Why push/pull zoom is not widespread?

Question

The Canon EF 100-400mm f/4.5-5.6 L IS USM has a push/pull zoom mechanism instead of a zoom ring. Other lenses, like the Nikon 80-200mm f/2.8 AF released in 1988 use this type of zoom too, but the lenses with a push/pull zoom are very rare (none among today Nikkor lenses, I believe).

This zoom seems ways more practical and it is especially much faster to move the front element instead of rotating the ring. The possible issues like the lens extending or collapsing when pointed to the sky or to the ground are solved by the tension ring¹.

Why is this not widespread on pro telephoto lenses? What are the drawbacks? Is the only reason to avoid to move/rotate the front element because of the air flow?

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^{¹ In Canon EF 100-400mm f/4.5-5.6 L IS USM, the tension ring is close to the focus ring, which makes it easy to move the wrong ring by mistake, but this particular issue is related to the lens itself and not the push/pull zoom mechanism, so it's out of scope of this question.}

Answer

Not speaking for any company/lens designer, but I think it's a lot less practical now than it was in the manual focus days when one hand on one ring controlled both zoom and focus. The push/pull zoom was less precise than a rotating ring (it took some small force to move from any rest position, so overshooting was easy for critical composition), but managing everything from a single ring made life simpler overall.

But to extend the question a bit: why did push/pull focus fall from grace? After all, for a unit-focus lens (a lens where all of the elements/groups are in a fixed relationship with one another, and the whole assembly is moved toward or away from the film/sensor as a unit), that's all you need (and you can, if you wish, use push/pull focus alone on most rail-type view cameras). Two things make it impractical -- a lack of precision on the one hand, and the fact that lenses on small-format cameras don't tend to be unit-focus these days (internal focus, which is actually a sort of zoom, and corrective elements that float in relation to the rest are closer to the norm these days).

With focus largely outsourced to the camera, the more precise two-ring arrangement is a lot less annoying than it used to be (like on my old Minolta 35-70mm f/3.5MD). With focus control being mostly a matter of a button-push with the right hand, the left hand is free to concentrate on the zoom. With a ring, the only real annoyance is backlash -- there's usually a small but detectable "dead period" -- just the tiniest fraction of a degree -- when you reverse the direction you're rotating the ring. With a push/pull, on the other hand, there's usually some stuttering as you overcome the friction (the "tension ring"), so making fine adjustments after you've done the gross positioning is sometimes difficult and frustrating. You might think that really, really small adjustments aren't nearly as critical as a lot of people think they are (and I'd be inclined to agree with you), but as long as photographers are convinced that they have no pixels to waste at all, precision is going to win out over speed as long as the speed penalty isn't outrageous.

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Does the size of the front glass mean anything?

Question

Considering the Nikon lenses:

Prime lenses:

• AF Nikkor 14mm f/2.8D ED has a huge front glass (●₃),
• AF-S Nikkor 50mm f/1.8G has a tiny little front glass, deep inside the body (∙₁),
• AF-S VR Nikkor 105mm f/2.8G IF-ED has a tiny little front glass (∙₁),
• AF-S VR Nikkor 200mm f/2G IF-ED has a huge front glass (●₃),
• AF-S Nikkor 300mm f/2.8G ED VR II has a huge front glass (●₃),

Zoom lenses:

• AF-S Nikkor 14-24mm f/2.8G ED has a large front glass (●₃),
• AF-S Nikkor 24-70mm f/2.8G ED front lens is still much larger compared to non-professional lenses of the same range (●₃),

• AF-S Nikkor 70-200mm f/2.8G ED VR II has a large front glass (●₃).

• AF-S DX Nikkor 18-200mm f/3.5-5.6G ED VR II front lens is much smaller than any of the three lenses above, but, well, it's a DX lens (•₂),

• AF-S VR Nikkor 70-300mm f/4.5-5.6G IF-ED front lens is small too compared to the lenses below (•₂),
• AF-S VR Nikkor 24-120mm f/3.5-5.6G IF-ED front lens is again quite small (•₂),
• AF VR Nikkor 80-400mm f/4.5-5.6D ED front lens is pretty small compared to 200mm or 300mm or the 70-200mm (•₂).

I don't see any relation between the size of the front glass and the focal length, focal range or image quality.

If we take only zoom lenses, there would be a link between the maximum aperture and the size of the glass, larger aperture requiring a larger glass. Actually, this is not true, since AF-S Nikkor 17-35mm f/2.8D IF-ED has a large maximum aperture, but a small front glass. Also, this doesn't work at all for prime lenses, where the lens with the largest aperture has the smallest front glass.

The quality of the lens doesn't seem to influence the size of the front glass neither, at least not for the prime lenses.

So what forces to make larger lenses with larger front elements?

Answer

Generally speaking, a larger front element is necessary to achieve a wider maximum aperture. More specifically, a larger front element helps achieve the necessary "entrance pupil" diameter required for a given lens, provides the necessary primary light-gathering power of a lens, and helps achieve the necessary angle of view of the lens. (The entrance pupil is the diameter of the physical aperture as viewed through the front of the lens.)

The physical diameter of a lens generally must increase as the maximum aperture increases, and once you pass f/2.8, each additional stop greatly increases the physical size of the lens. Additionally, once you pass f/2.8, each additional stop requires a considerably greater amount of light, and larger front lens elements are a key factor in gathering that additional light.

For ultra-wide angle lenses, such as the 14mm f/2.8, a larger lens element is often necessary to assist in capturing light rays from a wide enough angle of incidence, more so than for achieving a wide aperture (14/2.8 = 5mm physical aperture, quite small.)

For wider-aperture telephoto lenses, the physical aperture tends to be much larger, which tends to dictate the size of the front lens element more than the necessity of gathering wide-angle incident light rays. The 70-200mm f/2.8 lenses have a physical aperture of 71.4mm, some 14 times larger than the 14mm f/2.8 lens.

Lenses like the 70-300f/4.5-5.6 and 24-120 f/3.5-5.6 have much smaller maximum apertures for their focal lengths. 300/5.6 = 53mm, some 1.5 times smaller for 100mm greater focal length. A 300mm f/2.8 lens would require a 107mm aperture, which is twice as large as a 300 f/5.6, and would require a much larger front lens element to gather enough light to accommodate such a large aperture. The 80-400mm again has a fairly small maximum aperture at its longest focal length...400/5.6 is 71.4mm again, vs. 100mm for the 200/2 and 107mm for the 300/2.8. The 80-400mm lens has a larger front element than say the 14/2.8 or even a 50/1.4 due to the physical size of its aperture...which even at f/5.6 is considerably larger than any wide angle lens. A 50mm f/1.0 lens would have a physical aperture of 50mm, which over 20mm smaller than the 71.4mm of a 400/5.6 lens.

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