Why doesn't my Lens Flipper™ work with Sigma lenses?

Question

OK, so here's a weird one...

Several months back, I bought myself a Lens Flipper. It seems to be a really useful thing to have! More recently, I bought a couple of Sigma lenses. Last night, I discovered something baffling: Even though both the Sigma lenses fit perfectly on my Nikon camera body, neither of them will mount properly on the Nikon-fit lens flipper. One lens will mount on one end of the flipper but not the other, the second lens point-blank won't mount onto either end [at least, not without applying far more force than you should apply to a delicate piece of precision engineering!]

I don't understand how this is physically possible. The mount on the lens flipper and the camera are identical. All my Nikon lenses mount effortlessly on both. The Sigma lenses mount perfectly on the camera body... but somehow not on the lens flipper. What's going on??

Answer 1

Different lenses protrude into the body cavity more or less, so even though the mounting ring is identical you may find that the sigma lenses have a different shape inside this ring.

I have several pentax and sigma lenses with a pentax fit, and I find that not all dust caps work on all lenses - looking closely its down to the shape of the lens inside the mounting ring.