Re: Flash with pinholes

----- Original Message -----
From: "Colin Talcroft" <ctalcroft@yahoo.com>
To: <pinhole-discussion@spitbite.org>
Sent: Thursday, February 03, 2005 12:40 PM
Subject: [pinhole-discussion] Flash with pinholes

> Hi everyone,
>
> Quite a while back, someone posted some data about
> pinhole exposures using multiple flash firings. I
> can't seem to find it. Does anyone have a copy of this
> they could send me or post? I've been photographing
> for decades, but almost never use flash and it always
> confuses me. I'm thinking that, assuming an f number
> of about 250 for the pinhole, if the scene meters at
> say 8 seconds at f=22 (which is typical of the studio
> nudes I do) then the pinhole exposure would be in the
> neighborhood of 30 minutes, which works, I know from
> experience (although that can be tripled--which is
> very hard on a model!).
>
> Using flash at about 1.5m from the subject, how many
> times would I need to fire the flash to expose skin
> tones at a reasonable value? I'm guessing that f=5.6
> at 1/60th of a second is about right for an actual
> flash exposure with a lens. That suggests that, at
> f=250, I'd need to increase the exposure about 10
> times. Does ten flashes sound about right?

There are 11 stops from f/5.6 to f/250, that means you need to double the
exposure time 11 times or in other words you'd need to give 2048 times the
exposure of a lens at f/5.6, 1 flash for f/5.6 = 2048 flashes for f/250,
unbelievable when you think in number of flashes, but believable if you
think that if using available light you'd need 2048/60 = 17 seconds
(uncorrected for reciprocity time).

> Also, how does reciprocity come into this, I wonder?

Not reciprocity perhaps, but something else called intermittency, in theory
2048 flashes at f/250 are equal to one flash at f/5.6 but in practice you
would need a bit more due to intermittency.

Guillermo
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Received on Thu Feb 3 11:17:34 2005