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Pentax 645D

Sensor Color Uniformity

Medium format digital sensors often have non-uniform color rendition across the frame, typically leading to a blue or cyan cast into the corners of the frame. While lenses can also be involved (wide angle lenses in particular), longer focal lengths typically do not contribute much to the issue.

I had two lenses to test: the 55mm f/2.8, and a manual focus 75mm f/2.8. I tested both, with very similar results.

Example with 75/2.8

It’s curious that while Pentax offers lens correction for both distortion and chromatic aberration, it is not offered for this color shift into the corners. Nor does Adobe Camera RAW correct for it using the Adobe-supplied lens correction for the 75/2.8.

The image below is of a gray sky, with a 50 pixel Gaussian Blur to remove minor variations in density, taken at f/5.6 with the 75mm f/2.8. White balance was neutralized at image center, where the image is completely neutral, as measured in Photoshop.

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Variants gray, pureGray available in full article
Pentax 645D + Pentax-A SMC 75mm f/2.8 @ f/5.6, 1/200 sec, ISO 100

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