100-300 with Kenko extension rings - focus range

I have just run a series of tests at 100, 200 and 300mm with 26mm extension tubes to get an idea of the relative image quality at different apertures.

100mm - Centre sharpness is very good from F/5.6 onwards. The corners are horribly soft. Slight improvement in corners at higher F stops.

200mm - Centre sharpness is very good from F/5.6 onwards. The corners are softer with the extreme corners being very soft. Slight improvement in corners at higher F stops.

300mm - Centre sharpness is very good from F/7.1. No real improvement with higher F stops. Extreme corners a soft.

With my test target (pre-printed pattern on paper) I can't see a lot of difference in centre image quality between 100, 200, and 300mm at these F stops. Corners get much better at longer focal lengths. Magnification is approximately the same for all focal lengths!

F/7.1 is best at 300mm which is the same result that I have found with the lens on its own and is a lot better than F/5.6 (wide open). No great surprise I suppose. Given the corner softness at shorter focal lengths, I would advise setting F/7.1 and leaving it there for all focal lengths. I would probably only use the shorter focal lengths when I can get in close AND there is a need for a slower shutter speed (if indeed it that makes sense? If the magnification is the same at different focal lengths, can you use a slower shutter speed for shorter focal lengths and still avoid shake? Or do they all require the same shutter speed because the field of view is the same? Hmm...)