I finally decided to play the Sigma lottery. The first unit was clearly a returned one and had front-focus issue. The second copy i was sent seems OK but i just never seem to get a sharp enough shot at below 2.8.
Ah, so its the old Sigma issue - front/back-focusing going on here. Thanks everyone, i will try to microadjust but i am ready to return it and maybe wait for Sigma 50 Art series?
Giles, i agree 1.4 will be soft - but to miss the focus totally is another thing. My sigma seems to be focusing close to the focus point now after +20 MA. Check out the results:
Your first pic viewed at large 2048 shows front-focusing as well - you can see the hairs closest to the lens are somewhat getting in focus while rest is out of depth of field. I wanted the lens to focus on the eye if i put my focus point on the eye, not on the nose and that is the issue with this lens.
Also yes you are right about 24-105/4L as not really a kit lens, hehe. Its amusing to call it so - its impressive for a zoom. So is the 70-200/4 (i have non-IS).
I was a bit scared of the reported "front focusing" issues, despite the other wonderful features of this lens, so that I've performed quite a bunch of test shots to understand what happens at wide ( 1.4-1.8 ) apertures.
My statement, is that most of the reported "front focus" issues are wrong.
I'm not saying that this problem does not exist, but that is applicable to only a minor share of the lens.
Indeed, the real issues that likely many of us are experiencing are the combination of the "shallowness" of the focus plane and the sharpness behavior of the lens at wide apertures; as reported in some reviews, as in "dpreview" if I'm not wrong , the lens is softer in the center than in other areas (moving radially from the center).
I suggest, when verifying the "front-focus issue", to secure the focus plane is parallel to the subjects and ensure that subjects have enough contrast. Done that, if you get "in focus" areas at a different distance from where you expected (once again do not confuse focus with sharpness), then you most likely have a focus issue.
Why am I writing this? For a simple reason. Let's imagine that you shot, as i did, with the subjects not parallel to the "narrow" focus plane and you try to focus in the center of the lens (usually this is the case); then you look at the images and apparently the center will look in focus so-so, not so bad....but there is another area not too far from the center that "has better focus" ...is it? Actually is not properly the case; most likely the "better focused" area lays on the same focus plane of the center but has better sharpness due to the lens softness issue (at wide aperture), and the final result is that apparently the lens has focused a closer (or more far) subject.
In practical terms, the lens has to be used with care at wide apertures, to avoid weird results and may be problematic for usage in "macro" shots at wide apertures.
Apart from this (and some issues with purple fringing in particular light situation), the lens could provide outstanding results in most situations.
Hope it helps.
Posted 27 months ago. (permalink)
i was incorrectly testing on an angle, i tested at your settings 1.4 1/40
The other thing is that phase detect autofocus, while fast, isn't totally consistently accurate, and at 1.4, the DOF is so thin that the autofocus might just be missing. The LensRentals guy wrote a blog post about this. They were testing a Canon 50 1.4 and 1.2 with a 5D Mk II, and they found that 60% of the time it nailed the focus, 20% of the time it missed focus but the target was still acceptably sharp, and 20% of the time it completely missed focus. He definitely recommended using micro adjust/auto fine tune to correct consistent front/back focus issues, but said that phase detect autofocus is going to miss sometimes. It's built for speed, not accuracy.
The other thing is that the focus sensors are actually quite a bit bigger than the little rectangles you see on your focus screen. If the target (the eye, in this case) just barely fits the little rectangle, the camera probably doesn't have quite enough info to focus with, which will increase the chance of it missing.
