Please help - autofocus problem - so frustrating...

[michael_johnson17]

<p>Hi, I'm a long time reader, first time poster here on photo.net. I'm new to DSLR, though I shot some on a film SLR years ago. I just bought a Canon Rebel T3i and am having some issues with it. I'm shooting in Ap mode with the center autofocus point selected, on a tripod, using a 2 second shutter release delay. Shooting outside in the late afternoon (subject in sunlight), I will depress the shutter halfway, get a focus lock beep and snap a picture of, say, some trees at about 60 feet. I'm using a Sigma 30mm lens set at f/1.4, and the shutter speed ends up around 1/500. When I look at the LCD and zoom in on the center, where the focus was, the leaves of the trees are blurry. However, when i scroll down the zoomed-in image to the fence near the bottom of the frame (which happens to be at around half the distance that the trees are), the fence is RAZOR SHARP - I can see the grain, very clear knots, etc. The fence is NO WHERE NEAR THE CENTER FOCUS POINT.<br>

This exact thing happens on another Sigma 30mm 1.4 lens that I have access to, and a Canon 17-55 set at 2.8 aperture. The further away an object is, the worse this "near-focus" problem seems to be. For all of these lenses, if I turn the focus ring about 1/4 to 1/2 an inch after the autofocus locks (possible with all these lenses), I get great focus where I want it.<br>

I thought the above was due to my new camera having a front-focus problem so I returned it and received another one. The EXACT SAME THING IS HAPPENING with the new camera. I am so frustrated... am I profoundly unlucky and it's two bad cameras in a row? Are all three lenses bad, in the same way and to the same degree (takes about the same amount of focus-ring turning on all the lenses to get a clear picture after focus lock), or am I doing something wrong???<br>

Please, if anyone can help, I need some. I'm at a loss... I love the blur with the wide aperture and really like the low-light capability, but the largest aperture I can shoot at currently and expect my area of interest to actually appear in focus is f/5.6. Anything larger than that and what I want is out of focus while some object nearer to me is sharp.<br>

Mike</p>

[Matt Laur]

<p>Are you in single or continuous focus mode? Spot, or matrix-type focusing? Does this happen when you do NOT use the 2-second delay?</p>

[phule]

<p>Assuming you're using One Shot AF, is there a way to test the same scene with these lenses on a completely different model camera body?</p>

<p>Also, can you upload an example shot with this problem? </p>

 

[michael_johnson17]

<p>Matt - I am in single focus mode and I think I'm in spot focusing (I have selected the center focus point for my auto focus and that is the only point that flashes when I get a lock - is that spot focusing?) Although I haven't done extensive testing while not using the 2 second delay, that's where I first noticed that my focus seemed off - I was just shooting hand-held with no delay. I will test tonight with no delay. By the way - that's a beautiful picture, "Cowboy Accoutraments."</p>

<p>Rob - Yes, I could probably use a friend's camera this weekend to test it out (good suggestion, I was getting caught up with my own). I will upload an example shot tonight showing you what I mentioned.</p>

[john_deerfield]

<p>I would like to see the image(s) before commenting.</p>

[William Michael]

<blockquote>

<p>I'm <strong><em>shooting in Ap mode *1</em></strong> with the center autofocus point selected, on a tripod, using a 2 second shutter release delay. . . I am <strong><em>in single focus mode*2</em></strong> and I think <strong><em>I'm in spot focusing*3</em></strong> (I have selected the center focus point for my auto focus and that is the only point that flashes when I get a lock - is that spot focusing?)</p>

</blockquote>

<p>For clarity please confirm (user manual ref page):<br>

*1 you are in <strong>“Av” Mode</strong> (p 96)<br>

*2 you are in <strong>“ONE SHOT”</strong> mode (p 83)<br>

*3 you have selected the centre point to focus (p 86) and are <em><strong>NOT using</strong></em> “Live Mode” or “Live Mode Face Detection” (pp131-136), i.e. you are focussing, by looking through the viewfinder and NOT at the LCD screen.</p>

<p>***</p>

<p>For your testing: consider conducting the test on an object about 60ft away which is NOT tree’s leaves, but has a contrasty edge. See notes on p87 regarding low contrasts and repetitive patterns and AF problems focussing.</p>

<p>Do you have a filter on the lens?</p>

<p>WW <strong></strong></p>

 

[jasonweddington]

<p>Try a controlled test to see if your body is front-focusing across lenses. Use an AF test chart like the one here: <a href="http://regex.info/blog/photo-tech/focus-chart">http://regex.info/blog/photo-tech/focus-chart</a>.</p>

[benstanley]

