> From: Ken Norton <ken@xxxxxxxxxxx>
>
> Is the test chart set at an angle? There is a lot of variation between
> sides or center-to-edge.
My main reason for shooting these is to develop lens profiles for automagically
correcting chromatic aberration. So the shots are all optimized for that
purpose.
Correction: my main reason for shooting THESE is because a simple project to
generate a CA profile for the OMZ 21/2 got completely out-of-hand, until I
found myself shooting every bit of glass I could attach to the camera. :-)
If I had been doing more complete lens testing, I would have moved the camera
for each shot, and framed the chart perfectly, and used squares and laser
pointers to ensure a perfect 90 degree camera angle. I didn't mess with all
that, since I was primarily interested in having high-contrast areas in at
least one corner.
To that end, I set the focus magnification all the way as far as it would go
into the upper-left corner, and then used the centre of the splayed line cross
for focusing. So THAT should be the sharpest part, rather than the middle. This
may be MOST of the uneven focusing that you thought to be from "shooting at an
angle."
I did not use squares and lasers to keep things at 90 degrees, but the chart
was mounted on the wall of a building, and I eye-balled as square an angle as I
could. Since these were all f8 or smaller, I think DOF was adequate for my
purposes.
If you look at the OM 500/8, I don't think there's much edge-to-edge variation.
I can only assume that the others have significant curvature of field issues.
In ALL cases, I carefully focused on the cross-shaped lines in the upper left.
Another interesting thing that doesn't show up is the degree to which
horizontal and vertical lines were in focus. Although I can't claim to explain
or understand the physics of it, it appeared to me that some of these lenses
had slightly different focus points between vertical and horizontal lines. In
each case, it seemed easier to focus on the horizontal lines. That could be an
artifact of sensor/processor/viewfinder stuff, or slight wind currents, or my
imagination, or magic pixie dust.
Another interesting thing was the degree of POP when focusing. The OMZ 500 hit
focus like a sledge hammer, whereas I had to hunt and peck with the other
mirrors.
Anyway, like most lens "tests," this proves nothing. But it was fun to sorta
get a handle on these lenses' strengths and weaknesses in shooting a wall from
30' or so.
Jan
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