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Heavy vignette when shifting A7r on Cambo Actus

tylergreenphoto

New member
I've had the Cambo Actus for a few years and have almost exclusively shot BW infrared with it. Recently, I found the need to shoot color and have noticed the vignette is terrible when I shift the back (a7r) but almost unnoticeable when the back is centered (see attachment, 1 horizontal frame overlaid on 2 vertical shifted frames). Any ideas about what is going on here? Doesn't seem like normal lens vignetting to me. The lens used is a 47mm Schneider digitar at f8. Not an issue on my 135mm Sinaron.

I've thought about looking into preowned MF backs, would that resolve this issue, since I would not need to shift the camera to have the same image circle coverage?
 

Attachments

Alan

Member
Color cast from symmetrical lens. Look up Lens Cast Correction (LCC) with Capture One, or Flat Field(?) with Lightroom. You need to shoot a calibration frame through a white plastic card for each shift position. You'll have the same problem with most MF backs as well.
 

rdeloe

Active member
To add to Alan's point, you'll see it with wide angle lenses but not (or certainly not as much) with longer focal lengths. You also won't see it with SLR lenses.
Good article here for Lightroom users (but the basics are relevant to any software): https://www.reidreviews.com/examples/flatfieldnew.html

Shooting a calibration frame is worth it not only for lens cast, but also for colour shifts. You can fix them both in one go. Unfortunately, it's no a panacea. In order to fix illumination falloff the software is raising exposure, which means noise in severe cases.
 
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