From: jacobson@cello.hpl.hp.com (David Jacobson)
Subject: Re: Questions about everything
Date: Thu, 7 Jan 1993 05:11:37 GMT
Organization: Hewlett-Packard Laboratories

In article <1993Jan5.114632.29209@spider.co.uk> mikec@spider.co.uk (Mike Coren) writes:

>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?

I think the requrired diameter to avoid vignetting is the diameter of the
aperture (focal_length / f_stop) plus 
Sqrt[24^2+36^2]/focal_length*front_nodal_point_to_front_element_distance.
I didn't follow your arithmetic, but since you don't know the last
term, you can't get a hard number.

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

The same reason bananas are yellow.

>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?

Don't know.

>4)  Does the light in the refrigerator turn off when you close the door?
 
Yes.  (You knew the answer too.)

>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).

Doubled.

>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?

I too worry about sunlight on clear Fuji cannisters. 

>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.

He's nuts.  UV is for color.  I avoid skylight filters on the basis
that if I want to tint my pictures I'll put a filter to do that on,
and when I don't I won't.  Also not all skylight filters are the same
(I've heard) and if I mix brands, then various lenses will do slightly
different things.

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

See answer to #2 above.

>Michael D. Coren, Electrical Engineer             mikec@spider.co.uk

  -- David Jacobson