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up vote 24 down vote favorite
3

What is back-focusing? Is it something I need to be worried about, or can I just live with it? How can I tell if my camera/lens is sufferring from it?

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See also photo.stackexchange.com/questions/1/… adjustments you can make on higher-end cameras. – mattdm Mar 23 '11 at 3:13

4 Answers

up vote 17 down vote accepted

Back-focusing and front-focusing is when the auto-focus consistently is slightly off in either direction.

This problem has always existed as long as auto-focus has existed, but it has come into focus (no pun intended) with digital cameras where you can enlarge the image to pixel level and really see where the focus is.

You can test this with a simple setup of a ruler and a box of matches, or any similar objects. Put the ruler on a table and place the box standing right beside it. Focus on the box and take a picture, preferrably using a long lens and the largest possible aperture (lowest f-stop value).

box -> |                         o <- camera
       |
--------------- <-ruler

Now when you examine the photo you can see where the ruler is sharp. If the auto-focus is correct, the box is in focus, and the part of the ruler that is sharp is a section in front of and behind the box, centered slightly behind the front of the box. That is because the depth of focus is slightly longer behind the subject:

box -> |
       |
--------------- <-ruler
     ^  ^
     |__|sharp

Testing my EF 28-135:

auto-focus test

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Testing focus with a ruler is prone to error because you can't guarantee what part of the scene the autofocus sensor is triggering on. To avoid this, use one of the focus charts suggested by other answers. – Reid Jul 20 '10 at 14:11
1  
@Reid: Good point. I just set the center point to be the only active sensor, so that I know what it is focusing on. – Guffa Jul 20 '10 at 15:33
It's more than that - sensors aren't necessarily active exactly where they show in the viewfinder. So for example you could think you're focusing on the 30mm mark but in fact your sensor found the 34mm mark, which will show up as back focus but in fact isn't. That's the reason good focus charts have a clear, unambiguous thing to focus on. – Reid Jul 20 '10 at 16:18
2  
Reid: That is not a problem, as the area that I am focusing on is a lot larger than the sensor, and it's nearly parallel with the focus plane. I am of course focusing on the box, not the ruler, that's the whole point of having the box there in the first place. – Guffa Jul 20 '10 at 17:43
OK, that's fair. I still submit that a good focus chart is better, because it reduces the fiddliness of one's setup and comes with clear directions to follow. For example, if one did a similar setup but the side of the box was visible, that could capture the focus, or if you used a battery (a common focus target) there's no guarantee where on the curved surface the focus hits, or if you're not lined up exactly perpendicular with the box, there's a source of error. Another consideration is that a focus chart will give you numbers for both sides of the frame. – Reid Jul 20 '10 at 18:07
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up vote 11 down vote

There's a page at focustestchart.com that goes into enormous detail about this.

The quick summary: back focus is when the focus plane is further back than you were expecting it to be (so your subject is out of focus, but something behind it is sharp). This is usually operator error, but might possibly be a problem with the lens; the page linked above gives you a way to distinguish between the two.

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1  
to be more precise, the (excellent) explanation is here in the page : focustestchart.com/chart.html#BackFocus – Jean-Philippe Caruana Jul 20 '10 at 14:34
Matt, the link seems to be dead. Can you see if you can find if it's moved? I found this link which looks good, but not sure if it's the same information - regex.info/blog/photo-tech/focus-chart – MikeW Apr 3 at 5:22
@MikeW I've substituted for an archived version of the webpage for the time being... – Rowland Shaw♦ Apr 3 at 8:15
up vote 2 down vote

Back-focusing/front-focusing is when your auto-focus is wrong, and focuses too far or too close. This is most visible when you're shotting with fast telephoto lenses and in other situations with shallow depth of field. Most people don't like to have their photo out-of-focus, so if you're suffering from this it's best to fix it.

As Matt writes, you can check your camera+lens using chart at focustestchart.com. On some cameras front-back focusing can be tuned directly in their menu, but sometimes it's necessary to get the camera and/or lens calibrated in service center.

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See comment under Matt's answer, focustestchart link is dead - if you know of a new location for that page, pls update – MikeW Apr 3 at 5:23
Rowland Shaw was able to find an archived link, fixed it with that one. Thanks for the ping! – che Apr 3 at 17:03
up vote 2 down vote

See also the focus test chart at http://pentaxdslrs.blogspot.com/2008/06/part-1-autofocus-adjustment-for-pentax.html. It has a slightly more accurate focus target and measures the 45 degree angle for you.

Note that it's incredibly easy to do focus testing wrong. Be careful and disciplined in following the directions.

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