In camera multiple exposure technique
Posted 15 February 2015 - 03:12 PM
Also, if I'm shooting tri-color technique where each pass is Red, Green, or Blue how do I compensate for each filter so that the final composite is balanced?
Posted 15 February 2015 - 03:48 PM
I'm like 90% sure this is correct, but for double exposure, you underexpose one stop for each pass because that's half as much light each pass. Two stops under would be a quarter of the light as a "correct" exposure, so you would need four passes to get the proper exposure. So for three passes, I'd underexpose 1.5 stops, that should leave you with 33% as much light as a "correct" exposure. Is that right? Somebody correct me, please!
As for the filters, they should tell you how much more to stop down. Usually it's about 2/3 of a stop. So in all I would probably try underexposing about 2 stops for starters. That should get you near the correct exposure.
Posted 15 February 2015 - 05:09 PM
If the double-exposed images overlay each other, as opposed to being objects against a black background for example, then yes, you'd have to underexpose each pass because of the accumulating exposure. Three passes would mean each should supply 1/3 of the exposure, so Josh's guess of 1.5-stops under for each sounds about right though I'm not sure of the exact amount to achieve a 66% lowering of exposure.
When going through color separation filters, though, I don't think you necessarily have to make each negative 66% underexposed since they are only supplying information for one color record. If the object was a red apple, for example, it's going to naturally be more exposed on a red record than the blue and green ones, so I don't think you'd want to underexpose the red pass by 66%. However, there is bound to be some accumulative effect. Testing would be best, but I'd probably be conservative and only underexpose each pass by a 1/2-stop.
Posted 15 February 2015 - 07:25 PM
I made a mistake in my head with the filters because obviously you need more light not less. Half a stop sounds like the place to start. Definitely.
Posted 21 February 2015 - 09:07 AM
I've been thinking about testing shooting tri-color with three passes myself, does anyone happen to know any examples where it has been used?
Posted 21 February 2015 - 11:43 AM
Eventually reciprocity failure becomes significant and the calculation doesn't work. I've made 60 exposures and that did require a test.
Making three exposures onto color film using fairly selective R, G, B filters is interesting. It essentially creates a new color film, with new color reproduction characteristics. You will not reduce exposure when using these filters but increase it, quite a bit. Think it through. The light transmitted by the three filters adds up to much less than full white light. For example, when using Wratten filters #29, #61, #47 the total is about 25% of full white light. Very crudely, because this ignores the peculiarities of the color film's sensitivities and balance, your three filtered exposures together are only equal to one unfiltered exposure through an ND 0.6 filter. So you must add about 2 stops on average when making each of the filtered exposures.