[Designosophy]

I currently use manual focus lenses about as much as autofocus, and until recently, I used manual focus lenses more often than autofocus. I've had some front/back focus problems here and there - I have one lens that back focuses at is longest focal length, but not at its shortest length. Impossible to fix. Having just acquired a Sigma 30mm f/1.4 yesterday, I felt like it was front focusing a smidge, and I was able to correct it with a +3 adjustment. However, I was playing around with the lens in fairly low light and I found that it was front focusing like crazy. I was using the uppermost leftmost AF point, and on a lark, I decided to try the other AF points. Aside from the two outer, non-cross type AF points, all the others delivered spot-on focus. But that one point consistently front focuses. I'm going to try it again in bright light to see if the problem is limited to low light.

My question: Has anyone else seen this problem before, and if so, what are your thoughts on it?

[kenyee]

Cross points are always more accurate in low light. What conditions were you focusing in?

[JinDesu]

So to summarize, if I am reading this right:

Outer non-cross type AF points - not spot on focus
Most cross type AF points - spot on focus
Center cross type AF point - front focus

Is that a correct assessment? Sorry, it was a little bit confusing reading it in that paragraph.

[Designosophy]

Outer, single-directional AF points: Off badly, consistently in low light, Pretty bad in bright light too, based on a little testing at lunch today.
All inner cross-type AF points except the uppermost leftmost: spot on
Uppermost/leftmost cross-type AF point: front focus in low light

[GDRoth]

Boy, will this be a dumb question;

Why not focus using the center spot, then move camera if you wish to shift comp?

[wlachan]

Not surprising since the AF sensor is adjusted and held by 3 screws. AF isn't accurate because the sensor was loosely adjusted in the factory, then compensate through software. Since the optical path isn't aligned to begin with, each of the 11 points (not really points btw) will behave differently with different lenses. A QC disaster.

[Designosophy]

Boy, will this be a dumb question;

Why not focus using the center spot, then move camera if you wish to shift comp?

Not a dumb question, and it is a workaround. I just find that often, when working with a narrow depth of field, the simple act of recomposing after focusing can result in a misplaced plane of focus.

Not surprising since the AF sensor is adjusted and held by 3 screws. AF isn't accurate because the sensor was loosely adjusted in the factory, then compensate through software. Since the optical path isn't aligned to begin with, each of the 11 points (not really points btw) will behave differently with different lenses. A QC disaster.

Interesting. Perhaps someday I'll have to send the camera out for adjustment. But I'm not desperate - I'm just curious at this point. I'm also going to try some other lenses to see if the behavior is similar.

[GDRoth]

thanks

[JinDesu]

Alright, proper dumb question here then.

We can send in a camera for AF adjustment? Who do we send it to, is it warranty covered, and how good of a result should one expect from the adjustment? I see some people say that their cameras have been adjusted and they focus razer sharp now, etc. Never got to ask before.

[Anvh]

I would say that the sigma 30mm has a very curved field of dof lik most f/1.4 and faster lenses have, but i'm now just stating the obvious here

[Designosophy]

Alright, proper dumb question here then.

We can send in a camera for AF adjustment? Who do we send it to, is it warranty covered, and how good of a result should one expect from the adjustment? I see some people say that their cameras have been adjusted and they focus razer sharp now, etc. Never got to ask before.

I don't know the details. My camera will not be under warranty because I bought it used, so that's a moot point to me. I still need to try some other lenses. Another day...

[sterretje]

Is, what you have focused on (e.g. left top), in focus? If so, it's the lens design as indicated by Anvh.

[jk2011]

hello,
i've got a k5 7 sigma 30 f1.4 as well.
sometimes it does feel a bit soft as well. again, i think you need to really test the accuracy. ie
1. is on a tripod when you took the shot
2. is the camera set on spot focus or continous
3. how wide was the aperture
4. were you focusing on the same spot

i always try to blame to camera/ lens myself but in most case it's not.

all i can say, bolt on a sturdy tripod, lock it on one spot & test to you heart's content

[westmill]

My guess is that its just the lens ! The 30mm sigma is simply a poor lens. Centre quality Is very high indeed but the edges are very lacking.
The lens needs to be stopped down to 5.6 to 8 before the edges just about reach very good levels. So if your looking at anywhere other than the centre
it could be poor indeed.

[dlacouture]

My 30/1.4 has the same behavior... The outer points are simply useless.
Other lenses are perfectly fine...