The Nikkor 50mm 2.8 that you have will give a quite non-flat field when used for 16x20 and larger. Assuming your enlarger is aligned, you can focus the center and far edge by moving the enlarger head up and down (don't touch the focus knob). Then set the enlarger head at the linear midpoint between the two points of focus and stop down to about f16. You should be able to make a reasonable print, but if you intend to do a lot of images that size you might want to look at the HM Schneider or the Rodagon-G.
Originally Posted by M Carter