[Yakim]

[quote name='Klaus' date='03 June 2010 - 09:02 PM' timestamp='1275598966' post='157']

"Residual spherical aberrations" - the lens suffers from focus shifts when stopping down.

The focus information is obtained at max. aperture so it's slightly off at other settings. At close focus distances this is a relevant problem.



RSAs are not so uncommon in this class.

[/quote]



Apparently mine had focus shift even wide open. <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/sad.gif' class='bbc_emoticon' alt='' /> In fact, I sold it just because it was so annoying. Even calibration didn't work. <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/sad.gif' class='bbc_emoticon' alt='' />



Happy shooting,

Yakim.

[Klaus]

A focus shift at max. aperture isn't really possible by definition. In this case you've "just" a back-/front-focus problem.

That said it can be calibrated this way on purpose assuming that it hides the worst at smaller apertures.

[Guest]

[quote name='Klaus' date='07 June 2010 - 12:35 PM' timestamp='1275910547' post='304']

A focus shift at max. aperture isn't really possible by definition. In this case you've "just" a back-/front-focus problem.

That said it can be calibrated this way on purpose assuming that it hides the worst at smaller apertures.

[/quote]



Yes: I calibrated my Sigma to focus accurately at maximum aperture (which given that phase detect AF is effectively "seeing" f2.8 means deviating away from enutral) but the price, obviously, was worse performance stopped down.



But I also find that if I calibrate it to be good at about 2 metres, it's out of whack very badly indeed at infinity -- and vice versa.



So I now calibrate it for 1.5 metres at f 1.8 which is the useful environmental portrait range, where AF is useful with the thin DOF, and if I use it for general work I use manual focus.

[Guest]

[quote name='DavidBM' date='07 June 2010 - 02:21 PM' timestamp='1275916866' post='305']

Yes: I calibrated my Sigma to focus accurately at maximum aperture (which given that phase detect AF is effectively "seeing" f2.8 means deviating away from enutral) but the price, obviously, was worse performance stopped down.



But I also find that if I calibrate it to be good at about 2 metres, it's out of whack very badly indeed at infinity -- and vice versa.



So I now calibrate it for 1.5 metres at f 1.8 which is the useful environmental portrait range, where AF is useful with the thin DOF, and if I use it for general work I use manual focus.



I would love someone to make a 50mm 1.4 with good build, that is really decent wide open and stellar at middle apertures, and with accurate AF. Strangely no-one has done that. It's a pity that the Canon 1.2 is 1.2; I'm sure my brief could be filled by an L-prime with current technology if they were just a bit less ambitious about aperture. I'd pay 35 2.4 prices for that in a heartbeat (or five while I consult the domestic gods)

[/quote]

[Yakim]

[quote name='Klaus' date='07 June 2010 - 12:35 PM' timestamp='1275910547' post='304']

A focus shift at max. aperture isn't really possible by definition. In this case you've "just" a back-/front-focus problem.

That said it can be calibrated this way on purpose assuming that it hides the worst at smaller apertures.

[/quote]



Yes, I know that. It is just a figure of speech. <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/sad.gif' class='bbc_emoticon' alt='' /> But the fact remains that calibration was not successful for all distances and all AF points.



Happy shooting,

Yakim.