Comparing an El-Nikkor 50mm f2.8N with a C.E.Rokkor-X 50mm f2.8

Got both of these lenses recently, the El-Nikkor is brand new and is the N version, and the Rokkor is used with a couple specks. I want to easily compare these lenses to decide which one I'll use. I was thinking of just taking portions of a 16x20 projected image (center and corners) and comparing with eye, and just comparing a regular 8x10 from each by eye. all from 35mm film. is there a better way?

If you want to make 16x20s, make full 16x20s. There is a doughnut shaped pattern of ... squishyness ... that is normal when the lens is going out of its range.

You usually don't see it until you're getting between 15x and 20x. Even then, you may not see it unless your enlarger is perfectly aligned and the negative flat.

Go by the grain structure of your normal film and developer. You're trying to image the grain sharply and evenly across the field of the 16x20. Plus X in Rodinal is a good test negative.

Both should do well at 15x, both should fail by 20x.

You should work at f/4 or f/5.6.

Potential problems: uneven easel, inaccurate focus, this may be out of the enlarger's potential.

On the other hand, if both make nice looking 16x20s without much difference, flip a coin. They should be equal to the task. Normally, problems are not found with the lens.

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df cardwell said:

Potential problems: uneven easel, inaccurate focus, this may be out of the enlarger's potential.

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im working on getting everything perfectly aligned. no use in spending money on darkroom equipment and then not taking the time to make sure it is working as well as it can.
I've read an article on another section of Apug that compares the Rokkor with other lenses and including an older El-nikkor, and it comes out on top. I usually don't do testing of any kind, but I'd just recently come across the 50mm 2.8N for $10 so I tacked it onto the purchase of an 80mm f5.6N for $30. if anyone is looking for these lenses, I've found a seller who has stacks of these brand new in boxes that he is looking to get rid of.