At 12/31/2021 04:00 PM, Mike wrote:
>Nothing says Happy New Year like some serious pixel peeping.
>
>This Sigie lens has to be sharp in the corners wide open for astro landscapes.
> Sigma has quite good QA now but on FM about 1/3 end up being sent back. I
>was tempted by the sonnie 14/1.8 but there are more aberrations in the corners
>even at f2.8, the corners are just so-so at 1.8 and the copy variation is
>quite large. It is smaller and lighter and thus better for travel though.
>
>I focused on our chimney in the center of frame and then moved the cam so the
>chimney was in each extreme corner--200% crops. IBIS off and on tripod. These
>are SOOC jpegs.
>
>https://www.olyendomike.com/Sigma-14-24-f28-centering-200/
>
>I think modestly soft at 24mm in the rt lower corner but not badly decentered
>at 200%. Good enough or not??? I can send it back for 30 days.
>
>Mike
I recommend reading Jim Kasson https://blog.kasson.com/lens-screening-testing/
There are links to test targets you can download and print. I printed one on
ledger size paper (11x17inch) and use it for testing. For example of one of my
tests at 100%:
https://photos.app.goo.gl/Jctj4b71u8ARjCHa8
There is a minimum distance the subject needs to be for center focus and
shifting camera to the corners based on focal length. For instance following
this link in the article
https://blog.kasson.com/lens-screening-testing/whats-wrong-with-conventional-lens-screening/
he says (note the 4th point):
Target alignment is critical.
Target flatness is equally so.
The target size gets to be impracticably large if the lenses are short.
It's hard to practically impossible to test short lenses at realistic distances.
It's hard to light the target evenly
It's hard to find a space to set up the test with a big target.
...
Short focal lengths are harder because of the distance to the subject required
to keep it in the plane of focus when you shift the camera. Since you are
looking for good Astro lens wide open, this is the most important point.
Given all that I don't think your test is going to tell you what you want to
know. Perhaps go out to one of the marshes, angle the horizon and take a photo
at infinity, which seems to be the way they do it on FM
https://www.fredmiranda.com/forum/topic/1534737/
Not a lens test expert - WayneS
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