Why aren’t large-range constant-aperture zooms like 18-135mm f/2.8 common?
Asked 11/4/2014
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I understand that the f-number is the focal length divided by the effective aperture (entrance pupil), not just the physical opening of the blades. So why are constant-aperture zooms with a very large zoom range—such as a hypothetical 18-135mm f/2.8—rare or unavailable, while lenses like 70-200mm f/2.8 are common?
Is this mainly a physics/optical-design limitation, or would such a lens just become too large, heavy, and expensive to be practical? Also, is the key issue the zoom ratio (for example, 18-135 is 7.5x while 70-200 is under 3x) rather than simply the difference in millimeters?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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The main reason we don't have "super zooms" with a large constant aperture is size/weight/costs. Roger at LensRentals recently blogged about this in the post:
About That 25-300mm f/2.8 You Wanted
About How Big is that?
The lens is in a video housing, so that makes it a bit larger than an SLR designed lens of the same specifications would be. But it's 16 inches long, which wouldn't change much if it were an SLR lens. That's more than twice as long as a Canon or Nikon 70-200 f/2.8. It's just about an inch longer than a Nikon or Canon 500mm f/4 lens.
Those of you who shoot with filters might be unhappy with the 135mm front element, and that wouldn't be any different if it was a photo, rather than video lens.
It weighs in at 18.5 pounds. To compare with something most people have handled, the Nikon and Canon 70-200 f/2.8 lenses weigh in at about 3.5 pounds. Even the Canon 600mm f/4 IS weighs only 8.7 pounds. The Nikon 600 f/4 VR is closer, at 11.5 pounds.
It seems to me that even if I could afford one, I wouldn't want to carry it around.
Originally by user7271. Source · Licensed CC BY-SA 4.0
user7271
11y ago
0
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They’re possible in principle, but usually impractical.
The big issue is zoom ratio, not the simple mm difference. A 70-200mm lens is under a 3x zoom, while 18-135mm is 7.5x. With a relatively small zoom ratio, designers can keep the zooming groups arranged so the lens can maintain a constant entrance pupil more feasibly. With a large-ratio zoom, more of the magnification tends to happen behind the aperture diaphragm, which doesn’t help the entrance pupil stay large enough for a constant f/2.8.
To keep 135mm at f/2.8, the entrance pupil must be roughly 48mm. Doing that across an 18-135mm range would require a much larger, longer, heavier, and more complex lens than typical consumer zooms. That also drives cost way up.
So the reason you don’t usually see lenses like 18-135mm f/2.8 is not that physics makes them impossible; it’s that the resulting lens would be extremely bulky, heavy, and expensive for most photographers. Constant-aperture zooms are much more practical when the zoom range is modest, which is why 24-70mm f/2.8 and 70-200mm f/2.8 are common, while superzooms usually have variable maximum apertures.
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