Minolta 50/1.4 AF problems on Alpha 350

[mdcromer]

have a devil of a time with AF on this lens. My other lenses AF much more reliably. It will hunt even outdoors in good light with cats and other such. This is not an issue of FF or BF -- it is an issue of not locking focus (I have tweaked my AF sensor distance which is now pretty accurate on all my lenses, even this one, on a test target, which is about the only thing this lens will reliably AF on).

This is on an Alpha 350 with center AF point (also other AF points).

I am about to get an Alpha 550 from FedEx and it will be interesting to see if the AF works any better. I also need to try this lens on an Alpha 700 and/or 850/900.

Worst case I will need to sell and get a new Sony or Sigma 50/1.4 because the AF performance right now is just MAJOR SUCK! Other than the AF, I like the lens and it is plenty sharp enough when the focus is correct!

[David Kilpatrick]

Could just be that the lens contrast is not proving enough for a quick lock on. I now have a 50mm f/1.7 and a 50mm f/1.4, and also the new 30mm f/2.8 - speed and accuracy of focusing favours the smaller aperture f/1.7 and 30mm, not the 1.4

David

[mdcromer]

I'm inclined to agree with you David -- the 50/1.4 is a low contrast lens for sure, and it will lock if the target is good enough.

It will be interesting to see how the 550 does. . .

[bfitzgerald]

No problems here good light f.17 wise. The A200 has AF accuracy issues with it low light though or rather the central AF point appears to cover a larger area than other models I own I can get there I just have to keep an eye on things with it. Curiously the Km5d is almost deadly accurate AF wise with this lens even in very low light. So is the 5, 60 and of course film 7.

I can understand problems low light but not really in good light

[redsim74]

This is an interesting thread.

I have huge problems with focus accuracy (back focus) using the SAL50/1.4 on my A700 - to the point I don't use the lens - but my old KM5D does far better with it, pretty much bang on.

I'd thought it was a problem with the lens but the results on the 5D seem to exclude this. My other lenses don't appear to have any significant focus issues, though I have nothing else faster than f/2.8.

[David Kilpatrick]

The reason is that the 50mm f/1.4 has a really strong aperture-related focusing shift. On the old 5D, there is no f/2.8 centre sensor, so you are focusing in effect around a true aperture of f/5.6 to f/7.1 - it depends on lens geometry. If you shoot around f/5.6 to f/8, the AF focus should match the real focus. But if you shoot at f/2.8 or f/1.4 it will shift. It may just happen to shift on the 5D to a degree that matches BF/FF error. On the A700, this lenses focuses at f/2.8 on the centre sensor only, and should be dead accurate at f/2.8 but show some BF/FF shift as you open to f/1.4 or close to f/8.

The new Sigma 50mm f/1.4 is designed to have minimal aperture-related focus shift; most older lenses were not, and the true point of focus moves a lot as you go from f/1.4 to around f/5.6 (after that it does not tend to change much).

David

[redsim74]

Thanks for the explanation, David.

I'd normally have to stop down to maybe f/5.6 with the A700 to get acceptable focus. I'll try to do some tests around f/2.8 to try to understand what's happening with the shift.

I take it the effect would be the same on the FF cameras? I'm toying with the idea of picking up an A850 where the focal length is perhaps better suited.

[David Kilpatrick]

Yes, the effect is the same on full frame - but you can use the micro AF adjust to suit your most typical way of using the lens - i.e. adjust it for say f/2 best focus, so you can use it as a fast lens. Or even f/1.4 though that may be a bit soft.

David

[redsim74]

...but you can use the micro AF adjust to suit your most typical way of using the lens ...

Ah, of course. I hadn't considered that.

Many thanks!


Sim

[Wes Gibbon]

Does this problem apply to all versions (Sony & Minolta) versions of this lens?

[mdcromer]

The 550 seems to focus better with this lens (although certainly far from perfectly). I'm tempted to pick up a 50/1.7 and see if I have better luck with it. I'd rather not get the new 50/1.8 despite David's praise for this lens because I plan to go FF as soon as Sony offers a FF version of the Alpha 550.

P.S. the 550 is an INSANE fun camera!

[David Kilpatrick]

I have not praised the 50mm f/1.8 - not yet used it, the 30mm f/2.8 macro is the one I like.

David

[mdcromer]

Sorry David for the mixup. . .