Talk:Minolta STF 135mm f/2.8 T4.5

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"the only lens [natively] for Minolta AF without AF" -- I think I have seen a long Sigma tele offered natively for A mount (i.e. without using adaptor) but MF only. I just checked, maybe the mirror 600/8. Also, there is the 500/7.2, which is supposed to have AF, but I cannot imagine how the AF sensors will cope with 7.2. --88.74.189.22 (talk) 14:13, 20 May 2011 (UTC)
 * Yes, the STF is not the only MF-only A-mount lens. There was a whole range of Sigma Auto Program (AP) lenses and also the Cosina Voigtländer Macro Apo-Lanthar 2.5/125mm SL is not an AF design. Fixed that.
 * Being here already, I also removed the statement, that the STF lens would come without AF because the Minolta/Sony AF system would be "incompatible". This statement is wrong. Phase detection AF would be capable of focusing the lens, although a bit slower and perhaps with a bit more hunting than usual, although, I think, predictive focusing won't work very well with this lens, so that the camera would have to fall back to the older "scanning" method. Anyone can easily try it for himself by mounting the STF on a Minolta 7000 or 9000 equipped with the optional Minolta focusing screen 70/90PM (a center split prism screen with microprism collar). From long time personal experience with this combination I know, that there are no problems at all to focus the STF using the split prism focusing screen, in fact, it works like a charm; there are no black-outs of the split prisms (over the full manual range from T4.5 to T6.7 and then downto A9.5), and no softening of the inner edges of the image visible in the splitted area while the center portion of the image is nearly in focus (that is, when you use the split prism). If phase-detection AF, which is based on the same principle, would be principally incompatible with an apodization filter in the optical path, split prism focusing could not work either - but it does. Still, I see two possible reasons why Minolta might have chosen to make it a MF lens: 1) given the slow transmission aperture values reported to the camera, even a camera with special 2.8 sensors might never attempt to use them (unless it would special-case the STF and take this into account). This would cause a lens with a high effective geometrical aperture of 2.8 to be focused with the narrower measurement basis of the normal AF module, which is designed to work with lenses with effective apertures down to ca. 6.7. A narrower measurement basis means reduced focusing accuracy compared to the theoretically achievable optimum with the 2.8 module, however, it would not be different from using a camera, which does not feature any special 2.8 AF sensors at all. 2) I haven't noticed it so far, but perhaps the STF, as a matter of its optical design other than the apodization filter, has a significant focus shift when stopping down. Some lenses do, some don't, it would have nothing to do with the filter. However, if so, phase-detection AF, which always works at the largest aperture opening, might be off a bit when stopping down this lens using the lens' A-aperture. But even if so, this could not cause any problems while working in the lens' T-mode, where the aperture cannot be changed by the camera. All in all, both conditions also occur in conjunction with some other lenses, so they don't principally outrule the STF from being used with phase-detection AF. It was a design decision, but it is very well possible to design similar lenses with apodization filters, which would work with phase-detection AF, in the future. --Matthiaspaul (talk) 08:54, 10 August 2011 (UTC)