I have 11 Pentax 67 lenses, and use some of them on Canon bodies. Why? The EOS bodies have superb electronics, and very precise stop-down metering, so I prefer Canon 35mm bodies over older Pentax 35mm bodies.
1. Obviously, saving money is a good reason to use the lenses you already own, rather than buying the same lenses twice. If an artist is satisfied with a lens, by all means use it! Filters are not cheap either and can easily double or triple the cost of a lens, and then you've got to buy a lens hood because Canon would like some more of your money!
2. Every manufacturer has a quality mix. You can buy cheap Pentax lenses, or very high quality Pentax lenses, retailing for thousands of dollars. My 67 series lenses are an example of high quality Pentax lenses, superb design, construction and image quality. Pentax also makes some great 35mm lenses, including a shift lens, macro lenses, super telephoto lenses, large aperture lenses, and so on--certainly of professional quality.
3. Canon plans to rule the world! I have 6 Canon prime lenses and am very impressed by four of them! But think about it: 10's of millions of photographers are all buying the same cameras and lenses. We have the same production and design, the same glass and coatings, the same fluorite elements and plastic barrels. The same electronics. The same background blur and correction of distortion or aberration. You can really see the sameness of Canon optics in the world of mass media, so I enjoy using lenses that have a distinctly different optical personality.
Lenses that are equivalent in engineering terms should produce images that are very similar, but I find this is not the case. I suspect our eyes and brain are capable of finer distinctions than most people are aware, such that minor differences in technology substantially change our response. But isn't that what visual art is all about? (!!!)
4. Pentax and other lenses may be a better value than Canon lenses, which sell for premium prices. This is different than the other situation of getting the most from equipment you already own. If you are happy with your results using other Pentax lenses, the chances are good you will be again with that telephoto lens you've been saving hard-earned money to buy. Should you buy Canon for twice the price? Isn't it easier to just paint a red ring around the front of your lens. :-)
5. Saving the best for last, it is always content that wins the day, or ruins an image. There is no escape from having to use your mind in the creation of visual art, and no matter what your credit card limit may be--fine optics cannot help failing visualization. As Ansel Adams so eloquently put it: "There is nothing worse than a sharp image of a fuzzy concept".
Three cheers for Ansel!
I actually have 47 reasons for using Pentax lenses whenever I choose, in a variety of adaptations (like a Pentax 645N body, which accepts all 67 lenses as if they were made for the 645 format). Five reasons should be good enough!
Adapters for Pentax K-->Canon EOS are readily available on eBay. My first adapter was not a very good design--I could never keep track of a tiny locking pin and eventually lost it. No problem, for short money I bought another adapter that works very well.
I can also use Hasselblad lenses on my Canon, including 80mm f/2.8 Planar, 120mm f/4 Makro Planar, and 250mm f/5.6 Sonnar. These lenses are every bit as good as fancy Zeiss lenses you can buy to fit Canon that cost a fortune.
I have another adapter to mount the Pentax 67II camera on the back of my Sinar F camera! What a rig! It's a pretty radical arrangement, but there are advantages. You get the Pentax 67II metering, viewing system, 120/220 roll film, and the choice of either leaf shutter or focal plane shutter. So far, it's not a very practical rig for short focal lengths, but it may be possible to change that with recessed lens boards. I have some Fuji view camera lenses, but could also use enlarger lenses (I have some excellent enlarging lenses, which have a flat-field design, ideal for some subjects. They also have large image circles.)
By age 55, I have collected more gear than I need and periodically unload various items. But there is a point to all this. Adaptations are very important in photography, and putting a Pentax lens on a Canon body is the kind of thing people start out doing. Don't let Canon or other manufacturers call all the shots! Imagine, design, invent, create your own rigs to do whatever you like. What for? Because it's your idea, like every single photo you take!
