I just bought Sigma 15mm F2.8 EX diagonal Fisheye couple weeks ago,
to pair it with my 20D and used it to shoot outdoors with aperture
smaller or equal f/8. The images are just fine.
2 days ago I tried to shoot indoors with maximum aperture f/2.8,
then I realized that in Autofocus mode with aperture wide opened,
all images turned out to be soft (out of focus), until I readjust
the focus manually, more sharpness could be obtained.
Later I experimented with some shots on tripod, taken around 6 ft away
from the object (camera box), with centered focus point on
the word "EOS" (sample 5). Focus lock via * (Cfn 4 set to 1), and self-timer
since I do not own a wired remote shutter-release.
The findings:
1. AF with f/2.8, result: soft, slightly out of focus (sample 1 & 5)
2. Same focus position now with f/8, result: sharp focus (sample 2 & 5)
3. Readjust focus manually, f/2.8, result: sharp focus obtained (sample 3 & 6)
4. Same readjusted focus position as above (point 3), the f/8 is now (sample 4 & 6)
slightly out of focus.
My questions:
Is there something wrong with my fisheye lens?
Or this AF phenomenon belongs to the characteristic of fisheye lenses?
Which if set to maximum aperture, the AF will slightly run out of focus?
I've tried the AF with the rest of the richtlinear lenses I owned, they are
just working fine and no such a phenomenon occured.
I highly appreciate your response and comments.
Here are the sample images links, (cropped, auto levels adjustment, no USM): Sample 1, 2, 3, 4 Sample 5, 6, 7: AF Point, AF position, MF (adjusted) position
My Sigma 15 fish autofocuses just fine, I haven't heard to many complaints from other people about focusing on theirs either. You might want to send yours in to get it adjusted, or return it and get another?
That is definitely out of focus, not soft, but out of focus.
