OK it seems a little quiet on here, so here I go with my first topic!
I've noticed in some lens tests the issue of residual spherical aberrations (focus shifts when stopping down). However I've never seen it mentioned in a Pentax lens review/test, is this something that the Pentax lenses are free from, or is it just the case that the issue was not mentioned?
Regards,
John
[quote name='johnwhit' date='06 June 2010 - 03:33 PM' timestamp='1275831195' post='271']
OK it seems a little quiet on here, so here I go with my first topic!
I've noticed in some lens tests the issue of residual spherical aberrations (focus shifts when stopping down). However I've never seen it mentioned in a Pentax lens review/test, is this something that the Pentax lenses are free from, or is it just the case that the issue was not mentioned?
Regards,
John
[/quote]
Don't bet on this. Many lenses show RSAs. The primary question is whether it's field relevant.
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 name='Klaus' date='06 June 2010 - 03:39 PM' timestamp='1275838747' post='274']
Don't bet on this. Many lenses show RSAs. The primary question is whether it's field relevant.
[/quote]
Just curious really, I'd never seen it referred to in a Pentax test. I tend to concentrate on taking photos these days and try not to become too involved on the technical matters. I still enjoy reading the tests and the PZ tests are very thorough, keep up the good work.
Best regards,
John
[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