An imaging device may include single-photon avalanche diodes (SPADs). The single-photon avalanche diodes may be arranged in an array of microcells (such as a silicon photomultiplier). Each microcell may have an aspect ratio that is greater than 1. Each microcell may be covered by a microlens that also has an aspect ratio that is greater than 1. The microlens may have curvature in a first direction (parallel to the width of the microcell/microlens) and less curvature in a second direction that is orthogonal to the first direction (parallel to the length of the microcell/microlens). Forming non-square, rectangular microcells and microlenses in this fashion may allow for larger microcells that still have satisfactory microlens performance.