Zoomed-out to its shortest focal length, the 75-300mm is slightly soft in the extreme corners until the aperture is closed right down to around f11 to f16. The 18-70mm kit lens is much sharper in the corners at 70mm, so if you want the best results at around this focal length, go for the 18-70mm kit lens and zoom it all the way in.

Zoom the 75-300mm all the way in though and it performs much better in the extreme corners, although as seen in the crops below, it does exhibit some coloured fringing.