Re: Polaroid exposures

----- Original Message -----
From: "The Painted Horse" <paintedhorse@in-tch.com>

> Hi Guillermo,
> Thank you for your reply. Obviously, I miscalculated the math for my
exposures
> (*sigh*). However, just to clarify..on the calculations given below,
wouldn't
> it be f16 @ 1/100 secs and not 1/25 sec?

1/125 and 1/100 is for practical purposes the same thing, nevertheless, you
are right, the BASE exposure for sunny open skies conditions is f/16 @ 1/100
when the emulsion you use has an ISO-100 (or EI-100 for that matter).

> I'm trying to understand the basis for the formula and the 804 film is ASA
> 100. Given this, wouldn't the initial exposure (not factoring in
reciprocity)
> then be around 10 sec. rather than 40 sec?

That's right, 10 secs @ f/500 will give you the same exposure than 1/100 @
f/16 (disregarding reciprocity corrections). So using this table for 804 at
Polaroid http://www.polaroid.com/service/filmdatasheets/8_10/804fds.pdf ,
the correction for 10 seconds would be something like 2.25 f/stops that
translate to something like 48 seconds final exposure time.

If you were to meter the snow (under whatever conditions of light) and get a
reading of f/16 @ 1/100 secs (for instance), exposure that we now is
equivalent to f/500 and 10 secs, if you give that esposure, the snow will be
rendered with a grayish tone. To get it white you would need to increase
the exposure around 2 stops, so the 10 seconds become 40 seconds, to which
we'd need to add more time to correct for reciprocity.

I think I mixed sunny/16 and metered ideas in my last message, sorry.

> To answer your question I had taken a metered reading of the northern sky
> rather than the snow for these exposures. The reflection was just too
bright
> from the snow for me to think I would get an accurate reading. The
reading
> came to EV16. Now, here is the part that, looking back, I can't
> explain...somehow I came to an exposure of 15 minutes.

EV16 is f/16 @ 1/250, I may be wrong, but I think northern sky should be at
least 1 stop darker than the actual snow, which is the same to say that it
is at least 1 stop brighter than 18% gray. Therefore to get snow the "right
tone" you should have exposed for f/16 @ 1/125 seconds, which in turn is
about the same as f/500 @ 8 seconds (uncorrected).

> Please don't ask me how

OK, I won't.

Guillermo
Received on Thu Apr 5 18:17:49 2001