# Focus points - who cares? (honest question)

Started Sep 18, 2012 | Discussions thread
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Underlying math:

Focus Recomposing.. really its easier and simpler than noodling w/ AF points manually, hoping your camera magically knows what to focus on or dealing w/ crappy "edge" points that suck .. these are for full frame. (really wishing the EOS3 existed in digital.. but still focus recomposing does basically work)

how its calculated:

Inputs:

fl=focallength in mm

ap = aperture (1.4,1.8 blah blah)

coc = 0.029 is the circle of confusion value for 35mm film stolen from wiki (some manufacturers may use 0.03 when they calculate these things.. but i used wiki) :: This i believe is a function of print size actually.

s = distance from subject in meters

precision = 1/3; (this is how close to the center of the DOF the AF point gets.. its 1/3 for the "high precision" points for a canon)

Calculated bits:

H = (fl*10^-3)^2/(ap*coc*10^-3)
dofcorrected = (-H*s/(H + s) + H*s/(H - s)) * (1-precision) / 2

and finally the angle at which you can tilt your camera and remain in focus is simply:

ArcCos[s/(s + dofcorrected)]

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