Constant vs. Variable Aperture zooms

TylerW · April 7, 2007

I'm slowly getting to the point where all of my zooms are constant aperture - I like that - any time the technology in my hands allows me to not think about its own shortcomings, I'm a happy man.

However, part of my problem with this thought is that I don't really know how the two are different in their design. Why do variable aperture zooms lose light on the long end? And if that's the case, do constant aperture zooms simply establish a lowest common denominator baseline and make the rest of the zoom range adhere to it?

This is pretty much a purely academic discussion, but what are some of the core design differences between constant aperture zooms vs. variables?

ziggy53 · April 7, 2007

TylerW wrote:

I'm slowly getting to the point where all of my zooms are constant aperture - I like that - any time the technology in my hands allows me to not think about its own shortcomings, I'm a happy man.

However, part of my problem with this thought is that I don't really know how the two are different in their design. Why do variable aperture zooms lose light on the long end? And if that's the case, do constant aperture zooms simply establish a lowest common denominator baseline and make the rest of the zoom range adhere to it?

This is pretty much a purely academic discussion, but what are some of the core design differences between constant aperture zooms vs. variables?

There is a lot of mis-information on the Internet about how "constant aperture" zooms work, but the most lucid explanation comes from Bob Shell:

Comments from Bob Shell (January 8, 2003):
"An f-stop is the ratio between the focal length of the lens and the *apparent* size of the lens opening as viewed through the front. It must take into account the magnification factor of all lens elements in front of the diaphragm, because it is the size of the opening that the light "sees" as it passes through the lens, not the actual physical diameter of the diaphragm opening.
It is this fact that allows companies to make constant aperture zoom lenses which maintain a constant f-stop when the focal length changes, because such lenses are designed so that the magnification factor (diopter value) of all elements in front of the diaphragm changes as focal length is changed to hold the aperture value constant."

Duckys54 · April 8, 2007

I, too, have also wondered this quite often.

pathfinder · November 28, 2007

ziggy53 wrote:

There is a lot of mis-information on the Internet about how "constant aperture" zooms work, but the most lucid explanation comes from Bob Shell:
Comments from Bob Shell (January 8, 2003):
"An f-stop is the ratio between the focal length of the lens and the *apparent* size of the lens opening as viewed through the front. It must take into account the magnification factor of all lens elements in front of the diaphragm, because it is the size of the opening that the light "sees" as it passes through the lens, not the actual physical diameter of the diaphragm opening.
It is this fact that allows companies to make constant aperture zoom lenses which maintain a constant f-stop when the focal length changes, because such lenses are designed so that the magnification factor (diopter value) of all elements in front of the diaphragm changes as focal length is changed to hold the aperture value constant."

Ziggy,

SO the ratio of the "apparent" diameter of the front lens element, not that actual size.

Can you point me to a lens where the rated aperture is larger than that of the actual ( not apparent) front lens element?

If the actual size is not a limiting factor, then why don't manufacturer's use this to create fast, ( large fstop) but small lenses? We all know that lenses that are faster f 1.2 are much larger than f 2.0 for instance.

ziggy53 · November 28, 2007

pathfinder wrote:

Ziggy,

SO the ratio of the "apparent" diameter of the front lens element, not that actual size.

Can you point me to a lens where the rated aperture is larger than that of the actual ( not apparent) front lens element?

If the actual size is not a limiting factor, then why don't manufacturer's use this to create fast, ( large fstop) but small lenses? We all know that lenses that are faster f 1.2 are much larger than f 2.0 for instance.

Obviously, the lens has a limiting factor, and in a simple telephoto zoom the limiting factor is generally at the long end of the zoom (the least efficient effective aperture).

Using the Canon EF 70-200mm, f2.8L USM lens as an example, measuring in the same way as you appear to have measured your EF 300mm, f2.8, I measured the diameter of the front element in it's mount. I measured 71mm, (which I don't consider perfectly accurate.)

Using the formula of focal length divided by front element diameter I get 200/71 = 2.8169014084507042253521126760563 (not f2.8 but close enough for this argument.)

Since this is a zoom lens, the 70mm focal length also uses the same front diameter of 71mm, so it could be calculated at an effective aperture of 70/71 = 0.98591549295774647887323943661972, or about f1, unfortunately not the case.

The sacrifice in effective aperture on this lens is obviously at the short end of the zoom.

nstam · March 20, 2008

pathfinder wrote:

Ziggy,

SO the ratio of the "apparent" diameter of the front lens element, not that actual size.

Can you point me to a lens where the rated aperture is larger than that of the actual ( not apparent) front lens element?

If the actual size is not a limiting factor, then why don't manufacturer's use this to create fast, ( large fstop) but small lenses? We all know that lenses that are faster f 1.2 are much larger than f 2.0 for instance.

i hope i am correct for these statements!

as iggy has shown you with the mathematical value... also any zoom lens that does does not have a constant aperture throughout has a much larger front element than needed at the wide end. the diameter of the front element is its size for the tele end of the lens.

that goes the same for any multiplier, 1.4x or 2.0x as canon makes. when you add these multipliers to any lens.... you automatically lose alot of the glass that is made!