I'm using a Canon 8-15mm fisheye with a sony alpha 7R3 and a sigma adapter. I just noticed that at 8mm the circle of the image is not exactly centered; it is close to the top border of the image.

Is it a lens defect or a problem with the adapter?

enter image description here

  • Do you perhaps have a photo that shows the problem? And is this closeness to the border consistent or does it occur now and then? Jan 16 '20 at 17:35
  • added... Thank you! It does occur every time.
    – Bob
    Jan 16 '20 at 17:56
  • Oh, I see, it's actually very slightly closer to the top than to the bottom. What is the actual problem you encounter? Because to me it seems like a perfectly usable image, but perhaps I'm unaware of certain special use cases that are applicable to you. Jan 16 '20 at 18:02
  • 1
    I'm not too worried but I was wondering if it's a lens defect or fisheye lenses are never exactly centered...
    – Bob
    Jan 16 '20 at 18:16
  • Such things tend to be telltale of a heavy optical train bending under its own weight... Jan 16 '20 at 20:43

The most likely cause is an alignment issue that results in the lens' optical axis not being perpendicular to the camera's sensor. I'd place money that the adapter is not sitting perfectly flat with either the camera's flange, the lens' flange, or both.

  • It doesn't have to be a perpendicular misalignment (i.e., lens tilt). It could just be a bit of lens shift, due to loose tolerances in the adapter's mount(s). It could be sitting perfectly flat, just... off-center a bit.
    – scottbb
    Jan 17 '20 at 1:11
  • @scottbb It could be, but my money is on tilt. That's where most adapters are weakest. Especially when the direction of movement can be explained by gravity pulling on the lens when the camera is in landscape orientation.
    – Michael C
    Jan 17 '20 at 1:39
  • I don't disagree with the reasoning that tilt/sag is very likely. I was going to counter-argue my own comment, but now I'm going to double down. Here's my thinking: if it's tilt due to lens relaxation (i.e., gravity pulling it downwards), then downward lens tilt usually results in the image circle very slightly skewed upwards. But because the image is inverted at the image plane, that translates to a visible downwards shift in the viewed image. But a downwards shift of the lens moves the image circle lower on the image plane, resulting in an upwards shifted viewed image, just like in OP.
    – scottbb
    Jan 17 '20 at 1:47
  • (implicit in my above thinking is that either motion (tilt or shift) is due to loose tolerances that are affected primarily by settling, i.e., gravity. Tight-but-machined-out-of-parallel mount faces don't care about gravity, and that explanation is certainly possible and completely explained by your answer)
    – scottbb
    Jan 17 '20 at 1:51

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