Timeline for What is back-focusing?
Current License: CC BY-SA 3.0
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| Oct 4, 2014 at 15:26 | history | edited | Guffa | CC BY-SA 3.0 |
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| Sep 29, 2013 at 0:29 | comment | added | Max | @Guffa : heads up, your image yields a 404, you should update it or delete it. Thanks! | |
| Aug 2, 2010 at 20:13 | comment | added | ex-ms | 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. | |
| Jul 21, 2010 at 8:45 | vote | accept | Edd | ||
| Jul 20, 2010 at 18:07 | comment | added | Reid | 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. | |
| Jul 20, 2010 at 17:43 | comment | added | Guffa | 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. | |
| Jul 20, 2010 at 16:18 | comment | added | Reid | 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. | |
| Jul 20, 2010 at 15:33 | comment | added | Guffa | @Reid: Good point. I just set the center point to be the only active sensor, so that I know what it is focusing on. | |
| Jul 20, 2010 at 14:11 | comment | added | Reid | 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. | |
| Jul 20, 2010 at 13:16 | history | edited | Guffa | CC BY-SA 2.5 |
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| Jul 20, 2010 at 13:11 | history | edited | Guffa | CC BY-SA 2.5 |
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| Jul 20, 2010 at 12:25 | history | answered | Guffa | CC BY-SA 2.5 |