How are enlarger lens f/stops calculated? On cameras, lens f/stops are calculated for the focal length that results when the lens is focused on infinity. For any closer focus, in theory the marked f/stops are wrong, and you must calculate a 'bellows factor' to find out the real f/stop. Since enlargers are NEVER infinity focused, are the marekd f/stops always 'wrong'? Or are enlarger lens f/stops calculated at some intermediate focusing distance? Since enlarger exposures are based on trial-and-error, it's mostly a point of curiosity; it wouldn't matter what the markings were really. But if you used an enlarger lens on a camera, would the f/stops be accurate at infinity? But, this bellow-factor line of thought brings up another issue. Generally, when I change print size, I scale the exposure time proportional to the change in area of the print--the same thing as measuring the change in lens height and applying the inverse square law; I just find calculating the change in print area easier. BUT...either method requires you to refocus the enlarger at the new magnification, so in theory the aperture at the new print size is different than that at the old size. A full treatment would require me to 1. Calculate the change in print area (or equivalently, measure the change in lens height and apply the inverse-square law) 2. Measure the before-and-after bellows extension, and adjust my lens so that my aperture is the same at both heights. I had never considered the aperture 'problem' until I got a view camera, but when making 4x5 enlargements to 8x10 prints, bellows factor on the enlarger must be quite significant.