Godfrey
somewhat colored
I have the early M-Rokkor 90mm f/4 ... you can tell it's the early one because it says "Made in Germany" on the lens barrel ... It was made in Wetzlar on the same production line as the Elmar-C 90/4 and is identical other than for the name on the bezel and the filter thread. At f/4, it has a bit of light falloff in the corners and edges. By f/5.6-f/8, that's all gone and it has a lovely imaging signature. It's small, not particularly light, and feels great in use.
The 40.5 filter size is the same as the M-Rokkor 40mm f/2 (which I had once upon a time) and also the SMC-Pentax-L 43mm f/1.9 Special (which I replaced my other 40s with). For a two lens travel kit, the Pentax 43 and Rokkor 90 work well since I can use the same filters on both.
G
The 40.5 filter size is the same as the M-Rokkor 40mm f/2 (which I had once upon a time) and also the SMC-Pentax-L 43mm f/1.9 Special (which I replaced my other 40s with). For a two lens travel kit, the Pentax 43 and Rokkor 90 work well since I can use the same filters on both.
G
For lenses available for sale, check the shopping cart shop.CameraQuest.com
the M 90/2.8 APO is listed there, both M and AIS versions
Godfrey
somewhat colored
Looks like a very nice lens, at a reasonable price! 
G
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Graham Line
Well-known
Have had the Minolta CLE version of the 90/4 with the 40.5 filter ring for many years and have always been pleased by the contrast and resolution with both color and B&W. At the time I bought it used, pricing was less than the comparable Leica lens. Don't think the 2.8 Voigt was available at the time. I generally shoot it at around f5.6 when possible.