The advantage of the majority of m42 lenses, like the Super-Takumars for example, is that they have an Auto/Manual diaphragm switch, or slider, on the barrel, and it allows the lens to be focused wide-open, and then with one press of the Auto/Manual switch, you can close the lens down to the pre-selected shooting aperture, without the need to "count click-stops" to get to say, f/8. With a Nikon lens for example, it would be like 2/2.8/4/5.6/8, or "four clicks" that need to be counted in order to go from wide-open and down to f/8....with an m42 lens, ONE, single press! Also, the m42 adapter for EOS is glass-free. NO need for a glass element to achieve infinity focus. If you go to like the manual focus forum on-line, there are a lot of user reviews of various lenses. I think the Super-Takumar 135/3.5 has lovely bokeh. In the late 1960's, the Pentax Spotmatic and the Super-Takumar lenses were popular with professional photographers in Europe; these old lenses were "professional-caliber" lenses in their day, before the full ascension of Nikon, and before Canon really had anything decent. Canon cameras at that time were pretty clunky machines, and the Nikon F Photomic FTn and FTN models were big, ungainly beasts.