Subject: Re: Focus Target
From: Ernie Wright <[email-masked]>
To: [email-masked]
Date: Wed, 31 Jan 1996 17:18:15 -0800

Scott Burton wrote:

> Depth of field can be calculated fairly simply (simple for those people
> who know how to use that math stuff, it involves numbers...I think)

I've been known to use that math stuff on occasion.  Here's one way
(there are several) to calculate the near and far limits of focus.

   h    = ( f * f ) / ( a * c )
   near = ( h * s ) / ( h + s - f )
   far  = ( s * near ) / ( 2 * near - s )

where

   s is Focal Distance
   f is Focal Length
   c is Circle of Confusion Diameter
   a is Aperture (F-Stop)
   h is Hyperfocal Distance

The units don't really matter, but they all have to be the same, so
it's easiest to use meters.  Notice that at 30 feet, f/2.8, a 50 mm
lens on a 35mm SLR, with a circle of confusion diameter of .025 mm,
near is 23'10" and far is 40'5".  Pretty close to 1:2 (or 1/3 : 2/3),
but go to f/4 and the proportions change to 1:3 (1/4 : 3/4).

- Ernie