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03/06/2005 06:38:48 AM · #1 |
I have a canon 28-135 IS USM lens.
The last time i was taking pictures, i took a picture directly parallel, or perpendicular (however you look at it) to a flat surface. perpendicular is perhapsthe right word. Anyhow, the flat surface,I think, should be totally in focus.. the center is in focus, however the edges of the frame are badly out of focus. Is this normal???
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03/06/2005 07:06:44 AM · #2 |
Did you take the picture on 28 mm? I'm not any expert, but from what i've read, I understand that it is normal that, in both extremes of the lenses (28mm and 135mm), the autofocus doesn't get quite as sharp as in another distance set in between.
If this doesn't answer to your question, you might want to think if the lighting conditions were equal (on edges and centre), or if you adjusted your focus to center only. You even might wanna try correcting the autofocus, adjusting it manually. |
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03/06/2005 09:25:00 AM · #3 |
Originally posted by xonkas:
Did you take the picture on 28 mm? I'm not any expert, but from what i've read, I understand that it is normal that, in both extremes of the lenses (28mm and 135mm), the autofocus doesn't get quite as sharp as in another distance set in between.
If this doesn't answer to your question, you might want to think if the lighting conditions were equal (on edges and centre), or if you adjusted your focus to center only. You even might wanna try correcting the autofocus, adjusting it manually. |
well allright, i could buy the 28mm thing, perhaps, but you can't set a camera to ONLY focus on one part of the frame... so that i don't think is the problem.
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03/06/2005 09:34:55 AM · #4 |
Assuming the entire object was in the same focal plane, that seems to me to be a 'defect' in the lens or lens design.
Most reviews of lenses look at edge sharpness, because it's a fact of life that the lens sharpness is not uniform throughout. I don't know how that one fares, but even if it does well, it appears there's a lot of sample-to-sample variations on lens sharpess (though I don't know if that applies to edge sharpness).
On a DSLR, which doesn't use the entire image from the lens, this effect should be smaller than on a film camera.
You can find lens test sheets on the web and photograph them to do some of the same tests. I don't have a URL right now but if I find it later before someone else posts one, I'll post it.
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03/06/2005 09:41:47 AM · #5 |
| I have noticed the same problem with the image stabilized lens in my Canon Pro 90 IS. The effect is not consistant, varying somewhat from one frame to another. If I turn off the IS then there is no problem. |
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03/06/2005 10:49:24 AM · #6 |
That lens will be somewhat soft at the corners, when shooting wide angle with the aperture wide open.
If you shoot at f-8 it should look very good over the whole frame.
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03/06/2005 10:55:53 AM · #7 |
As previously posted, wide open you should expect some softness in the corners. there might also be some curvature of the focal plane, and that results in the same look but the reason is different.
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03/06/2005 11:48:41 AM · #8 |
Originally posted by leaf:
I have a canon 28-135 IS USM lens.
The last time i was taking pictures, i took a picture directly parallel, or perpendicular (however you look at it) to a flat surface. perpendicular is perhapsthe right word. Anyhow, the flat surface,I think, should be totally in focus.. the center is in focus, however the edges of the frame are badly out of focus. Is this normal??? |
It's normal for some zoom lenses that cover a broad range to be soft in the corners when wide open at largest aperture and shortest focal length. Optical coinsiderations for wide-angle lenses and telephoto lenses arte very different, and no lens that bridges the two can be sharp as a tack at all settings across the entire plane of the image. A dedicated wide angle lens will always perform better. This is distinctly true of the lens on my 5700, btw.
Regarding terminology, (parallel versus perpendicular), the key concept is that the image plane needs to be parallel to the object plane, so that's hwo we speak of it in the large-format world anyway; you took a shot with the camera "parallel" to the object.
Robt.
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