[Rafael]

While I love my autofocus zooms, I also love my old MF primes for obvious reasons. However I picked up a Bell& Howell 28mm f2.8 prime last week and it behaves differently enough from some of my other lenses that I became confused and now seek guidance.

All thee of the lenses pictured here have a slightly different take on the P/K mount.

My "other" 28mm f2.8 lens, the Vivitar, doesn't auto-stop down: As you adjust the f-stop the lens stops down no matter what. The new Bell & Howell stays wide open until releasing the shutter, but the green button only stops down momentarily. My trusty old 50mm f-1.7 Sears super special stays wide open and stops down when pressing the green button as I would expect.

I noticed that all three lenses have different "takes" on the guard for the aperture tab, and I figure that's the difference. What I'd like to know is, is there some place that explains the differences in these mounts, and are there modifications I can do [e.g. filing down the bump in the gaurd, etc.] that I can do to make these lenses more usable? Thanks!

[boriscleto]

The Bell & Howell may not be shorting the pin it needs to. If you take a picture and look at the EXIFit will probably say M42 or no lens. You should be able to use some two sided foil tape to short the pin or file off some of the black paint to expose the metal underneath.

[Lowell Goudge]

The difference might be the black painted lens mount. Sand it where the contact pins would be so the pins are shorted