What does is a "cam" inside a lens?

[j.weegee]

"There's a cam inside the 24-105mm lens that is designed to maintain an accurate focus when the lens is zoomed from tele towards wide." [Chuck Westfall, Canon USA]

I was trying to find a good parfocal zoom lens when I stumbled on to this quote regarding the EF 24-105mm f/4L lens. What exactly is a "cam" and what does it do to enable parfocal zoom in a lens?

-J. Weegee

 

[Big Mike]

A cam is a rotating or sliding piece in a mechanical linkage used especially in transforming rotary motion into linear motion or vice-versa.

Cam - Wikipedia, the free encyclopedia

It sounds like there is a cam in the lens that will adjust the focus as the lens is zoomed.

 

[Studio7Four]

Zoom lenses are comprised of multiple internal optical elements; these elements move to achieve different fields of view (zooming). Generally speaking, these optical elements are arranged into a few groups. The elements within a group travel as one, but the different groups move in relation to each other. That is, as you zoom, one group may move faster than another group, or may even reverse direction over the zoom range. The cam is what takes the rotary motion (turning your zoom ring) and converts it into linear motion (moving each group properly along the lens barrel).

I'm a little surprised to see this called out as a point of interest for that particular lens. I would expect that all but the simplest, lower quality zoom lenses (with only one moving group) have a cam, and even then I'd suspect a cam rather than other methods.

 

[Overread17 5]

Actually its fairly uncommon in DSLR zooms. Basically with AF and the fact that most photos are taken at a single focal length (you might zoom but the photo itself is taken at a single focal length only) there has just not been the demand for the focus point to remain fixed. Even some top end zooms (eg the 70-200mm f2.8 IS L MII and the 25-70mm f2.8 L) are not parfocal.

I suspect it will grow in importance as video becomes a stronger part of the DSLR package.

 

[prodigy2k7]

Couldn't this be useful for videos? Zooming without changing focus.

 

[j.weegee]

Thanks for your help guys. I;m really trying to understand the mechanics of lenses. This is a step in the right direction.

 

[Studio7Four]

Actually its fairly uncommon in DSLR zooms. Basically with AF and the fact that most photos are taken at a single focal length (you might zoom but the photo itself is taken at a single focal length only) there has just not been the demand for the focus point to remain fixed. Even some top end zooms (eg the 70-200mm f2.8 IS L MII and the 25-70mm f2.8 L) are not parfocal.

I suspect it will grow in importance as video becomes a stronger part of the DSLR package.

Thanks for the clarification - my experience in lens design isn't so much on the commercial end. That's a great point about AF being a quick enough refocus.

 

[Big Mike]

I suspect it will grow in importance as video becomes a stronger part of the DSLR package.

Canon has a handful of new 'Cinema EOS' lenses
Canon Cinema EOS | Products: Lenses-EOS

 

[KmH]

Parfocal zoom lenses were more common in the pre-auto focus era, because it really helps do manual focus when you can zoom in on your point of focus, focus, and then zoom back out without the focus changing.

Be prepared to spend siginificant money for a quality parfocal zoom lens.