Now my widest rectilinear lenses have the focal length of 28mm (or equivalent). These lenses are the Pentax K 28/35, the Pentax DA L 18-55/3.5-5.6 (for APS-C), the Olympus 14-42/3.5-5.6 (for M4/3), and the fixed 18.3mm lens of the Ricoh GR (APS-C).
In real life, I rarely need a wider angle. I used to have the Variozenitar 25-45/2.8-3.5 for those rare cases, but I sold it because I didn't like its poor mechanical design.
While experimenting with the Pentax 18-55/3.5-5.6 on the full frame sensor, I found that the 24mm produces visible vignetting. But is the result acceptable if the vignetting is corrected by software?
To correct the vignetting, I decided to use the "Flat-field" option of the ART raw image processing program. In theory, this gives a better result than using the coarse "Vignetting Correction" option.
Sometimes people say that a flat-field image should be all white. But the color is not important, what is important is the solidity of the color. So if there is no cloud, the sky can be used to make a flat-field image (but be careful with the sun).
When taking a flat-field image, the focal length, focusing distance, and aperture must be the same as for the primary image.
Some examples of the 24mm of the Pentax DA L 18-55/3.5-5.6 lens on the full frame sensor (Sony a7c, 24MP).
1. At f/4, focus far away.
The image without correction:
The image with flat-field correction:
The flat-field image:
2. At f/4, focus near.
The image without correction:
The image with flat-field correction:
The flat-field image:
See also related notes:
- Pentax DA L 18-55/3.5-5.6: Using on FF (2024-03-19)
- Pentax DA L 18-55/3.5-5.6 (K) (2024-02-15)
- Вариозенитар 25-45/2.8-3.5 (2022-12-23)
- RawTherapee (2022) и ART (2022-03-30)
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