Pentax DA 35mm 2.4 AL on a Sony A7rIII using the LA-KE1

[J Caples]

I was surprised when I found that this ASP-C lens is capturing at full frame throughout the F Stop range on my Sony A7RIII using the LA-KE1 adapter. 

On my Pentax full frame camera there is considerable vignetting at the wide open F stops, as well as edge softness. 

I am wondering if the distance that Sony places lenses from the sensor helps to make this lens suitable throughout the F stop range.

[Pieter]

No: the distance between the rear lens element and the photosensitive plane (sensor/film) is exactly the same on the Sony A7Riii as it is on your old Pentax camera. The adaptor is designed to exactly fill the difference between the flange-focal distance of Sony E-mount (18mm) vs Pentax K-mount (45.46mm). So the adapter is 27.46mm thick.

Could it be that your camera automatically switches to APS-C mode as it detects the vignette? Check the size of the images you get out of this lens and whether you have auto crop mode enabled.

Edited September 4, 2024 by Pieter

[Cameratose]

I know the R III automatically switches to APS-C when an E-mount lens is used, but don't know if the adapter you're using has that feature. 

I was thinking maybe flange size since Sony is smaller than other FF cameras, but that makes no sense because the sensor size is the determining factor.

I'm with Pieter. Check the image size to see if you're getting a FF image or cropped.