How to Mount Pentax K-Mount on Canon?

Question

I have an old Pentax 50mm 1:2 lens (from a Pentax Asahi K-1000) and a Vivitar 28mm f2.8 that I am trying to adapt to fit on my Canon Rebel t2i. I purchased two of these adapters but I cannot figure out how to attach them to my lenses. I've watched every video I could find on YouTube, and all of the demos show the adapters simply clicking into place. But when I line up the red dot on the back of the adapter with the red dot on the lens and twist, the adapter gets stuck. See photos for starting and end-stuck positions.

According to the videos I watched, the metal spring should move past the metal ridge until it locks down into place (about 30° I think). I've tried depressing the metal spring and twisting, but that doesn't seem to do anything. I don't have an eyeglasses screwdriver, so I stuck an Xacto knife between the spring and the metal ridge, but the adapter still wouldn't budge.

Both lenses mount on my Pentax K-1000 just fine. What am I doing wrong?

Answer 1

Did you see the following review on the Amazon page for the product you linked to?

As noted by other people who have bought this adapter, there are issues with its ability to securely mount the Pentax lens. After doing some cross-referencing with similar adapters, it is obvious that the retaining spring is on wrong! Someone had one job to do…. Spring needs to be taken off, bent back in the opposite direction and reinstalled with the slot in the spring toward the camera

Answer 2

Is the spring 'just a little bit too long'? I had that same problem. Unscrew it and move it a little. usually there are slots in it. The problem is that the bayonet flanges are not all of the same size. The camera uses a pin to lock the lens in place. I have multiple Pentax lenses and and a Sigma. The size of the flanges is not the same.