Front/back focus fiction or fact?

<p>Hi,<br />I have been walking around for quite some time with a general question about front or back focus issues. I have read many posts regarding the subject, but have not yet found the answer to my question.<br />Perhaps the question will establish my lack of intellect, but I'll give it a try anyway.<br>

<br />Why, if the camera's AF sensor uses the lens for focussing, and one can see through the viewfinder that the object is in focus, why o why can front or back focus occur?<br>

<br />I mean it is TTL AF we're talking about. So if the sensor says it's in focus, it should be sharp right? I have a hard time accepting that on pro level bodies front/back office issues occur.<br />Thanks!</p>

<p><em>So if the sensor says it's in focus, it should be sharp right?</em></p>

<p>The autofocus is DSLRs is typically not done by the primary imaging sensor but a separate CCD which is only used for autofocus. The light seen by this sensor is not exactly the same as the light that the primary imaging sensor sees, therefore there can be discrepancies. I'm afraid these issues are very real.</p>

<p>Ok Ilkka,<br>

Thanks for the reply. It seems that I have to do some serious testing in the coming days.</p>

<p>Most of the claims I've read on the web regarding front/back focusing were due to misunderstandings about how to effectively use the camera. But, sure, there have been some proven cases of front/back focusing, as confirmed by methodical testing. Not nearly as many as the web lore would have you believe tho'.</p>

<p>Minor mirror alignment problems can lead to problems with critical focus. This problem existed with all SLRs long before the digital era. I once found a tiny bit of debris adhering to the mirror stop that cause a minor manual focus error that was noticeably only in macro photos. Just carefully brushing away the debris solved the problem. But this too would be a fairly uncommon problem.</p>

<p>There are some well documented and illustrated tests for evaluating your camera's focus performance. Are you familiar with any of those? If not, I'll try to find my old bookmarks for those tests. Haven't used 'em myself since I got my D2H in 2005, after satisfying myself that the occasional focus errors I saw were my own fault.</p>

<p>To add: it is not so easy to test for these things. The positions of the AF sensors may not be exactly as marked on the focusing screen. The sensor area is finite so it might focus to the front or back in the area, and still work as intended. There may be random focus errors due to tolerances and yet this is not the same as systematic front or back focus. In my experience significant<em> systematic</em> focus errors are rare but they do happen with some bodies and lenses; most errors are user/usage/technique and precision related, or that has been my experience.</p>

<p>Lex, Ilkka,</p>

<p>Thanks for the replies. I have to admit, coming from film and now using a D700, I have come across many unsharp pictures that were caused by me not using my brains, hence the lack of intellect.</p>

<p>Lex, I found the focustestchart and will give it a try one of these days, thank you!</p>

<p>Funny, using film I usually thought about what I was doing before pressing the shutter release, calculating shutterspeed for focal length, checking DOF, but now I seem to have gotten a bit lazy. And, my primary subjects changed from landscape to three very, very energetic kids - which doesn't help.</p>

<p>The reason I doubted the front/back issue thing, is that all my lenses are capable of producing razor sharp results, but I noticed that the focus has to be spot on, where as with film there seemed to be a little more room for manouvering.</p>

<p>One more thing, stopping down a lens should solve the problem, shouldn't it? Yes I realize that a shallow DOF sometimes makes the difference between a snapshot and a stunning picture, but for landscape it should be somewhat of a cure, right?</p>

<p>Thanks again,</p>

<p>It`s far easier to find focus issues on digital than on film.</p>

<p>In my experience, to have perfect focus on non-static (posed) people photography is really difficult when shooting at wide apertures. A slight subject movement turn to unfocused what I expected to be perfectly focused eyes... sometimes I`m not close enough to the subject and cannot distinguish if I have placed the AF sensor mark on the glasses, overall face, etc. with the same problem.</p>

<p>Stopping down could solve the problem but the aesthetics are different, due to the "deeper" DoF. Sometimes there isn`t other remedy. On landscape shots there are other issues, like the lens performance at infinity, plane of focus, haze, etc. that also could result on a not as sharp as expected pic... again, looking at the screen at pixel size is easy to find everything.</p>

<p>Using the backfocus test chart, from Tim Jackson, tim@md.co.za, way back in 2004, I was able to prove back-focus with certain lenses on my old D70. It was quite frustrating. I had the same initial thoughts as Arthur, if it's in focus on the screen, why is the resultant image un-sharp. I even took back a couple of bodies and had a fairly non-productive correspondence with Nikon. Our local Nikon rep ran the other way when she saw me coming. My D200 was way better, and my D700 is fabulous. I do not know if manufacturing tolerances have improved, or if autofocus technology has improved. It seems less of a problem to me thses days.<br>

Good luck.<br>

Eric</p>

<p>Side question ...<br>

Barring issues of camera shake or subject movement, does it follow that if one is using the live mode in some of the new DSLRs that the front/back focusing issue cannot happen? I'm coming from a frame where I don't uderstand how that new stuff all works. Thanks.</p>

