On Sat, 20 Aug 2011 16:53:29 -0700 (PDT), Ron Cuaz
wrote:

Supposed I want to know how big a portion of object a certain distance
away would appear in a pixel in a camera.. with and without lens
magnification, what exact formula do you use?

Supposed the ccd chip resolution is 120x120 with pixel pitch of 25
micron.

And the focal length of the lens is 6.76 millimeters, with field of
view of 25 degrees and F-number of 1.5 with aperture of 4.5
millimeters.

Pixel scale is commonly calculated using

A = 206265 * d / F

where A is the angle per pixel is arcseconds
d is the pixels size (or pitch)
F is the focal length of the lens
(206265 is the number of arcseconds in a radian)

In your example,

A = 206265 * .025mm / 6.76mm = 762 arcsec/pixel = 12.7 arcmin/pixel

(which isn't a very realistic equipment combination for most imagers).