Originally posted by stevebrot I have puzzled over this and wonder if it has to do with field curvature. Dedicated macro and bellows lenses are designed to have very flat fields.Steve
I will make no claim to have any but the most elementary understanding of optics, the "why" of optical phenomena, but I do know some actualities. Lens formulae have an ideal ratio of subject-to-lens
vs lens-to-image-plane distances. This is most obvious in deep macro lenses, those designed for reproduction ratios above life size. There is an optimum magnification for these,. At either lesser or greater magnifications overall IQ declines. They are totally unsuitable for use at infinity, or even ordinary close-up range such as 1/5 life size. It has been common knowledge that when photographing something at greater than life-size with a regular camera lens, it's worth trying the lens in reverse position to improve IQ. This is because of the subject-to-lens versus lens-to-image-plane ratio. At greater than life size the lens is acting like a projector lens. The Tessar four-element lens is surely one of the great basic lens designs. At one time some manufacturers offered "macro-Tessar" lenses in which, you guessed it, the sequence of Tessar elements is reversed.
Flat field is generally desirable in ordinary camera lenses just in case there is something at the edge or in the corner of an image that should be sharp. Flat fields in macro lenses, those used in the range of perhaps 1/10th life size to life size, was originally regarded as desirable because these lenses were once commonly used for copying documents or photographs or paintings. The 4-legged copy stand devices that threaded into a filter thread is evidence of this as such a device was not intended for photographing insects, flowers and the like, what most of us use our macros to do. When photographing flowers or insects close-up, the subject is not "flat field." In fact, the periphery of the image field for such subjects is commonly background where bokeh is far more important than flat-field sharpness, and fortunately, most macro lenses have excellent bokeh.
I am sometimes surprised that acutance has dropped out of lens evaluation. It probably has a much greater impact on the impression of IQ than field curvature.