I've recently bought some adapters that convert M42 universal thread mount lenses to Canon EF mount cameras. Since the 42 millimeter screw thread lens was made most famous by Pentax, this mount is often called Pentax screw mount. It is also known as universal mount among some people,and the mount style has been used on cameras marketed by Pentax,Praktica,Chinon,GAF,Ricoh,Fuji,and some other manufacturers also.
Why would anybody want to use "old lenses" on a new Canon digital camera? For me the answer, "Just because it's fun to use something new and different and cheap to get interesting pictures," springs to mind.
HOW TO FOCUS MANUALLY:The best way to focus by eye is to set the lens to infinity and to quickly and smoothly rack the lens closer until it "looks good", and then to STOP turning the focusing ring. Usually, that initial stopping point will be very,very close to the correct distance. Many times however,you will have run a little bit past the optimal distance,and a slight reversing of direction of the focusing ring, or a little hitch, will be enough to get you to perfect focus. Trust me: on almost any lens, starting at INFINITY and rapidly cranking the lens closer only until the image "looks good",and then stopping, will result in the most accurate focus,under most conditions. Older,manual focusing lenses by top optical companies (Nikon,Canon,Pentax,and Olympus) were designed to focus by hand and eye and often have relatively longer arcs of travel than autofocus lenses are designed with. Older manual focusing lenses often begin with an Infinity focus distance marking,followed by marked distances of 200,100,and 50 feet being found engraved on the focusing scales of 200mm lenses,for example. Newer AF lenses are designed to be focused by a computer working in conjunction with a small,high-precision motor,and most newer AF lenses have very,very poor manual focusing performance due to a typical Infinity focus distance,and then a mere 10-20 degrees of focusing ring travel until one is focused at 10 meters. Manually focusing a manual focusing lens is easier than focusing most autofocus lenses manually,in my experience.
I recently bought a trio of lenses in M42 mount. I bought a nearly identical-looking pair of Asahi Optical Company lenses, the shorter of which is a 135mm f/3.5 Super-Takumar and the longer a 200mm f/4 Super-Takumar. Both Super-Takumars have the mechanical AUTO-Manual diaphragm stop-down control on the rear part of the lens,which allows one to focus wide open and then to merely "flip" the lens down to shooting aperture without need for counting click stops or visually confirming that the lens has been stopped down to taking aperture. Both Super-Takumar lenses have purple-ish antireflective coatings on the front elements. Both Super-Takumars are all-metal designs,and both are in excellent mechanical and optical condition.
The Super-Takumar lenses have lengthy, scalloped,ribbed metal focusing rings which rotate somewhat slowly but very smoothly,with ample degrees of turning between Infinity, 200 feet,100 feet,and 50 feet on the 200 mm f/4 lens allowing the photographer to set focus by "eye" over an ample number of degrees. While the focus is somewhat "slow", the ample number of degrees of turning allows the lens to be focused without the hair-trigger touchiness of many of today's low-priced autofocus zooms, where the lens has a 45 degree arc of travel lock-to-lock.
The 200mm f/4 Super-Takumar focuses well by hand on the 5D. It has markings for Infinity, 200 feet, 100 feet, and 50 feet, over a span of something like 20 degrees,and that is just the far focusing range; the rest of the footage range has many degrees' worth of turn. The focusing throw is rather long,and it allows you to get reasonably close to "on" without risk of being grossly "out" on focus using the 5D's large but low-magnification finder. I do not have a lens shade for this lens,and when shooting toward the sun in the late afternoon, I've gotten some bad flare from it-arcing, half-frame bad. The 200 is sharp close-in. At 10 to 15 feet, it renders scenes with high detail on the 5D. The focusing is slowish and geared for focusing by hand and eye,and even though the lens is totally new to me, I'm getting 4 out of 5 shots well-focused. I've shot the 200mm mostly at f/5.6 and at that f/stop it renders very good,sharp images on the 5D.
The third M42 thread mount lens is a Vivitar 55mm f/2.8 Auto Macro made from the late 1970's to early 1980's. I did some on-line research and one fellow wrote that this Vivitar macro retailed for around $275 in the late 1970's,and was thus one of the more-costly third party lenses of the period. This lens extends a ridiculously long way, achieving full 1:1 image magnification with no need for a separate extension tube. It focuses from Infinity to 1:1 in about 690 degrees of travel,or ALMOST two full 360 degree turns of the focusing ring! The lens grows from 2.5 inches when focused at Infinity to just a hair over 4 and 7/8 inches in length when extended fully to its closest focus of 1:1 magnification.This Vivitar 55mm macro has a nice complete set of marked magnification ratios,engraved and filled with bright orange-painted markings and indexes with clearly legible settings of 1:1, 1:1.25, 1:1.5, 1:1.75, 1:2 or 1/2 life size, 1:1.25, 1:1.3, then 1:4, 1:5,1:7, 1:10, 1:20. At the closest macro-range magnification of 1:20, the lens is focused at a marked 4 feet, while at 1:10 the lens is focused at a little bit over two feet. At a foot and half, the magnification is approximately 1:7. This lens is difficult to focus accurately and repeatably at ranges of over about 10 feet on my EOS 5D. The focusing is very,very critical past 10 feet,and this is hair-trigger focusing outside the macro range is typical of most all macro lenses. At close ranges however, this lens focuses reasonably well in decent light,even when stopped down to f/5.6,as I am typically going to be doing with this lens.Bokeh is pretty good for a macro lens,especially foreground bokeh.