What is back-focusing?
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?
See also http://photo.stackexchange.com/questions/1/what-is-the-best-way-to-micro-adjust-a-lens adjustments you can make on higher-end cameras.
Now it is easy to check FF or BF. on ebay you can find lens focusing jig. This is very helpful in checking the lens
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 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: Good point. I just set the center point to be the only active sensor, so that I know what it is focusing on.
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: 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.
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.
Great explanation. It's also not limited to autofocus, but applies equally to any focusing mechanism, e.g., a bad rangefinder cam, a misadjusted mirror on an SLR, and so forth can all cause back-focus. Some photographers have regularly had their lenses/cameras individually calibrated to each other for just this reason.