Re: image circle relative to focal length/fstop

But if your aim, Thom, is to have a circular image in the middle of your
paper/film format, the bellow mentioned rule will not work for you, in such a
case you should perhaps assume a 5 times the focal length or even more. Use
also thick material to make your pinhole.

Guillermo

PS: it was somewhat hard to answer this nature of question without recurring to
use "derivative calculus" which BTW, I thought it was called trigonometry,
taught in grade 8 to me. My friend Juan use to say: "why are we learning this
stuff we would never use", he was probably right ;-)

----- Original Message -----
From: "Tom Miller" <twmiller@mr.net>

> The rule of thumb is that, on a flat film plane, the circle will be
> 3.5 times the focal length. So, if the focal length is 1" the circle
> will be 3.5", if the focal length is 2" the circle will be 7", and so
> on. The f-stop doesn't really matter for this rule of thumb.
>
> ----- Original Message -----
> From: "Thom Mitchell" <tjmitch@ix.netcom.com>

> How can I quickly determine the size of the image circle for a given
> focal length and f/stop. I want to be able to keep an image circle
> from getting too big, i.e. I want it to be 2' or 3''. Any quick help
> would be appreciated as would simple rules of thumb as opposed to some
> of the derivative calculus I sometimes see. Thanks for your help in
> advance. Thom
Received on Mon Mar 25 22:53:36 2002