It has been shown that excessive error may be introduced in small angle x‐ray scattering by the use of an incident beam of finite cross section. To account for this error the usual procedure has required multiple integrations for rays reaching a single pointPon the film from all points within each slit. However, the common effect from both finite slit height and width is a range of scattering angle for rays reachingP. The integrations over each slit dimension are reduced to a single integration over scattering angle, which entails finding the limits of scattering angle.The assumption of a constant distribution of intensity throughout the range allows the range of scattering angle &dgr;&egr; to be taken as a measure of the collimation error. Minimization of &dgr;&egr; permits determination of optimum values of slit height‐width ratio. For two slit collimation, it further leads to determination of optimum slit spacing and optimum relative slit sizes.