Noise propagation in iterative reconstruction can be reduced by exact data projection. This can be done by area-weighted projection using the convolution method. Large arrays have to be convolved in order to achieve satisfactory image quality. Two procedures are described which improve the convolution method used so far. Variable binning helps to reduce the size of the convolution arrays without loss of image quality. Computation time is further reduced by abbreviated convolution. The effects of the procedures are illustrated by means of phantom measurements.