<p>I have heard there are more problems with this kind of thing with Sigma lenses, but I don't know if its true or not. Apparently its a bit of a lottery - its either good or it isn't.</p>

<p>Can you try repeating the test but using Liveview? Make sure liveview focus uses its normal contrast detection AF, not "quick focus" which uses the phase detection autofocus. You'll know its using contrast af if you hear/see the lens narrowing down to the correct focus with a bunch of trial&error focus movements.</p>

<p>There are 2 types of autofocus in most modern digital SLRs, they use different bits of hardware/software. Basically contrast detection af is what you normally use in liveview, it is also the way most compact cameras work - it is slow but is not affected (I don't believe) by any calibration issues. Whereas phase detection autofocus (which is faster) can be affected by your lens or camera having calibration issues.</p>

[michael_johnson17]

<p>William - thanks for requesting clarification.<br>

*1 Yes, I'm in Av mode;<br>

*2 Yes, I'm in one shot mode;<br>

*3 I have selected the center point to focus and I am not using live mode - I am using the viewfinder to compose and focus.<br>

I had a filter on the lens but removed it for today's tests.<br>

Jason - I tried the focus test in the link that you provided and will attach some of the results, but I'm not sure I did the test correctly...<br>

Ben - I will try what you recommend with Liveview.</p><div></div>

[michael_johnson17]

<p>The next image resulted after autofocus did its thing and I adjusted the focus ring by about 1/4 inch to increase the distance.</p><div></div>

[michael_johnson17]

<p>The next two photos my focus was on the spark arrestor on top of the chimney. The first photo is with the normal focus operating, the second after I adjust the focus ring about 1/4 inch turn to increase the focus distance.</p><div></div>

[michael_johnson17]

<p>Here's the adjusted focus of the chimney.</p><div></div>

[michael_johnson17]

<p>The last picture is one of a focus test chart - again, I don't know if I took this correctly.</p>

[benstanley]

<p>Can you do one using liveview as well?</p>

[michael_johnson17]

<p>I will take one with liveview late this afternoon with the good light. Oops, it appears I didn't attach the focus test chart. I'll do that tonight as well.</p>

[michael_johnson17]

<p>Attached is a picture with the Sigma 30mm, f/1.4, normal focus.</p><div></div>

[michael_johnson17]

<p>The next photo is the same image, using autofocus with my Sigma lens, then turning the focus ring about 1/4 inch.</p><div></div>

[michael_johnson17]

<p>Wow, this is using Liveview... it's about as sharp as I can make it when I treak the autofocus. Whoever suggested that I try this was spot on - what does it mean that the camera can focus clearly with liveview but not with the standard autofocus system? I have a lot more pictures confirming this - liveview is about as clear as my tweak after autofocus.</p><div></div>

[michael_johnson17]

<p>Here is one of the focus test charts.</p><div></div>

[William Michael]

<p>Ha! :) Go Ben!</p>

<blockquote>

<p>what does it mean that the camera can focus clearly with liveview but not with the standard autofocus system?</p>

</blockquote>

<p>Michael - re read Ben's post he explains it.<br>

I concur, Ben: I note Sigma Lenses seem to have this problem more often than other lenses. <br>

I don't have any Sigma lenses but have interrogated results from some of the Sigma Primes used shooting wide open.</p>

<p>WW</p>

<p> </p>

[michael_johnson17]

<p>Thanks to all of you for the questions and suggestions - I take it that I can send my lens and camera to Sigma for calibration, and once calibrated it should focus as well with the standard (phase detection) autofocus as with liveview (contrast detection) - that's my take away. I would never have thought to test the camera in this way to ferret out where the problem lies.</p>

<p>It also turns out with the new camera body I no longer have any front focus problem with the Canon lens at f/2.8, just with the two Sigma lenses. With the Canon lens the focus is right on and I get somewhat sharper pics at narrower appertures, probably normal for the lens.</p>

<p>This is a great relief. I contacted Sigma and will send my lens in for calibration. In the meantime I have a trip to Europe and will use liveview for focusing at wide appertures with the Sigma, until I can get it calibrated.</p>

<p>Again, thank you all very much. I'll look forward to researching more about phase and contrast detection autofocus!!</p>

<p>Much appreciated,<br>

Mike</p>

[michael_johnson17]

<p>Just an update: I sent my camera and Sigma lens (just the one I wanted to keep) to Sigma and they calibrated the lens to my camera. Had everything back within 2.5 weeks from when I sent it. Focus is now spot on and I'm very pleased with the lens.<br>

Thanks to all for your help in troubleshooting!<br>

Mike</p>

[William Michael]

<p>:)</p>

<p>WW</p>