Metering for multi-expsosures

[seany65]

Hello all,

I'm aware that doing multi-exposures on a single frame will lead to over exposure if you use the original exposure reading for each shot, which means you should deliberately under-expose, but if we want to do two shots on one frame do we under-expose both shots by one top or just the second shot? What about a third or even fourth shot?

Any help would be much appreciated.

 

[Joao]

Trichromy (sort of)

Trichromy (sort of)
Triple exposure of Portra 160 (with red, blue and green filters). Mamiya C330. Standard commercial development, some resizing, no colour changes.



Regards

Joao

 

[Joao]

To get the above image I just exposed each shot 1/3 of the total required exposure time.
This is quite empiric but I get the results I want.
Joao

 

[Richard G]

I’ve done ok with accidents with the same exposure as for one image with black and white.

 

[seany65]

To get the above image I just exposed each shot 1/3 of the total required exposure time.
This is quite empiric but I get the results I want.
Joao

Hmm, seems pretty much to be what I think I vaguely remember of what I think I've read about it many years ago, but the memory was so vague I thought I'd better check. Thank you.

Richard, nearly all my shots look like accidents and I've not done any multi-exposures yet, lol.

 

[seany65]

Joao, I'd better double check, as I'd generally be thinking in apertures when adjusting exposures for M.E.'s:

The meter suggest f4 for 1 shot, so for a double exposure I'd be exposing both shots at f5.6 and for a triple exposure I'd be exposing all three shots at f8?

 

[pvdhaar]

f8 would be for four shots (two times two). For 3 shots it would be halfway between f5.6 and f8. So at f6.7.

It's a valid question though, because at the extreme, you're running into the limitations of the film. There's a minimum exposure required to push the film's response above its threshold. But that's only an issue when you're really doing something extreme, like stacking tens of exposures on a single frame. In such cases, pre-flashing the film up to threshold level may be required.

 

[Joao]

@sean65 A quick search finds several links with techniques for ME. Most deal with tricks to get doble exposures in cameras that prevent it.

A Multiple Exposure Experiment: 20+ Exposures in a Single Film Image » Shoot It With Film

Emma Lloyd in “Analogue Wonderland” sumarizes the technique (Quote)

• Correct exposure at shutter speed 1/125 would need to be set to 1/250 for a double exposure (2 exposures, each 1 stop underexposed), and 1/500 for a quadruple exposure (4 exposures, each 2 stops underexposed)
• Correct exposure at aperture f8 would need to be changed to f11 for a double exposure (2 exposures, each 1 stop underexposed) and f16 for a quadruple exposure (4 exposures, each 2 stops underexposed).

(End of quote)

It would be interesting if you post your results mentioning what you have done

Regards
Joao

 

[seany65]

Thank you Peter and Joao for the further replies and info.

a single f4 would be at f5.6 for a double, (1 stop down from a single) but a triple would be f6.7 (1 and a 1/2 stops down from a single), and f8 for a quadruple (2 stops down from a single).

But to my two braincells, if the exposure for a triple is only 1 and 1/2 down and not 2 as I'd thought, then each successive shot should be cut by a smaller amount each time, eg. if the first decrease is 1 full stop, and the second half a stop, then the third decrease (fourth shot) should be cut by 1/4 of a stop.

To any readers: don't mind this stuff, it's just my ramblings, which may or may not help me get my head around the decreases being less than I thought they'd be for each shot after the second.

Anyway, the photos I have in mind will only be having three shots on each frame at most, that is if I ever get to do them. Unfortuantely the camera I want to use only has 3 apertures and no between settings, though I have a set of ND's I can use. Would using a 1 stop ND on the third shot have any effect?

 

[pvdhaar]

...
But to my two braincells, if the exposure for a triple is only 1 and 1/2 down and not 2 as I'd thought, then each successive shot should be cut by a smaller amount each time, eg. if the first decrease is 1 full stop, and the second half a stop, then the third decrease (fourth shot) should be cut by 1/4 of a stop.
...

The same compensation is applied to all exposures. In the example with a base single exposure at f4, all three would be shot with the same shutter speed and all three at f6.7. Don't confuse multiple exposures with layers in a photo editing program where each higher layer needs higher transparency to build up the total image.

 

[seany65]

Thanks Peter. Yes, I understand that, but I didn't explain myself very well (as per usual, when "thinking" is involved, due to the fact that I'd started writing before the full idea of what I wanted say to had formed. 😳). Instead of "each successive shot" I should've written "each successive set of shots for one frame", going from a "set of 2" which has both shots 1 stop under, I now find a "set of 3" has all three at 1.5 stops under (instead of 2 stops under as I'd expected) through to a "set of 4", which to me, I'd expected a "set of 4" to be three stops under. Looking at the second of Joao's "Emma Lloyd" examples, a "set of 4" would only be 2 stops under, meaning the decrease gets smaller for each "set". eg. 1stop each for a "set of 2", a further decrease on that of a 1/2 stop for each for a "set of 3" and a further decrease on that of a 1/4 quarter stop for each of a "set of 4".

I think that's what I mean.