Patent: Canon EF 2.8x Teleconverter

[Blaze]

A 70-200 f/2.8 at 70mm has a maximum opening 25mm in diameter, agree?

A 70-200 f/2.8 at 200mm has a maximum opening... 25mm in diameter.

AFAIK, there's no physical mechanism opening the blades wider as the focal length increases. Rather, the zooming in optically magnifies that 25mm aperture such that it appears to be 71mm in diameter (at the 200mm example).

I see no reason they couldn't employ that principle in teleconverters.

That said, sure, I probably I read it wrong, hence me asking what they meant by that bullet point.

I don't think you understand the fixed f-number zoom lenses properly. If the 70-200mm had a maximum opening of 25mm at 200mm, then it would only be f/8 zoomed in. There is in fact a physical mechanism opening the blades wider as the focal length increases.

No, it has 2.8, because the pupil is magnified by the front element moving relative to the pupil.

If they could make the blades open to 71mm in the same form factor, they sell it as a 70-200f/1-2.8.

Zooming a lens doesn't mechanically widen the pupil. It optically magnifies it.

http://www.dgrin.com/showpost.php?p=523730&postcount=2
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."

Hmmm. It seems I'm wrong then. Thanks for the correction.

It still doesn't completely make sense to me though. If it's possible to magnify the image while maintaining the same f-number, then couldn't they make TC's that go in front of the lens without sacrificing aperture? And why even make variable f-number zooms?

 

[AmbientLight]

1) Putting a teleconverter in front of a lens won't really work for two reasons. First you will not be able to attach it to different sized lenses, for example a 300mm f2.8 and a 300mm f4 lens have significantly different front elements. This alone should be a rather obvious reason. The second part is the optical setup. Here I can only point to the article about lens heritage, to throw some light at the reasons. Let us just say that technically this won't work with multiple different lenses either.

2) Zoom lenses with variable aperture are being built, because they are cheaper to make.