What is a cam?

[wintoid]

OK I've been reading about lenses and cameras for years, but some of the terminology has gone over my head.

I started reading about the Minolta 40mm Rokkor, which has a CLE and a CL version. There is apparently a difference in the cam between the two versions.

Can someone explain to me what a cam is and what it does?

 

[ferider]

The RF cam of a Leica lens is the part of the lens that moves the RF roller of the camera back and forth.

"cam" is a more generic mechanical term. For example, cams are used to move valves in most 4-cycle combustion engines.

Roland.

 

[tlitody]

it's (usually) a piece of curved metal with a very specific profile (curve) which is in contact with another piece of (usually) metal. As the lens focus is altered the cam on the lens moves the piece of metal it is touching which alters/moves something else. In the case of range finders it is moving the rangefinder mirror/prism to adjust the alignment of the rangefinder patch.

In a car engine the cam rotates above the cylinders and the curve on the cam controls the opening and closing of the engine valves. By adjusting the shape of the curve on the cam you can control the opening/closing/timing of the valves in relation to the firing.

So if your lenses have a cam profile designed for the rangefinder on particular camera and you use those lenses on a different make of camera, the adjustment the cam profile gives may not be match the rangefider adjustment of that other cameras design. Therefore you would need the cam modifying to suit the different camera.

 

[sevo]

http://en.wikipedia.org/wiki/Cam

As many lens couplers transfer force in only one direction, cams are very suitable for them, and many mechanical couplers on lenses are implemented as two-dimensional cams on the lens circumference or bottom plane.

Leica mount RF cameras in particular have a coupler (or cam follower) on the body side which tracks the cam motion of the lens and translates it into rangefinder movement. In the most trivial case, normal lenses, the whole inner barrel/optical block acts as the cam. To focus other focal lengths (where the barrel movement by distance is different), the lenses have to be equipped with a dual helicoid - one moves the cam (a vertically travelling inner ring or ring segment inside the barrel) which interfaces with the rangefinder, the other moves the inner barrel.

 

[wintoid]

Thanks guys.

 

[ornate_wrasse]

I'm glad this question was posted. I've been wanting to know this for awhile.

Thanks for the excellent explanations.

Ellen