front focusing? need help troubleshooting.

[rice1]

<p>hi all:<br>

i am helping a friend troubleshoot what seems to be a front focusing sigma lens. canon body is a 7d, lens is a 120-300 f2.8 sigma. in AF mode, AV priority, all photos are blurry. in each photo there seems to be a focused area in the front of the subject. this also seems to be worse at longer zoom lengths. see pic below for sample. would like to hear opinions. thank you.</p><div></div>

[rice1]

<p>another example...</p><div></div>

[scott_ferris]

<p>Yep front focus. I believe the 7D can store information for microauto focus adjustment for some third party lenses, if the Sigma is one of them I would<a href="http://www.northlight-images.co.uk/article_pages/cameras/1ds3_af_micoadjustment.html"> do this setup.</a> It is far and away the most accurate way to set up micro AF and it is free!</p>

[brett_w.]

<p>hard to tell with the small pics, I think it would be better to do a focus test on a stationary subject first then if it checks out try moving subjects</p>

<p> </p>

[jto1]

<p>It does indeed look like serious front focusing, but you should check the shooting technique and ensure that a focus point wasn't active and hitting the ground in front of the soccer player. That would easily cause this error (I've done it myself by accident at my son's soccer game). So, be sure you're testing with only the center focus point.</p>

<p>As suggested above, test with a stationary subject.</p>

<p>Also as suggested above, that link for testing is by far the best one out there for micro-adjusting a lens, but it takes some time and effort. You might also screw up the focus somewhere else in the zoom range, so it's a touchy subject.</p>

<p>Good luck!</p>

<p>John (<a href="http://www.johnolszewski.com">http://www.johnolszewski.com</a>)</p>

[craigd]

<p>Not to be a smartass, but one way I've found to avoid front-focus/back-focus problems is simply not to buy Sigma lenses. I often read reviews of Sigma lenses where the reviewer complains of front- or back-focus, or had to go through three copies of the lens to get one that worked well enough to review at all. This doesn't seem to be an issue with the other top lens manufacturers (specifically Tamron and Tokina, in addition to the camera makers themselves), at least not to anything like the same degree. Sigma also seems to have more compatibility issues (having to get lenses re-chipped to work with newer cameras) than the other major AF lens makers. There is a fair amount of history in this area by now, and the evidence seems pretty convincing to me that Sigma's manufacturing quality control is inadequate, and their reverse engineering of the EF mount's electronic signaling is rather poor.</p>

[steve_dunn2]

<p>First off, before suspecting a defective lens and/or camera (either of which is possible), let's simplify. Manually select one focus point, put it on a stationary target at a distance (and with the lens set to a focal length) similar to the situation in which you're seeing the focusing problem shown above, and see if focus works properly. If not, then yes, you have a focus problem which can probably be cured using microadjustment.</p>

 

<p>But if this test works, then either it's a problem with tracking the moving subject, or with focusing on the wrong thing (the ground in front of the subject). I don't know how quick the AF system on that lens is so I don't know if the lens is having trouble adjusting quickly enough; I suspect that's probably not the case. The AF system on the 7D is more than up to the task of tracking a soccer player, particularly if it's set to AI Servo mode (as opposed to AI Focus, in which it basically starts off assuming the subject is stationary and only switches to servo once it's noticed that the subject is moving).</p>

 

<p>My guess is that somehow the ground is being chosen as the subject. Perhaps zone AF or automatic AF point selection is being used?</p>

 

<p>P.S. according to the EXIF data, both pictures were shot with a 1D III, not a 7D. Maybe the editing software is messing that up.</p>

[arie_vandervelden1]

<p><em>Not to be a smartass, but one way I've found to avoid front-focus/back-focus problems is simply not to buy Sigma lenses.</em></p>

<p>That doesn't work. My Canon 17-55/2.8 front-focuses badly. So the only way to avoid front/back focus issues is to not buy any lenses at all.</p>

<p>Anyways: to the OP. Try this. Set up the camera and lens on a tripod and point at a distant stationary subject. Turn on live-view with max magnification and set live-view focus mode to quick-focus (mirror drops down and regular AF module is used). Now, defocus the lens and then half-press the shutter button to autofocus. Look at your live-view display and see if you can improve on autofocus my manually twisting the focus ring. Repeat the above ten times. If there is consistent front-focus then dial in a AF-microadjustment. Try the above again ten times, and repeat until AF gets it right.</p>

<p> </p>

[jto1]

<p>I agree with what Arie said, and also using the chart mentioned above to check this. It's the easiest way to see what's really going on and to eliminate shooting error.</p>

<p>But be careful...As mentioned above, an AF-microadjustment change to one focal length at one shooting distance on a zoom lens might cause problems elsewhere in the zoom range. I'm not saying it WILL, but it COULD....so just watch out for it after you make any adjustments before you use the lens for anything important.</p>

<p>John</p>

 

[rice1]

<p>thanks all, i will pass the info on to my friend. i was hoping for a more "beginner level" answer for him since he is new to sports photography. we will do our best. thx again.</p>

[rice1]

<p>steve dunn, you were correct, the pics posted were from my 1DIII because i tested his lens on my body. his 7D was doing the same thing. this led me to believe it was the lens.....we hope (easier to return or fix).<br>

any other thoughts?</p>