Constant F-stop Zoom - how?

[Helen B]

I can't believe that no one mentioned that. It's not the physical size of the aperture blades. It's the apparent size of them that changes when you zoom.

It was mentioned in posts #2, #4 and a few others.

It's one of the reasons why it is important to explain that the f-number is calculated from the diameter of the entrance pupil, and not from the physical diameter of the iris.

One could say that there are four classes of zoom lens, as far as aperture goes:

Variable aperture, in which the iris does not change physical size when the lens is zoomed. The movement of the front elements does change the magnification of the iris (ie the diameter of the entrance pupil changes), therefore the ratio of f-numbers at the extremities of the zoom range does not match (is not inversely proportional to) the corresponding change in focal length.

Variable maximum aperture, constant thereafter. For example, a fast movie zoom might be f/1.6 max aperture when wide, f/2.2 when zoomed in, but be constant aperture beyond f/2.2. These used to be quite common, and they help when using an external lightmeter. The iris does not move when the lens is wide open, and therefore there is a change in the maximum aperture from one end of the zoom range to another. After that either the lens design (magnification of the iris) or that coupled with iris movement keeps the aperture constant.

Constant maximum aperture at all apertures, using a combination of iris movement and variable magnification of the iris.

Constant maximum aperture at all apertures, only using variable magnification of the iris.

Best,
Helen

 

[Infidel]

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."

Just for the record, this is the exact content from the link I posted earlier in the thread.

 

[prodigy2k7]

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."

Just for the record, this is the exact content from the link I posted earlier in the thread.

So it is. Too bad nobody read it (including me) haha...
I actually found it through a google search. Same forum post.

 

[Helen B]

It's not a complete answer, however. As I mentioned, not all constant aperture zooms achieve constant aperture solely by changing the magnification of the iris.

Best,
Helen

 

[prodigy2k7]

It's not a complete answer, however. As I mentioned, not all constant aperture zooms achieve constant aperture solely by changing the magnification of the iris.

Best,
Helen

Nice to see you on the forums! I love reading your posts.

Isn't another way is to move the physical location of the aperture forward or backward?

 

[epatsellis]

not likely, a better way would be to cam the actual iris opening to the zoom setting, varying the actual iris diameter.

 

[prodigy2k7]

not likely, a better way would be to cam the actual iris opening to the zoom setting, varying the actual iris diameter.

Can you explain this further please? I would like to know more.

 

[Garbz]

I think he's referring to physically making the iris open wider for a given focal length.

I'm not quite sure how that would work though. Size restrictions aside if you can do that it sounds like you'd be actively limiting your max aperture at the telephoto side if you do. Kind of like turning an 18-55 f/3.5-4.5 into an 18-55 f/4.5. If I'm envisioning this correctly.