How does maximum aperture change across a variable-aperture zoom lens?
Asked 9/17/2016
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On a variable-aperture zoom, such as an 18–55mm f/3.5–5.6 or 18–105mm f/3.5–5.6, does the maximum aperture change linearly as you zoom? For example, can you estimate the maximum aperture at 55mm by taking a midpoint between f/3.5 and f/5.6, or is the relationship more complicated? I’m also wondering whether that would imply a longer zoom could be noticeably brighter than a shorter one at the same focal length.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
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No, I doubt most lenses will be linear that way. Here is a Nikon 16-85mm result. It is a f/3.5 to f/5.6 lens.
The results reported in EXIF data are:
16mm zoom - reports 16mm at f.3.5
24mm zoom - reports 24mm at f.4
35mm zoom - reports 35mm at f/4.5
50mm zoom - reports 50mm at f/5
70mm zoom - reports 68mm at f/5.6
85mm zoom - reports 85 mm at f/5.6
16 to 35mm is roughly double, and the difference is 2/3 stops.
24 to 50mm is roughly double, and the difference is 2/3 stops.
35 to 70mm is roughly double, and the difference is 2/3 stops.
But don't assume linear. Modern lenses do all kinds of tricks inside, shifting elements to implement zoom and focus; both modify focal length.
If you want to know intermediate values for your lens, instead of guessing, you can simply do this same check and see what it reports. Do make sure your focused distance is several feet, say 6 or 8 feet anyway, because the focal length also changes with distance. The marked focal length is only valid for infinity focus. It will be different (but not reported) if closer. If up pretty close, the lens probably won't open to maximum aperture number (because the new true focal length computes a different number).
Originally by user38978. Source · Licensed CC BY-SA 4.0
user38978
9y ago
0
Generated from our catalog & community — verify before relying on it.
No—don’t assume a simple linear relationship between the numbers printed at each end of the zoom range. On variable-aperture zooms, the widest available aperture usually changes in steps as focal length increases, and the exact points depend on the lens design.
In practice, many lenses progress something like f/3.5 → f/4 → f/4.5 → f/5 → f/5.6 as you zoom in, rather than following a straight-line interpolation. Also, f-numbers are a ratio of focal length to entrance pupil diameter, so the behavior is not best understood by averaging the endpoint f-numbers.
For example, one Nikon 16–85mm f/3.5–5.6 reports approximately: 16mm f/3.5, 24mm f/4, 35mm f/4.5, 50mm f/5, 68mm f/5.6, 85mm f/5.6. That shows the maximum aperture changes at specific zoom positions, not continuously in a simple midpoint sense.
So your calculation of 55mm = f/4.55 is not a reliable way to compare lenses. A longer zoom may or may not be brighter at 55mm; you need the actual lens behavior. The easiest way to check is to zoom the lens and read the reported maximum aperture in-camera or in EXIF metadata.
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