Angel Star Photography Loc: Tacoma, WA

B&H sells the Fotodiox Pro products, too.

Blurryeyed Loc: NC Mountains.

David, the Pentax cameras including the M42 Spotmatics have a longer flange distance than do the EOS cameras. The flange distance is the distance from the camera mount flange to the focal plane (sensor), the reason that I collect Pentax Takumars is because they take a very simple adapter that is nothing more than a spacing ring so the lens is spaced the proper distance from your sensor. With the older Canon FD lenses or Older Minoltas for instance the flange distance is actually shorter than the Canon EOS so you need a corrective lens to accommodate mounting to your EOS camera because obviously you can not have the rear of the lens inside your camera body.

You can spend too much on these adapters as some of them can be somewhat expensive, but what you need is a simple spacing adapter, I do recommend the flangeless adapters, most come with a flange at the bottom of the thread which with some Takumar lenses will pinch the aperture activator pin on the bottom of the lens, when that happens the diaphragm will not properly function and sometimes the focus throw becomes difficult to operate.

Here is the one I use, the last time I purchased these I only paid $11 for them, they have obviously gone up in price but I highly recommend them.

I have been collecting and using these lenses for a long time, not just some guy shooting my mouth off about something I don't know anything about.

https://www.amazon.com/gp/product/B00CZ7EO5I/ref=oh_aui_detailpage_o05_s00?ie=UTF8&psc=1

DavidM Loc: New Orleans, LA

Wow, I really appreciate you posting this. One question because I'm trying to learn more and not doubting your experience.. What's the difference between the adapter you noted and the ones listed by amazon? Would these not take into account the issue you mentioned with the activator pin? I'm just wondering why these would be listed if they would encounter this problem?

https://www.amazon.com/s/?ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=pentax+m42+to+canon+EF+adapter&rh=i%3Aaps%2Ck%3Apentax+m42+to+canon+EF+adapter

Blurryeyed Loc: NC Mountains.

It's really hard to say why some have flanges and some don't, if you look closely at those adapters that you linked the Fotodox Pro does not have the flange. The problem is not present with every lens but when it does occur it is a real problem, the first time I went through it I thought that the seller I had purchased the lens from had ripped me off, but I was wrong, he explained the problem to me but it was one of the first of these lenses that I had purchased so of course I was mistrusting and returned the lens. Bummer because it was a rare lens. The next time I experienced it I searched and found the flangeless adapter which were hard to find back then and it fixed the problem for me. I have several adapters as you might imagine because I have so many of these lenses, so I have some of both kinds but if I were to purchase more it would be the flangeless. Don't forget that focus confirm chip is very helpful, those adapters you linked do not have it.

Just a heads up, with most lenses when you stop down you will have to make exposure adjustments, your camera's meter is not going work correctly with these lenses, my common experience has been as you shoot with smaller apertures that you have to go to a negative EV adjustment. I don't know how long you have been shooting and what your experience level is but I can tell you that shooting with these lenses will slow you down quite a bit and if you don't completely understand the exposure triangle you will learn it while using these lenses. You will probably pay better attention to your composition as well because of the time that you spend finding the focus on a manual lens.

Have fun and don't forget my offer if you find you don't use them much.