Canon EF 50mm f/1.8 seems to front-focus at f/1.8 on EOS 650D — technique or lens issue?
Asked 4/11/2013
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I’m using a Canon EOS 650D with the EF 50mm f/1.8 II. At f/1.8, autofocus often appears to miss, even when I use single-point AF. In my test shots on a focus target, the sharpest area seems to land about 1 cm in front of the intended target.
Example settings: f/1.8, 1/250s, ISO 250.
Is this more likely due to shallow depth of field and beginner technique, or could the lens/body combination be front-focusing? What’s the best way to tell?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
13y ago
2 Answers
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At f/1.8 the depth of field is very small - any movement (of subject or camera) between focusing and taking the shot can push the subject out of focus - so you want to take the photo as quickly as possible after focusing and don't use the "focus and recompose" technique - not at f/1.8
The auto-focus on the 50mm f/1.8 is very slow, if the camera is set up to take the photo even if focus isn't locked it's possible you take pictures before the lens finished focusing - when the camera focuses it will beep and a small circle will light up in the view finder.
And last - and definitely least probable - your camera or lens can be miss-calibrated (at least for each other), there are questions on this site about focus testing (like this). I believe the 650D does not have any way for you to calibrate focus and you need to take the camera and lens to a service center (or at least a good camera shop).
Originally by user2481. Source · Licensed CC BY-SA 4.0
user2481
13y ago
0
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It could be either technique or calibration, and with this lens both are plausible.
At f/1.8, depth of field is very thin, so even slight camera/subject movement after focus lock can shift the sharp plane. Avoid focus-and-recompose, use a single AF point directly on the subject, and take the shot immediately after focus confirmation.
Also make sure the camera is not firing before autofocus has finished; wait for focus confirmation in the viewfinder.
Separately, the EF 50mm f/1.8 II is known to be softer wide open, so some of what looks like missed focus may simply be reduced sharpness at f/1.8. It generally looks sharper by f/2.8.
To test for a real AF issue, compare viewfinder AF with Live View AF on a tripod. If Live View consistently focuses correctly but viewfinder AF repeatedly front-focuses, that points to a calibration issue between the lens and body. One user also reported focus accuracy varying depending on where the lens started its focus travel.
Since the 650D does not offer AF microadjustment, a consistent front-focus problem would usually mean service or trying another copy of the lens.
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AI13y ago
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