Nonsense, Ian. Maybe you shoot a lot of wide angle negs which are thin around the corners, or otherwise just got lucky with a helpful diffuser. Print a very hard grade paper through it without any
neg in position, develop to max contrast, and look at what you actually have, unless you have a rare
enlarging meter capable of actually measing across the projected field. A spotmeter might be if you have a bright enough source. But any mfg published chart, like the one in the previous post, will tell
you the facts. It's pretty difficult to get fully even illumination even with an ideal diffuser short of 180
focal length. Below that, the diffuser needs to compensate by having more density in the middle. The
difference between a 135 and 150 might be modest, but between there and a still longer lens is often
appreciable.