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Autofocus with the FA 31mm and FA 43mm limiteds

Digby22
Posted 10/04/2011 - 22:47 Link
I am now the proud owner of the FA 31mm and FA 43mm limiteds (the latter very recently).

I have heard this mentioned before, but I have much more difficulty auto-focussing with these lenses than other lenses such as the DA 21mm and the DA 40mm.

Does anyone have any tips or workarounds to improve focus performance?

For instance, I have read that some people improve the FA 31mm flare performance by adding a stop down ring or filter rings as a makeshift hood. Would this also improve auto-focus?
Steve Chasey
Posted 10/04/2011 - 22:51 Link
Got both and neither given me any problems. Nor have I seen anything about either lens having AF issues.


Steve
In the Pack - Gripped K5 (SE),K7 & K20, Gripped MZ-S(SE)& MZ-S,DA10-17, DA12-24, DA14, DA*16-50, 50-135, 60-250 & 300mm; FA31mm/43mm/77mm Ltds; Sigma 8-16, 135-400 & 150-500
Half Backs: K10+BG,DA16-45, DA50-200
Backs: LXs,Super As and lots of A, M & K lenses
Impact Subs: 28mm Shift, K 135-600 (the Banahan of Pentax zooms ), 400-600 Reflex
Mannesty
Posted 10/04/2011 - 22:56 Link
Which body are you using them on?
Peter E Smith - flickr Photostream
Digby22
Posted 10/04/2011 - 22:58 Link
Thanks for the post. I should add that I am using a k20d (maybe the K5 is better) - and I am not limiting tips to equipment - technique also.

The green hexagon flickers a lot more with these lenses - so that I can't take the pictures (if in AF.S). This is especially true for quite close subjects (single portraits) in low light.

Steve Chasey wrote:
Got both and neither given me any problems. Nor have I seen anything about either lens having AF issues.


Steve

Hyram
Posted 11/04/2011 - 08:27 Link
I don't have any problems with the 31 on a K20D and use it a lot as a walk around lens.
Hyram

Bodies: K20D (2), K10D, Super A, ME Super, Auto 110 SLR, X70, Optio P70
Pentax Glass: DA* 300, DA* 60-250, DA* 50-135, DA* 16-50, DA 70 Ltd, FA 31 Ltd, DA 35 Ltd, DA 18-55 (2), DA 12-24, DA 10-17, M 200, A 35-70, M 40, M 28, Converter-A 2X-S, 1.4X-S, AF 1.7, Pentax-110 50, Pentax-110 24
Other Glass: Sigma 105 macro, Sigma-A APO 75-300
Flash: Metz 58 AF-1 P, Pentax AF160FC ringflash, Pentax AF280T
Mannesty
Posted 11/04/2011 - 08:42 Link
If you suspect a front/back focussing problem, you can adjust the body to suit up to 20 different lenses.
Peter E Smith - flickr Photostream
Digby22
Posted 11/04/2011 - 10:54 Link
I'll print out a FF/BF test sheet and look at it tonight.

Do you think it is possible that front/back focus problems could cause the green focus indicator to flicker?
Tyr
Posted 11/04/2011 - 12:25 Link
Digby22 wrote:
The green hexagon flickers a lot more with these lenses - so that I can't take the pictures (if in AF.S). This is especially true for quite close subjects (single portraits) in low light.

That will be because you are swaying backwards and forwards. The depth of field in the viewfinder and to the AF sensors will be very thin. If you sway a centimetre the camera may loose focus lock. Hold the camera steadier, steady yourself, brace yourself, use a tripod and control your breathing. Don't know if you've ever been shooting but you can steady your aim quite a bit by controlling your breathing.
Cayman
Posted 11/04/2011 - 13:16 Link
i have both those lenses and a K20D. no probs with AF although i did need to adjust for ff/bf with the 43mm.
Digby22
Posted 11/04/2011 - 14:37 Link
That's a good point. That amount of sway is quite possible. I'll also need to trigger inbetween heartbeats because my heart seems to make my body jump by about 1mm - perhaps enough to cause disturbance at these narrow depth-of-fields.

Tyr wrote:
Digby22 wrote:
The green hexagon flickers a lot more with these lenses - so that I can't take the pictures (if in AF.S). This is especially true for quite close subjects (single portraits) in low light.

That will be because you are swaying backwards and forwards. The depth of field in the viewfinder and to the AF sensors will be very thin. If you sway a centimetre the camera may loose focus lock. Hold the camera steadier, steady yourself, brace yourself, use a tripod and control your breathing. Don't know if you've ever been shooting but you can steady your aim quite a bit by controlling your breathing.

pentaxian450
Posted 11/04/2011 - 15:36 Link
Digby22 wrote:
Do you think it is possible that front/back focus problems could cause the green focus indicator to flicker?

No. Once the AF has locked on something, it won't try to "change it's mind"
Yves (another one of those crazy Canucks)
womble
Posted 11/04/2011 - 20:42 Link
Which focus mode are you using? Do you get the problem with AF-S or AF-C?

K.
Kris Lockyear
It is an illusion that photos are made with the camera… they are made with the eye, heart and head. Henri Cartier-Bresson
Lots of film bodies, a couple of digital ones, too many lenses (mainly older glass) and a Horseman LE 5x4.

My website
Digby22
Posted 12/04/2011 - 19:51 Link
Thanks for all your replies!

I have now checked the lens focus with a simple focus test chart and made an adjustment of -2. It was slightly front focussing.

It occurs to me that if I am taking a picture of a person (not an obedient model) at close range, then there would be combined swaying between me and the object - even if seated.

I tested the lens in the sun today and all seemed well - it took a little while to focus, but the af indicator didn't flicker so I wasn't prevented from taking shots. So either (a) good light, or (b) a still subject sorted this out.

The af problem I am sensing may be just the swaying combined with the FA 43mm not focussing as fast as my DA 40, which I understand is a fast lens.

So the new request is: any tips on slightly moving subjects at close range in low light?

I realise this may be some kind of holy grail of focussing...
Digby22
Posted 12/04/2011 - 21:14 Link
AF-S. It's the shots missed because focus won't lock that annoy me most. Obviously AF-C will help, but my guess is that I would end up with lots of oof shots.

womble wrote:
Which focus mode are you using? Do you get the problem with AF-S or AF-C?

K.

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