It's not the focal length but the design. Compare the F/8 90mm speced at F/22 to the F/4.5 90mm speced at F/16. Supposedly Nikon is claiming with the faster lenses you don't need to stop down to F/22.
It's not the focal length but the design. Compare the F/8 90mm speced at F/22 to the F/4.5 90mm speced at F/16. Supposedly Nikon is claiming with the faster lenses you don't need to stop down to F/22.
"why the shorter lenses would be best at F16 and the longer at F22"
diffraction
Glenn - my comment was intended as a joke, hence the "winkie".
Trade-offs in design. Note that the ideal in any case is a lens that is diffraction limited wide open but these would presumably be very expensive to design and manufacture. For taking lenses, the lens designer makes the reasonable assumption that a longer focal length will be stopped down a little more to compensate for the lower DOF from the longer focal length and therefore tries to optimize (make the lens diffraction limited) at that f-stop. That presumably frees up degrees of freedom. Thus, an effort to optimize shorter lenses at larger apertures and longer lenses at slightly smaller apertures. You could try to make a lens of any focal length diffraction limited at larger apertures but it can become expensive - the f/4.5 90mm (optimised at f/16) is bound to be more expensive than the f/8 90mm (optimised at f/22) - and if the work you are doing utilizes smaller apertures, both lenses will provide similar performance (i.e., the cheaper one will perform as well as the more expensive one). This can be contrasted to the situation for enlarging lenses where the lenses need to be diffraction limited as close to wide open as possible (to keep printing times short and reduce diffraction loss). Good quality enlarging lenses will typically be fast (large wide-open apertures) and will be diffraction limited wide-open or almost wide open. Cheers, DJ
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