I got a bit of a surprise today. I was shooting with the K-1 and Tamron 300 with Tamron 2x TC for 600mm ƒ5/6.
Eventually I started shooting with the K-3 and the Tamron 300 and 2x, just to check the image quality, I'm not sure with the 2x on the K-3 I've been getting some pretty spotty results.
Not terrible but also, I have better, shot with the Pentax 1.7x.
While I was testing the K-3 combo I put the K-1 on my lap, with the DA 55-300 PLM on it. (I didn't bring camera bag out.) I figured, frame loose, crop the corners.
Surprise. At 300mm, there was no need to crop. I did but I didn't have to.
One was taken at 230mm still no vignetting on the corners. This is the image un-cropped so you can see for yourselves.
When I get chance I'l have to check and see when the vignetting starts. So far it's good to 230mm.