n = 300

 

Averaging formula (now A = 1 and a = 200, so):

[1(200) + 2(b)]/3 = 300

[200+ 2(b)]/3 = 300

200 + 2(b) = 900

2(b) = 700

b = 350

 

You have calculated that you can make the first exposure at ISO 200 and the second and third exposures at ISO 350.

 

2b. Example of (more) Complex Multiple Exposures:

 

“I want to make a triple exposure using ISO100 Film and the first exposure will be most prominence and the second have less prominence and the third have less prominence again. I think that I want the first exposure to be at ISO 200."

 

(use the simple one first) n/3 = 100, therefore n = 300

 

Averaging formula (now A = 1 and a = 200 so):

[1(200) + 1(b) + 1©]/3 = 300

200 + b + c = 900

b + c = 700

You now need to choose values for b and c. For example, if you choose b = 250, then c = 450.

 

***

 

3. Formula not working, because you ‘ran out of’ ISO:

 

If the formula does not work (giving a negative ISO) then you have “run out of” ISO to use (i.e. you likely begin to overexpose the FINAL IMAGE).

For example:

 

“I want to make a triple exposure using ISO100 Film and the first exposure will have more prominence and the second less prominence and the third less prominence again. I think that I want the first exposure to be at ISO 200.”

 

(use the simple one first) n/3 = 100, therefore n = 300

 

Averaging formula (now A = 1 and a = 200 so):

[1(200) + 1(b) + 1©]/3 = 300

200 + b + c = 900

b + c = 700

You now need to choose values for b and c, for example, if you choose b = 800, then c = -100.

!!!

Formula does not work because you cannot have a negative ISO so therefore you CANNOT choose b = 800.

 

*

 

4. Formula not working, because the result will not be as what was originally required:

 

You need to check the relative ISOs that the formula gives to you and then relate those to ensure what you will achieve, will be what you set out to achieve.

 

Example:

 

“I want to make a triple exposure using ISO100 Film and the first exposure will have more prominence and the second less prominence and the third less prominence again. I think that I want the first exposure to be at ISO 200.”

 

Averaging formula (now A = 1 and a = 200 so):

[1(200) + 1(b) + 1©]/3 = 300

200 + b + c = 900

b + c = 700

 

You now need to choose values for b and c, for example, if you choose b = 500, then c = 200.

!!!

Formula does not work, because if c = 200, then it will have the same prominence as the first exposure made at ISO200, therefore you cannot choose b = 500.

 

WW

I just can't tell you how much I appreciate this information, I'm very very happy and I will be putting it into practice first thing tomorrow. Thank you so much!

You're welcome. Glad that I could convey it in an easy to understand manner.

 

Nice abstract.

 

Break a leg.

 

WW

William, If you divide by three (Simple sample #1), wouldn't all three exposures wind up being under exposed? Also, ISO 300 is not 1/3 of ISO 100. ISO 800 would be 1/3 the exposure (100-200-400-800). So I'm confused. Could you clarify.

Alan,

 

First question: "If you divide by three (Simple sample #1), wouldn't all three exposures wind up being under exposed?"

Yes each individual exposure onto the film will be 'underexposed', yet the combined all three exposures will usually make a reasonable image.

Think of it this way, if all three exposures were of exactly the same scene, and the shutter were opened three times, each time of 1/3 of the 'correct' shutter speed for that scene, then although each individual of those three exposures will be under, yet the combined of all three will make for a 'correctly exposed' image.

 

***

 

Second question: "Also, ISO 300 is not 1/3 of ISO 100. ISO 800 would be 1/3 the exposure (100-200-400-800)."

I've never worked multiple exposures by ISO Ratings, I have always calculated the exposure for each of a series multiple exposure in Stops.

For example:

> for a multiple of two exposures, each with equal prominence, then each needs to be -1 STOP

> for a multiple of three exposures, each with equal prominence, then each exposure needs to be: -1.5 STOPS

> for a multiple of four exposures, each with equal prominence, then each exposure needs to be: -2 STOPS

and so on.

 

***

 

The OP wanted a calculation representing the manipulation of the ISO Rating when s/he sets the Lightmeter.

To achieve this I developed the formula based upon the "STOPS" that I have previously used.

Using the ISO Rating is actually quite convenient, as it makes for a calculation involving simple arithmetic using the ISO numbers.

(I suggest that we forget about the idea of "1/3 of the exposure")

Look at the simple example, if we move from ISO100 to ISO300, then that is a move of 1.5 Stops"

I hope that explains it, in another way.

 

***

 

It is important that we realized these are guides only and much depends on the experience of doing it and feeling one's way with it.

 

For example, in the 1970's there was a great passion (fad) for Brides and Grooms to have their Portrait superimposed in a Brandy Balloon and also on the Wedding Certificate.These negatives were created as a multiple exposure, i.e. each was made as a Double Exposure. Experience taught me that for the former I would tend to make drop the Brandy Balloon a bit LESS than -1STOP (i.e. give it more exposure to make it a tad dominate), and for the Wedding Certificate I would drop the exposure a bit MORE than -1STOP (i.e. give it LESS exposure and make it less dominate).

 

WW

... I was just thinking that I wrote "and so on" a few times in the previous comments -

 

I have not ever made more than four exposures in any one multiple exposure, so my experience does not go beyond that.

 

This is relevant because "and so on" does NOT imply what exactly the progression should be for: five six and seven (etc) exposures.

 

My guess is that one could try continuing a reduction in half stops, for each additional exposure - in which case the formulae I wrote above for adjusting the ISO does not fit - on the other hand, using that ISO formula could be OK.

 

Mind, I really don't think there are many situations where more than four exosures are used, for a multiple exposure.

 

WW

Think in fractions, not multiples of ISO.

The exposure always adds up to 1, right?

So if you give your main subject half the exposure (200 ISO), then the other half of the exposure has to be divided among all the other multiple exposures.

So, say you're aiming to do 5 multiple exposures in total. That's one exposure getting half of the light, and the other four getting one-eighth of the light. (1/2 + 4/8 = 1)

 

This is pretty easy to translate into ISO speeds. You just multiply the ISO by the reciprocal of the fraction. Assuming a film speed of 100 ISO in the above example: The main exposure is set at 200 ISO (100 / 0.5) and the other four exposure are set at 800 ISO (100 / 0.125).

Also, ISO 300 is not 1/3 of ISO 100.

Yes it is Alan.

ISO speeds are linear, and setting an EI of 300 with ISO 100 speed film would result in 1/3rd of the correct exposure being given.

 

Well, unless you really want to do some mental gymnastics and use the logarithmic version of ISO. Then you'd have to add on the base 10 log of the reciprocal fraction to 21.

Edited February 21, 2019 by rodeo_joe|1

Sounds like you're going to have to experiment. If it were me, I'd take the five exposure in separate frames and then do a little Photo Shop

 

If you are digitizing the images, this is by far the easiest way to combine images and you can adjust in the post processing, instead of doing it the old time way.

 

Here's some earliet P.net discussion, too… How to take multiple exposures with analogue camera (Zorki 4)?

as well as a Wikipedia article Multiple exposure - Wikipedia

Think in fractions, not multiples of ISO.

The exposure always adds up to 1, right?

etc...

 

Simple. Love it.

 

WW