06-06-2010, 08:02 PM
[quote name='wim' date='06 June 2010 - 03:49 PM' timestamp='1275839398' post='276']
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
[/quote]
I can see how that could be desirable in a portrait lens such as the 85L, assuming you use it for portraiture.
Kind regards,
John
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
[/quote]
I can see how that could be desirable in a portrait lens such as the 85L, assuming you use it for portraiture.
Kind regards,
John