# looking for the formula

• 10-25-2011, 10:43 AM
CollinB
looking for the formula
When I look on any lens or camera there is a focus scale that is logarithmic.
How is that scale scaled -- how is it arrived at for each focal length?
What I am wanting to do is consolidate all of my LF formulas into a single application --
lens f/l + focus distance + exposure compensation.
It will make a useful Android app.
• 10-25-2011, 10:59 AM
Ian C
We discussed this a few days ago. Exposure compensation Δf for a lens extended beyond its infinity position can be reckoned in terms of focal length f and lens-to-subject distance s.

http://www.apug.org/forums/forum44/9...r-formula.html
• 10-25-2011, 11:13 AM
Ian C
If you’re interested in the distance the lens must be extended Δ forward its infinity position to focus at a lens-to-subject distance s,

Δ = sf/(s - f) – f

Example: f = 90mm, s = 3 meters = 3000mm

Δ = 3000*90/(3000 – 90) – 90 = 2.78mm

You’d extend the lens 2.78mm forward of its infinity position to focus lens-to-subject distance s = 3 meters.
• 10-25-2011, 08:30 PM
CollinB
Thanks!
• 10-25-2011, 10:36 PM
polyglot
Keep in mind that that formula applies only to lenses that are not telephoto or retrofocus, in which cases different extensions are required to achieve a given focus. Fine for most LF lenses though.