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# Thread: looking for the formula

1. ## 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.

2. 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

3. 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.

4. Thanks!

5. 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.

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