Archived from groups: rec.photo.equipment.35mm,rec.photo.digital (
More info?)
Ken Tough wrote:
> Alan Browne <alan.browne@FreeLunchVideotron.ca> wrote:
>
>
>>Digital cameras typically pre-flash and measure the light using the
>>viewfinder meter (some film cameras do this as well). It appears that
>>the OTF method does not work with digital.
>
>
> It's funny that. You'd think they could pop in a few direct
> pixel outputs to a real-time averaging circuit to make a nice
> on-the-fly system. Pre-flash can sometimes be a hassle,
> including reducing overall flash power. Does preflash still
> apply for the dSLRs? (Can't say as I've noticed it, but perhaps
> it's extremely quick).
CCD is a 'charge up' device. You can't read them while they're
charging. (I'm not sure about CMOS). They could have just put in a
couple dozen pixels dedicated to TTL flash metering, but the read design
of the chip probably precludes single pixel reads.
I was thinking that a simple way would simply be to have little
reflectors in between the pixels on the CCD/CMOS to reflect light out to
the classic OTF TTL detectors. However the glass over the sensor would
probably prevent this from working well and would result in light spill.
Kabosh that idea.
There is nothing wrong with the pre-flash other than integration with
slaves is difficult (There are slaves that will fire on the 2nd (or nth)
flash in a series).
I'm not sure how much energy is lost on the preflash pop. Not much I
would guess, eg: if they use 1/32 for the pre-flash, that much missing
energy would not be noticeable when shooting at 1/1, and at other levels
there would be plenty of power in reserve.
Cheers,
Alan
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