Why would a fisheye lens test sharper at the edges than in the center?
Asked 5/31/2022
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I tested a Sony A7R IV with a 12mm f/2.8 fisheye in Imatest and got unexpectedly higher MTF50 values near the image edges than in the center. Published charts for this lens suggest the center should be sharper.
I focused manually using the camera’s focus assist and photographed a flat test target. I initially wondered whether I was misreading MTF50, focusing incorrectly, or being affected by uneven lighting. After retesting, the top and bottom improved, but the left and right still looked slightly sharper than the center.
What could cause this kind of result when testing lens sharpness, and how can I reduce or eliminate it?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
4y ago
2 Answers
3
It looks like you're seeing the effect of tilt.
There are two main possibilities that can cause tilt:
- Your camera and test chart are not aligned properly. If the camera's sensor plane is not perfectly parallel to the perfectly flat test chart, then one side or the other or, as in your case, the top or the bottom of the test chart will be closer/further from the camera than the opposite side. Whichever side is closest to the lens' focus distance will be most in focus.
- Your lens is not aligned properly. This could be due to a misaligned lens element, to the flange ring on the lens not perfectly perpendicular to the lens' optical axis, or the camera's mount ring not perfectly parallel with the sensor. Any flange/mount misalignment will be more noticeable with very wide angle lenses than with longer focal length lenses.
Just based on the difference in curvature between the top and bottom of your test chart, as well as the difference in curvature between the left and right edges of the chart, I'd say that your main problem is one of camera/chart misalignment. It appears that your camera is closer to the upper left corner of the test chart and further from the lower right corner. It also appears that the lens was focused at the longer distance of the lower right corner.
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
4y ago
0
Generated from our catalog & community — verify before relying on it.
The most likely cause is tilt, not that the lens is truly sharper at the edges.
With a flat test chart, the camera sensor plane and chart must be perfectly parallel. If they’re even slightly misaligned, parts of the chart will be closer to the focus plane than the center, so those areas can measure higher MTF50. With an ultra-wide 12mm lens, small alignment errors are especially noticeable.
Possible sources of tilt:
- the camera and chart aren’t perfectly aligned
- the lens itself is slightly decentered or tilted
- the lens mount or camera mount/sensor alignment is slightly off
To reduce the problem:
- make sure the test chart is truly flat
- carefully align the camera so the sensor plane is parallel to the chart
- refocus after alignment changes
- repeat the test to see if the sharper side changes with setup
If the same side consistently tests better even after careful setup, that points more toward lens or mount misalignment than user error.
So your MTF50 interpretation is probably not the main issue; test geometry and possible optical/mount alignment are the key suspects.
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