Choice of Aperture / Depth of Field

Just as with color photography, most lenses require stopping down 2-3 stops to achieve optimal resolution and contrast, especially in the corners. Stopping down has the further advantage of mitigating any focus error and negating the effects of curvature of field, a lens characteristic which is often overlooked (center/corners do not focus in a plane). For 3D (non-planar) subjects, depth of field is of course useful for increasing sharpness front-to-back.

Depth of field behaves the same way in infrared as it does with visible light, except that (in theory) the diffraction limit is reached sooner due to the longer wavelength of infrared light, approximately 800 nanometers, as compared with 400 nanometers for deep blue light. In theory, that means a diffraction limit of up to 2 stops less. In practice, the optical performance of many lenses in infrared is such that diffraction can safely be ignored to f/11, and even f/16 in many cases (on the Canon EOS 5D). Don’t bother reading the technical mumbo-jumbo—it might be technically correct, but test your lenses and trust your own eyes!

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