When I did my lens screening test on the Fujifilm 45-100 mm f/4 a few days ago, I noticed the corners were a bit soft wide open. Not soft for a zoom, mind you, but still substantially softer than the on-axis sharpness. I wondered if the softness was inherent in the lens, or was due in part to field curvature. So I performed a test with the same Siemens Star, this time focusing on the target for each shot, so that field curvature would not be a factor in the results.
I tested at f/4. f/5.6 and f/8, and at 45 mm and 100 mm focal lengths. Here’s the test scene, with the lens set to f/4 and 45 mm:
Target distance was 13 meters for the 45 mm shots, and 17 meters for the 100 mm ones.
- The heaviest RRS legs
- Arca Swiss C1 head
- ISO 100
- Electronic shutter 1/10 at f/4, 1/5 at f/5.6, and 1/2.5 at f/8
- 2-second self-timer
- AF-S, medium spot size
- 3 sets of shots at each test condition
- Developed in Lightroom
- Picked best shot of each test condition
- Sharpening amount 20 radius 1, detail 0 (much less sharpening than the default)
- Adobe Color profile
- White balanced to grey background
We’ll look at some tight crops at about 180% magnification.
First, at 45 mm in the center:
It looks like the sharpest stop is close to f/8.
In the upper-right corner:
That is substantially sharper than the corner shots in the screening test, indicating that focus curvature did play a role in the corner sharpness there.
This lens is quite sharp in the corners at 45 mm!
At 100 mm in the center:
And in the upper-right corner:
That’s about the same as in the screening test, indicating that focus curvature was not a factor at 100 mm there.
Stopping down doesn’t help much.