From: mikec@spider.co.uk (Mike Coren)
Newsgroups: rec.photo
Subject: Questions about everything
Date: 5 Jan 93 11:46:32 GMT
Organization: Spider Systems Limited, Edinburgh, UK.


My apologies if this is a repost.  My link to the internet appears to have
died briefly over the holidays, so I don't know if this ever got out.

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OK, folks, now that you're all fresh and relaxed from your holidays, here's
some questions to get the ol' brains working again.  These came up when I
was reading through a photo magazine in a very thoughtful and emotionally
unstable mood.  I don't think any of these are FAQ's.  If they are, then
there are some very sick SICK people on this newsgroup!  Much obliged.


1)  Why do fast zoom lenses need rediculously wide filters when compared to
their slower counterparts?  For example, compare the specs on the Tokina
AT-X 28-70mm f/2.8 versus the SZ-X 28-70mm f/3.5-4.5.  Both have a maximum
field of view of 75 deg, 20 min at 28mm, but the AT-X (f/2.8) uses a 72mm
filter, while the SZ-X (f/3.5) uses a 52mm filter.  On the one hand, these
numbers seem way too small, since the 92mm long AT-X would need an opening
142mm in diameter to capture a 75 degree field, while the 65mm long SZ-X
would need an opening 100mm in diameter for the same 75 degree field.  On
the other hand, though, at the 70mm end, to give f/2.8, the AT-X only needs
a clear aperture of 25mm diameter, and the SZ-X at f/4.5 needs even less.
Is there a simple explaination for this, or is this one of the subtleties
of modern computer-aided optical design?

2)  Why does film speed (ASA) increment in 1/3 stops, while shutters and
apertures typically increment in 1/2 stops or whole stops?

3)  Why doesn't exposure value take film speed into account?  An EV by
itself seems useless if you don't know what film speed is being used.  Is
there something like an EV-ASA product that's commonly used?

4)  Does the light in the refrigerator turn off when you close the door?

5)  Guide numbers on flashes are typically given for 100 speed film.  If I'm
using 400 speed film, should the guide number be quadrupled, or only doubled,
since light intensity decreases with the square of the distance, or something
completely different?  (Actually, I think I know the answer to this one based
on EV's, but humor me anyway).

6)  I recently bought a bunch of 35mm Fuji film, which comes in clear plastic
cannisters.  If I want to put those cannisters into the film loops of my
camera bag, should I be worried about having direct sunlight hit them (not
that we get much direct sunlight in Scotland, but...)?  Should I put a few
at a time into black Agfa or Kodak cannisters before going out?

7)  Are UV filters for black & white only?  I read where this guy was fairly
adamant that UV filters are only for B&W, and you should use a skylight filter
for color.

8)  What is the air speed velocity of an unlaiden swallow?

Thanks in advance,
Mike

-- 
Michael D. Coren, Electrical Engineer             mikec@spider.co.uk
Telecommunications Techniques Corporation, Germantown, Maryland, USA.
Temporarily at Spider Systems Limited, Edinburgh, Scotland, UK.
Newsgroups: rec.photo