This type of lens is needed for large magnification. Too much curvature of field when using 'regular' lenses at large magnification.

Main problem with wide apertures is there is very much light falloff. As you can imagine at large magnification, the lens is very close to the film, so this makes falloff worse from cosine effect. Check the light falloff curve on the Schneider site and see it is pretty bad. I did a print a few weeks ago and forgot to stop down and it really is pretty bad (falloff, not sharpness).

I need to use my 45 schneider HM for 16x20 and larger. Smaller prints work better with a 'regular' 50mm lens.