Method and system for detecting whether an optical system is in focus using an improved interpolation technique

Claim 1

A focus detection system in which light from a object is incident on first and second photosensor arrays via an optical system to form a first image on the first array and a second image on the second array to obtain a relative displacement amount X between the first image with respect to the first array and the second image with respect to the second array, so as to determine whether said optical system is in focus or not, each array generating a respective pattern of signals in response to light falling on the photosensors of the respective array, said system comprising: means for storing a function f(i) expressing inconsistency between the pattern of signals from the first array and the pattern of signals from the second array when the patterns are shifted with respect to one another by a shift amount i and compared with each other, the shift amount i being an integer which represents the difference between a photosensor position in the first array and a photosensor position in the second array; means for obtaining function values f(i 0 ), f(i 0 -1), f(i 0 -2), f(i 0 +1) and f(i 0 +2) corresponding to i 0 , (i 0 -1), (i 0 -2), (i 0 +1) and (i 0 +2), were i 0 is the value of the shift amount i when f(i) is at a minimum; means for computing an amount I =X/p (p =pitch of the arrayed photosensors in the photosensor arrays) from said function values, said amount I being given by formulas (1) and (2) below: I=i.sub.0 +1/2-{f(i.sub.0 +1)-f(i.sub.0)}/A (1) where A=2{B 1 +K 1 (B.sub. -B 2 )} B 1 =f(i 0 -1)-f(i 0 ) B 2 =f(i 0 +2)-f(i 0 +1) k 1 =a constant between 0 and 0.5 I=i.sub.0 -1/2+{f(i.sub.0 -1)-f(i.sub.0)}/C (2) where C=2{D 1 +K 1 (D 1 -D 2 )} D 1 =f(i 0 +1)-f(i 0 ) D 2 =f(i 0 -2)-f(i 0 1) K 1 =a constant between 0 and 0.5 means for comparing said function value f(i 0 -1) with said function value f(i 0 +1) to judge whether (a) f(i 0 -1)>f(i 0 +1), (b) f(i 0 -1)=f(i 0 +1), or (c) f(i 0 -1)<f(i 0 +1); and means for obtaining said amount I given by said formula (1) in the case of (a), or given by I=i 0 in the case of (b), or given by said formula (2) in the case of (c).