<p>Jim</p>

<blockquote>

<p>To add: it is not so easy to test for these things. The positions of the AF sensors may not be exactly as marked on the focusing screen. The sensor area is finite so it might focus to the front or back in the area, and still <a href="http://www.photo.net/bboard/#" target="_blank">work</a> as intended. There may be random focus errors due to tolerances and yet this is not the same as systematic front or back focus. In my experience significant<em> systematic</em> focus errors are rare but they do happen with some bodies and lenses; most errors are user/usage/technique and precision related, or that has been my experience.</p>

</blockquote>

<p>Well-stated. I tried some of the focus-chart tests. While one chart indicated front focus errors, other charts and subsequent follow-up tests indicated that the focus was maximized in the original position. I suspect that, as Ilkka has suggested, the AF sensors may not line up exactly with the marks that we see in the viewfinder.</p>

<p>My D700's manual claims that AF fine tuning is NOT meant for general use. Given this and the inconsistency of my test results I've decided against tweaking this adjustment unless a particular lens is giving obviously out-of-focus results. So far, so good.</p>

<p>I did have one case of one lens which would back focus. I went through testing it & even though everyone tried to say I was wrong - - serious testing of the lens & camera combination according to given instructions on how to test for it proved beyond any doubt that I did indeed have one lens way back when which did back focus from certain local lengths from certain apertures on.<br /> I traded the lens in back to the store I bought it from within two weeks. For all the lenses I've worked with since, I've not run into that problem again. That was 3,5 years ago now.<br /> So - yes it does happen, but I would say it's rather rare & mostly it's pilot (read photographer's) error. In my case - it was the lens. Tested with single focus point on tripod with remote release etc etc.<br>

To me this is nothing compared to all my Canon shooting friends who keep sending in their lenses & camera bodies for calibration on a continues basis to Canon. That would drive me nuts.<br>

Lil :-)</p>

<p>I have a related question I'll ask here. Given that the autofocus system is inside the camera body and it looks through the lens (adjusting the lens via a feedback loop), shouldn't back focus or front focus be a property of the camera <em>body</em> rather than the lens? If focus errors are caused by slight misalignment of the AF sensors, then how in the world can it cause only particular lenses to front or back focus? Or is it that the error is only noticeable on certain lenses, with wide apertures?</p>

<p>This has always confused me, as there are stories all over the internet about how this or that lens is backfocusing and had to be traded in...</p>

<p>Ben, I believe it has to do with a particular lens/camera combination. Both parts of the combo will have a certain tolerance - the combination of which may put the combo "out of spec". My 80-200/2.8 AF-D had a severe focusing problem on my D70 - wide open, close focus, and at and near the longest zoom setting. No problem with other lenses though. And the lens is doing fine on both my D200 bodies as well as the D300. Same problem for a friend of mine, D80 and his 80-200/2.8 - he sent it to Nikon and it came back fixed (but without explanation of what had been done). </p>

<p>Hi Arthur and Ben,</p>

<p>I had exactly the same questions. This article answered them to my satisfaction:<br /><a href="http://en.wikipedia.org/wiki/Auto-focus">http://en.wikipedia.org/wiki/Auto-focus</a></p>

<p>Most Nikon cameras use phase detection <a href="http://en.wikipedia.org/wiki/Auto-focus#Phase_detection">http://en.wikipedia.org/wiki/Auto-focus#Phase_detection</a> which uses "the light rays coming from the opposite sides of the lens and divert it to the AF sensor, creating a simple <a title="Rangefinder" href="../wiki/Rangefinder">rangefinder</a> with a base identical to the lens's diameter." This is where the lens comes into play.</p>

<p>That brings up an interesting question. Would one see the same back/front focus problems using the contrast system of live-view?</p>

<p>Wow,</p>

<p>Thanks for sharing your thoughts. I think I'll stick with Dan's conclusion. AF tuning is not for general use. First I'll start shooting several (thousands) of pictures and then see what happens.</p>

<p>Just to thank you all, please enjoy the sunset I just captured in front of my house.</p><div></div>

<p>Come to think of it, 40 years ago three guys were taking pictures as well. I wonder what they felt like.....</p>

<p>Good point Arthur. I expect they were having a fine time - even though they were only provided with Hasselblads.</p>

<p>The AF Fine Focus Adjustment made a big difference for me on some of my lenses -- and they were Nikon's best.<br>

If it was unnecessary, then I don't think Nikon would have put it in their high end cameras.<br>

However everybody's standards and lens sets are different and YMMV.<br>

I did a check at 10' focussed on a newspaper with a large central cross as the focus point, and adjusted the fine focus through its whole range. I then chose the image which gave the best sharpness. This was done for all lenses wide open. Zooms were done at both ends of the zoom range.<br>

I then repeated the process focussing on a chain link fence at about 200'.<br>

My 50mm 1.8 and the 80-200 2.8 did not require any adjustment at any range.<br>

However the 24-70 needed +8, and the 85 1.4 needed +18!! A Sigma 120-300 required +5 at 200'.<br>

The 85mm deserves some comment as this is supposed to be a magic lens. I never liked it until this adjustment. The only point in using it is wide open to get the soft background, but mine never could focus on the desired point anyway.<br>

I figure that at close range, each +1 adjustment in fine focus is worth about 1/2" on the target, so instead of focussing on the eyes of a model, mine was focussing 4" away. No wonder it was unimpressive.</p>