Focus problem with Canon "nifty fifty" at f1.8...

Started Apr 24, 2013 | Questions thread
Olaf Ulrich
Contributing MemberPosts: 942
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Re: Focus problem with Canon "nifty fifty" at f/1.8 ...
In reply to RasiWick, Apr 24, 2013

Umm ... I think you're the first person I'm aware of who tags the Canon EF 50 mm 1:1.8 as a 'great lens.' It is pretty good, and affordable, but not really great.

Generally, lenses for cameras are optimised for long subject distances (unless they are dedicated macro lenses). At or near the minimum focus distance, performance will drop, especially with fast lenses, especially at or near full aperture. To mitigate the close-up performance loss, expensive lenses have floating elements, internal focusing, or rear-part focusing. The EF 50/1.8 has neither of these.

So sharpness being less-than-perfect at distances like 5 ft or less and at full aperture is to be expected. If you still think, yeah, but my copy is even worse than what's to be expected, then there are a few tests you can do. If possible, always use a tripod for lens testing. First of all, check if the plane of sharpness actually falls where you focused at. Use a subject with lots of detail across some depth, pick one detail, let the camera focus, shoot, and then check if the maximum sharpness in the picture is at the intended point, or somewhat before or behind it. Do that for various distances, such as 1 m, 3 m, and 10 m, and use a wide aperture for narrow depth-of-field.

Then check the sharpness for symmetry. Shoot a brick wall and see if the sharpness fall-off from center to corner is about equal for all four corners. Make sure the wall really is parallel to the camera's sensor. Do that at various distances again, and at various apertures. In most real-world lenses, the fall-off won't be perfectly even but no edge or corner should glaringly stand out.

Finally, if you see other people's pictures taken with the EF 50/1.8 at full aperture which show the sharpness you're missing then check out if they were using an APS-C-format camera (helps with edge fall-off of sharpness and illumination) and how much sharpening they've used in post-processing.

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