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Problems with 50 f1.4 & 135 f2.0 on 1.6x crop sensors

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Hi everyone

Recently I've tested the 50mm f1.4 on a full frame (5Dii) and on a 1.6x crop sensor (T2i/550D) and realized that at f1.4, f1.8 and f2.0 the lens produces a perfectly clear image on full frame but a seriously unusable image on the cropped sensor. Seriously unusable as in severe chromatic aberrations and almost out of focus image at the center (where it was focusing). The 50mm only became usable on the crop body when stopped down to f2.8.

Has anyone experienced something like this?
Is it a bad copy of the 50 f1.4? (I don't have another at hand to test right away)
And most important: has anyone experienced something like this with a 135L f2.0?


No problems with either lens on either a 7d or 1.3x 1dIV.

No problem with the sigma 50mm f1.4 on my 7d.  I rented the canon 135mm f2.0 but didn't have a lot of luck.  All my fault with a slower shutter speed than I should have used for the focal length resulting in camera shake.  I was trying to shoot a dance recital in very low light.  Valuable lesson learned.

dr croubie:
So the obvious questions first:
Autofocus or manual focus?
Tripod or handheld?
Is it only the 50/1.4, or has any other lens done this to you?

A good test would be to set up everything on a tripod, manual focus on live-view, take some test shots at a few apertures, then switch camera bodies, and do exactly the same tests (same iso, shutter speed, etc).

There should in theory be no difference between using the same lens on a different camera body, whether it's FF or crop, except if there's an AF-problem, the mount isn't seated properly (both of which will be fixed by using MF in live-view), or something very weird (like the aperture isn't stopping down to the correct value if the contact is dirty.
Sure, crop-sensors have higher pixel-density, so any value of CA (in mm) will show up as more 'pixels' wide, but that shouldn't be the difference between 'perfect' and 'unusable', it should just be 'slightly more noticeable'.


--- Quote from: dr croubie on February 21, 2012, 10:25:06 PM ---So the obvious questions first:
Autofocus or manual focus?

--- End quote ---

+1.  Fast primes suffer from longitudinal CA, throughout the frame (unlike lateral CA) and most noticeable in OOF areas (LoCA is aka bokeh fringing).  Fast primes shot wide have thin DoF, meaning more OOF areas.  If the AF isn't working properly on the APS-C, the results you describe make sense.  By 'not working properly' I mean not well calibrated to the T2i body (but a good match to the 5DII).  That's the situation that AF microadjustment is designed to correct, but since that feature isn't available on the T2i, sending the lenses to Canon is the only option.

The way to confirm that's the issue is to use Live View as suggested, either contrast AF (the slow one where the mirror does not flip up) or manually focus at 10x in Live View.


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