How do constant f-stop zoom lenses work?

Blackwood · March 17, 2008

I'm not sure if this is the right place to ask this question, so if not, I apologize.

I'm trying to visualize the mechanism.

Say I have my 24-105 f/4 set to f/4 at 24mm. When I zoom to 105, the lens opening must bigger to remain at f/4. How does that work? Is the "screw" action that increases focal length also geared into the aperture blades? And what about push/pull zooms?

ziggy53 · March 17, 2008

The answer has to do with the concept of "apparent aperture" and the "apparent opening" of the lens.

A more complete answer here:

http://www.dgrin.com/showthread.php?t=58299

Blackwood · March 17, 2008

ziggy53 wrote:

The answer has to do with the concept of "apparent aperture" and the "apparent opening" of the lens.

A more complete answer here:

http://www.dgrin.com/showthread.php?t=58299

Cool, thanks ziggy.

claudermilk · March 18, 2008

Blackwood wrote:

And what about push/pull zooms?

Are there any constant-aperture push-pulls? The only "trombone" currently available that I can think of is the 100-400, and that's variable aperture (also a rarity in the L line).

Blackwood · March 18, 2008

claudermilk wrote:

Are there any constant-aperture push-pulls? The only "trombone" currently available that I can think of is the 100-400, and that's variable aperture (also a rarity in the L line).

I'm not sure. It was an afterthought.

Ric Grupe · March 18, 2008

claudermilk wrote:

The only "trombone" currently available that I can think of is the 100-400, and that's variable aperture (also a rarity in the L line).

And...the 28-300...one I've been considering.

pyry · March 18, 2008

claudermilk wrote:

Are there any constant-aperture push-pulls? The only "trombone" currently available that I can think of is the 100-400, and that's variable aperture (also a rarity in the L line).

Canon's line includes another trombone (

), the 28-300 L. That however isn't constant aperture. This is.

If they can make mechanics that extend by rotating a barrel, they can make it happen the other way too.

claudermilk · March 19, 2008

Interesting. I wonder why there isn't much (any) discussion of that lens. Looks like a fairly nice ultra zoom. The PZ test shows it's a decent performer. Looks like operation is about the same as the popular 100-400, which means I won't care for it.

Now back to the original question. I hit google & came up with this:
http://photography-on-the.net/forum/showthread.php?t=86975#post777253 (see post #4)

Robinivich · March 19, 2008

claudermilk wrote:

Are there any constant-aperture push-pulls? The only "trombone" currently available that I can think of is the 100-400, and that's variable aperture (also a rarity in the L line).

I think you might find that some very old xx-2xx zooms with constant apertures would be push pull, my dad had an OM mount vivitar that was a push pull 70-210, I don't think it was constant aperture, but it might have been, there are also some older canons, including a 100-300 in both regular and L incarnations that were push pull f5.6s

ziggy53 · March 19, 2008

I have a Canon FD 100-200mm, f5.6 that is a push-pull design and constant aperture.

Art Scott · March 19, 2008

Robinivich wrote:

I think you might find that some very old xx-2xx zooms with constant apertures would be push pull, my dad had an OM mount vivitar that was a push pull 70-210, I don't think it was constant aperture, but it might have been, there are also some older canons, including a 100-300 in both regular and L incarnations that were push pull f5.6s

That vivitar was prob the Series 1, which was a fantastic lens....I shot many a concert, portrait and wedding with the Series 1.....mine was a f2.8-4butthe best part was it stayed focused while pusing or pulling......

I do not think any is doing a single action is anymore......all the zooms I look at have a zoom ring and a focus ring.....which is too bad....it lly cuts down on stress when everything can be done with just one ring and having the subject stay in focus while manually zooming is fantastic......

Robinivich · March 19, 2008

Art Scott wrote:

That vivitar was prob the Series 1, which was a fantastic lens....I shot many a concert, portrait and wedding with the Series 1.....mine was a f2.8-4butthe best part was it stayed focused while pusing or pulling......

I do not think any is doing a single action is anymore......all the zooms I look at have a zoom ring and a focus ring.....which is too bad....it lly cuts down on stress when everything can be done with just one ring and having the subject stay in focus while manually zooming is fantastic......

It was a fantastic lens to feel, do I ever wish we still had that build quality. Sadly my dad's kit of an OM1, 50f1.4, 24f? and that vivitar, all eventually succumbed to salt spray and humidity (we're both sailors). Proof that in some ways, plastic can actually be tougher. (also that the tropical seas are the worst environment you can have for anything technological :cry)

How do constant f-stop zoom lenses work?

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