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residual spherical aberrations (focus shifts when stopping down)
#1
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
#2
[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.
#3
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='Tongue' />



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 ....
#4
[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
#5
[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='Tongue' />



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
  


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