Originally Posted by Keith B
No, this is not quite accurate; Nikon has made a 50/1.2, 55/1.2, and 58/1.2, all manual focus lenses in variations on the F-mount. However, they have not made any such lenses with AF capability.
Strictly speaking, the f-number relates only to the focal length (at infinity focus) divided by the diameter of the entrance pupil; the flange diameter does not directly relate to this ratio. Otherwise, one would not be able to construct lenses like a 300/2.8, in which the entrance pupil has a diameter of approximately 107mm, much larger than any 35mm format lens mount. A telephoto lens has a pupil magnification ratio of less than 1 (i.e. exit pupil dia. / entrance pupil dia. < 1), and it is this asymmetry that relates to the light-gathering ability of the lens. Hence a small mount diameter does indirectly impose constraints on the optical formula, in the sense that it requires asymmetric construction of fast lenses for shorter focal lengths. This is part of the reason for Canon's decision to design the EF mount with a fairly large diameter.
In short, while it is not absolutely necessary to make the mount especially large, it does have the effect of expanding the possible design parameters of lenses for the system.
Something else that must be taken into consideration when designing large-aperture lenses with AF is that the design is further constrained by the requirements of the AF motor in two important ways:
- The AF motor and supporting electronics require additional space inside the barrel.
- The focusing group must be designed to permit efficient operation with the AF design.
The first point should be fairly obvious, but the second is more subtle. What I mean by this is that an AF lens must be designed in such a way that the focusing group's travel cannot be very long or its mass too heavy. Manual focus lenses often have focusing rings that turn well over 200 degrees from MFD to infinity, and they are geared in a way that permits very fine control of focus via the ring since this is the only way to focus the lens. An AF lens cannot afford this luxury as it compromises the AF speed, as does a design in which the focusing group is made of many elements or heavy elements. An example of such a lens is the EF 85/1.2L II USM, which is a front-focusing design that uses Canon's most powerful USM motor to move well over a pound of solid glass with pinpoint accuracy. It is the only EF lens, other than the supertelephotos, to use the 77mm ring USM.
This is why I often feel it is unfair to compare AF lenses against MF lenses in terms of "feel." They necessarily feel different because they are constrained by different requirements--the former has to have a short focusing helical to be "snappy," and the latter has to have a long one to facilitate accurate focus.
In light of these considerations, I believe that the real reason why Nikon has not designed a f/1.2 AF lens has less to do with the mount diameter than it has to do with their relative lateness in developing AF motor technology. This conjecture is supported by the fact that there still remain relatively few AF-S FX lenses in the Nikon lineup.
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