I recently ran into doubt about exposure. Are four 15 sec. exposures equal to one 1 minute exposure? Thanks!

And if Aaron will forgive me for tagging onto his question, will firing the flash in an otherwise dark studio 16 times at f/32 give the same exposure as firing it once at F/8?

Four 15 sec exposures are equivalent to one 60 sec exposure provided neither the camera nor the subject moves.

But there is still reciprocity failure to consider, depending on the film.

I think this is a real good question. I'm to lazy to check but did Fred Picker write that say, four two second exposures did not equal one eight second exposure? Now that was with printing paper and enlarging. I think that a full minute with film you would not be able to find a change.

In both cases, I'd say, "Probably not." But, that really depends on the film and its specific reciprocity characteristics.

Hi Aaron,

Are you asking about four 15 sec. exposures with an enlarger on printing paper, or with a camera on film?

I routinely use multiple light bursts when printing with my enlarger, usually 4 or 5 seconds, each. There is a slight warm-up period as the lamp is turned on and off. Therefore, in the case of the enlarger, four fifteen second light bursts would actually give slightly less total exposure than a single continuous one minute exposure. However, it really doesn't matter, as long as the test strip was also made with the same light bursts.

With a camera and film, There is no warm-up period as the shutter is fired. So yes, four fifteen second exposures would be equal to one minute of total exposure time. However, shutter accuracy, camera shake, and subject movement would need to be considered.

Hi all, thanks for the response. Eugene, sorry, should have mentioned film exposure. Recently I made multiple exposures on a single film and it turned out a little underexposed. Could have made a calculation error.

"Will firing the flash in an otherwise dark studio 16 times at f/32 give the same exposure as firing it once at F/8?" Mark Sawyer

Mark,

I believe Ralph and Eugene's answers are correct. If the flash put out the exact same burst of the same duration every time and 'warm up' wasn't a factor, I believe you would need 8 bursts at f32.

1 for f8

2 for f16

4 for f22

8 for f32

If I'm wrong, someone will surely let us know :-)

DG

Someone correct me if I'm wrong (which would be unsurprising, really), but if you're making four 15 second exposures on film then you have to correct for one minute of recipricatory failure (or have one minute be the result of your recipricatory calculation for a shorter exposure).

Of course, this makes me ask a question of my own: Would the recipricatory factor change if you waited a day between exposures? An hour? A minute? I suspect it doesn't matter, but perhaps someone out there knows for sure.

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