How do constant f-stop zoom lenses work?

Blackwood

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

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

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

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

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

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

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

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

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

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

Art Scott

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

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)