# looking for the formula

Discussion in 'Exposure Discussion' started by CollinB, Oct 25, 2011.

1. ### CollinBSubscriber

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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.

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3. ### Ian CMember

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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. ### CollinBSubscriber

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Thanks!

5. ### polyglotMember

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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.