Why can’t a variable-aperture zoom like 18-140mm open wider at the short end?
Asked 11/4/2024
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I’m looking at the Nikon AF-S DX NIKKOR 18-140mm f/3.5-5.6G ED VR. Since f-number is focal length divided by aperture diameter, it seems like at 140mm and f/5.6 the lens needs an effective opening of about 25mm. So at 18mm, shouldn’t that same lens be able to open much wider than f/3.5—at least in theory? What prevents a zoom lens from offering a much larger maximum aperture at shorter focal lengths?
Originally by Saurav Majumder. Source · Licensed CC BY-SA 4.0
Saurav Majumder
1y ago
2 Answers
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Because it is too hard to design a retrofocus UWA lens that is also sharp when using light from large angles (wide apertures). Because of design limitations you end up with a lot of optical aberrations when you try (soft focus, chromatic aberrations, coma, etc).
Also note that what matters is the size of the entrance pupil, which isn't really the objective element. It is the apparent size of the aperture restriction as viewed from the objective end, and as magnified by the optics at that end. If you look you will see that with a retrofocus design the entrance pupil is smaller than the objective element.
As the various lens elements move within a zoom lens it can change the magnification of the aperture restriction (constant f# zoom), or it can keep the magnification constant (variable f#). Or it can do something in between (smaller range variable), which is what your lens is doing.
Originally by Steven Kersting. Source · Licensed CC BY-SA 4.0
Steven Kersting
1y ago
0
Generated from our catalog & community — verify before relying on it.
Because the f-number is based on the lens’s entrance pupil, not simply the physical front element diameter. The entrance pupil is the apparent size of the aperture as seen through the front optical groups, and in a zoom lens that apparent size changes with the optical design.
At the wide end, especially in a retrofocus wide-angle design, making the lens much faster is difficult because light enters at steeper angles and optical aberrations rise quickly: softness, coma, chromatic aberration, and other issues become much harder to control. So even if the front of the lens seems large enough, the lens may not be able to deliver acceptable image quality at something like f/1.4 or f/2 at 18mm.
In zooms, moving lens groups can either help keep the effective aperture more constant or allow it to vary. A lens like 18-140mm f/3.5-5.6 is designed as a practical compromise among size, cost, zoom range, and image quality—not to be extremely fast at the wide end.
So the main limits are optical design and aberration control, not just simple opening diameter.
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