Focal Length/Lens for 16x20 prints

Randy Stewart said:

Nikon made a very nice 40mm 4.0 El-Nikkor just for your purpose. It is designed for optimum performance at greater enlargements. It is comparable to the Rodagon-WA discussed in this thread, but would not cost a ridiculous $1,100. Lenses like these are not regularly used, so they tend to be in like-new condition when found.

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I ran into a similar situation and ended up finding a pristine 40mm EL-Nikkor lens. But it does not seem to have a straight-forward wy to connect it tom my Omega D5 enlarger. According to KHB's lens charts, it should take the same mounting plate as the 50mm. KHB also recommends it only for diffusion heads. Why would that be? Is a condenser head too focused to cover the entire frame with that lens?

The Leitz Focotar 40mm F/2.8 lens that is standard on the Focomat v35 enlarger should solve your problem.