Why thet 70-200 4 L is not a variable aperture lens of 1.4-4

[kphoto99]

I'm enjoying the 70-200 F4 IS L lens I recently got, but it made me wonder why the lens is a fix aperture.
The front element looks like it is 50mm, so using the formula of D*A=f (D diameter, A aperture, f focal length) I get the following theoretically possible:
70mm - 1.4
100mm - 2.0
140mm - 2.8
200mm - 4

Even if it would not be a 1.4 at the wide end, it could be 2.8 till about 135mm. Would that not be better then the fixed aperture?
What are the reasons for keeping it everywhere at 4?

 

[Canon Rumors]

Size and cost. A zoom lens is going to be a lot bigger than a prime at similar focal length and aperture.

 

[Loswr]

The formula you refer to applies only to true telephoto lens designs, the 70-200 is a mixed design. Consider the 35/1.4, the front element is much larger than 35 / 1.4 = 25mm.

Keeping it a constant f/4 simplifies design (= lower cost ), and results in better IQ (the additional glass needed for decent optical aberration corrections of a 70/1.4 - 200/4 zoom would likely result in a lens larger, heavier, and more expensive than the 70-200/2.8...then there's the issue of IS on an f/1.4 lens...).

OTOH, a 70-200mm f/1.0-2.8 would be pretty cool. 8)

 

[Bdube]

I'm enjoying the 70-200 F4 IS L lens I recently got, but it made me wonder why the lens is a fix aperture.
The front element looks like it is 50mm, so using the formula of D*A=f (D diameter, A aperture, f focal length) I get the following theoretically possible:
70mm - 1.4
100mm - 2.0
140mm - 2.8
200mm - 4

Even if it would not be a 1.4 at the wide end, it could be 2.8 till about 135mm. Would that not be better then the fixed aperture?
What are the reasons for keeping it everywhere at 4?

Your formula is valid when the lens is only a single element and the aperture stop is at the surface of the lens. Obviously in a 10+ element zoom lens it is not applicable.

It is true is that a 200mm f/4 lens has an entrance pupil 50mm in diameter and that EPD would produce a 70mm f/1.4 lens.

Where the zoom motion happens determines if the lens is constant aperture or variable aperture. The entrance pupil is used to define f number and is simply the image of the aperture stop as seen through the lenses in front of the aperture stop. If you zoom in front of the stop, the change in focal length zooms the lens and also "zooms" the entrance pupil, so the f# is unchanged. If you zoom behind the stop, the change in focal length does not affect the entrance pupil, so the f# changes with focal length.

For marketing reasons, constant aperture is preferable. It is often more difficult to design in that way within physical size constraints, but marketing wins out. Often the design is too difficult and the lens is cheated. They will use at least some zoom motions behind the stop and program the diaphragm to open or close with the zoom motion to maintain a constant aperture. There are several examples of this, off the top of my head I know the Nikon 14-24, Canon 16-35 IS, and Sigma 24-35 all do this. The "normal range" f/2.8 zooms likely do it as well. I am not so sure in the telephoto space.