Borut Furnlan sets out to answer the following question:  “… what f-stop should I use at underwater close-up photography with a reproduction ratio of 1:1 (life size) to obtain the best compromise between depth of field and the list drop in sharpness due to the diffraction of light on aperture blades?” Borut also answers the following question: do objects underwater look 1.33 times bigger or 1.33 times closer? [Thanks, James]

Borut uses film, but the test is still an interesting read for digital guys (digital systems have different sensor sizes and pixel densities, which affect circle-of-confusion calculations).