Originally posted by microlight The KX is a good suggestion, but bear in mind that the mechanical KX, KM and K1000 were directly developed from the Spotmatic F, so probably had the same basic prisms and focusing screens.
That is an interesting thought. I don't own a Spotmatic F, though I do have a mid-60s vintage Spotmatic and a Spotmatic II as well as a KX. I did a quick comparison of all three with 55/1.8 lenses mounted. The two Spotmatics were about the same and the KX appeared to be about half again (1/2 stop) brighter.
FWIW, the viewfinder specifications for the K-series are mildly different from each other and from the Spotmatic F.
K2: 0.88x magnification, -0.8 diopter, 95% coverage
KX: 0.88x magnification, -0.8 diopter, 93% coverage
KM: 0.87x magnification, -1.0 diopter, N/A coverage*
K1000: 0.88x magnification, -1.0 diopter, N/A coverage*
Spotmatic F: 0.89x magnification, -1.0 diopter, 93% coverage
The other variable would be the focus screen. It is hard to say whether that is consistent over time or within the K-series. My comment would be that none of my pre-1978 bodies (Pentax and non-Pentax) have amazingly bright screens and that none are particularly dim.
For the OP...probably the brightest viewfinder on an M42 SLR would be the Voigtlander (Cosina) Bessaflex TM ($$$). An adapter for the MX might be the better idea unless you want the (highly recommended) convenience of using the automatic aperture actuation your lenses provide. My preference is M42 lenses on M42 bodies unless I am content with working SLOWLY.
Steve
* The camera manuals for both the KM and K1000 lack viewfinder coverage specs. The "
K-mount page" provides 87% and 88% respectively, though it is not clear where those numbers came from.