LensRentals has loaned me a Hasselblad XCD 55mm f/2.5 V lens for testing. Today I tested illumination falloff off-axis. I focused the lens to infinity, and made four photographs of a white wall with the camera rotated 90 degrees between each image. I used ISO 64 and the mechanical shutter. I brought all four images into Lightroom as a smart object, and averaged them to mitigate any unevenness in the wall illumination.
I examined the four raw files and found that the average corner illumination for a 200×200 pixel patch was 44% of the in the center of the image. That’s a bit more than a stop of light loss.
You can see that the lens corrections introduces some patterning into the image.
In order for lens corrections to work, it needs to boost the gain in the corners of the image. So I made a tight crop of the center and the corner, which I’ll show to you at about 300% magnification. These images have Lr’s contrast set to +100 to make the noise more visible.
You can see that the corner is noisier. This isn’t much of an issue with a mid-tone target at ISO 64, but it will be a bigger issue if more shadow boosting is required and the exposures are less generous.
Does Phocus do a better job?
That is smoother than the Lr version. Here’s the corner under the same conditions as the corner crop above.
The noise is similar.
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