06-06-2010, 03:49 PM
John,
As Klaus says. RSAs don't necessarily mean focus shift. It may contribute to focus shift, or rather focus shift in a field relevant way, but this also depends on the way it is handled in a lens design, or even the electronics of a lens.
F.e., little known fact is that the Canon 85L also has focus shift. To a degree the floating elements design (all but 1 element, the rear one, float, i.e., move when focusing), takes care of that, IOW, lens design, but calibration of the lens also does, and that is for a large part electronics.
RSAs often provide a lens with very good bokeh, or at least bokeh that is considered pleasant by many. That is no exception with the 85L... <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='
' />
With the 85L, at least with my own copy, it shows focus shift between F/1.4 and F/2.5, inclusive, of a maximum 1/4 DoF at F/2, close to MFD. Since this is well within DoF, and well within specs of the AF system, this is not a field relevant problem.
Kind regards, Wim
As Klaus says. RSAs don't necessarily mean focus shift. It may contribute to focus shift, or rather focus shift in a field relevant way, but this also depends on the way it is handled in a lens design, or even the electronics of a lens.
F.e., little known fact is that the Canon 85L also has focus shift. To a degree the floating elements design (all but 1 element, the rear one, float, i.e., move when focusing), takes care of that, IOW, lens design, but calibration of the lens also does, and that is for a large part electronics.
RSAs often provide a lens with very good bokeh, or at least bokeh that is considered pleasant by many. That is no exception with the 85L... <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='
' />With the 85L, at least with my own copy, it shows focus shift between F/1.4 and F/2.5, inclusive, of a maximum 1/4 DoF at F/2, close to MFD. Since this is well within DoF, and well within specs of the AF system, this is not a field relevant problem.
Kind regards, Wim
Gear: Canon EOS R with 3 primes and 2 zooms, 4 EF-R adapters, Canon EOS 5 (analog), 9 Canon EF primes, a lone Canon EF zoom, 2 extenders, 2 converters, tubes; Olympus OM-D 1 Mk II & Pen F with 12 primes, 6 zooms, and 3 Metabones EF-MFT adapters ....