Zoom lens mechanics

[uplander]

I got a Canon EF 24-70L f/2.8 yesterday and was mildly surprised when I noticed that when turning the zooming ring from 70mm down 24mm the lens barrel extended. To honest I was quite confused to say the least at first because all the other lenses that I have, the barrel extends when zooming in the other direction (retracted at the wide end and extended at the long end).

There must be a lot of engineer in because I can't let this go as "Okay that's how it is". My mind needs to know why? It's never been enough for me to know how to do something, I've always needed to why it works that way. An example of this is could be, lets use unsharp mask in photoshop for instance. I know if I adjust the sliders for amount, radius and threshold, I can manipulate the image. But if I understand what each slider does and why and how it effects the image, I can better use this function to get the desired results.

I getting off on a tangent here, so back to lenses:er:. I started to think why? Does this have to do with this lens being a constant aperture?

I grab my 70-200 f/2.8 which doesn't extend externally but does so inside the barrel and aha! This lens operates as does the 24-70. So now my mind is thinking, these lenses operate this way because that is how they can keep a constant large aperture.

So now I take my Tokina 11-16 f/2.8 and look to see if I can see the same phenomenon. Hmm, I can't really say so visually.

So, is there anyone out there who can explain why these lenses work this way? My curiosity gnaws at me!

 

[Big Mike]

I can't explain it...but an odd one is the EF-S 18-55mm. I seem to remember that the barrel extends while zooming out...and contracts while zooming in...but it changes directions mid way through. The actual shortest length of the lens is probably somewhere around 25mm or something like that. :scratch:

 

[rufus5150]

From my reading, the 24-70 was designed as such so that the effective cover and blocking of the lens hood would be appropriate at the appropriate focal lengths (that is, by extending the tube, the lens hood effectively becomes shorter due to where it attached to the lens) so as not to create vignetting on anything less than the maximum focal length.

And Big Mike's right, the EF-S does change horses in the middle of the stream, if you were.

 

[rufus5150]

Here's one source ( Canon 24-70mm f/2.8L review ):

Contrary to what you would expect, both of these lenses are longest at the wide-angle end and shortest at 70mm. Zooming towards the wide-angle end, I noticed the front element moving up into the front of the hood, since it is mounted on a non-moving portion of the lens. Cutouts in the hood prevent it from vignette at the wide end. If you zoom back to 70mm, the lens retracts, resulting in a nice, deep hood at the long end. It's a fantastic design.

When referring to 'both' the reviewer is comparing the old 28-70 to the new 24-70.