Why doesn’t a zoom lens’s physical aperture opening get larger as focal length increases?
Asked 3/30/2021
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I’m using an old Granit 80–200mm f/4.5 manual zoom on a Canon 550D, and I’m confused about how the f-number works while zooming.
I expected f/4.5 to mean the actual iris opening is focal length divided by 4.5, so at 80mm it would be about 17.8mm and at 200mm it would need to be about 44mm. But on this lens, the physical aperture opening seems to stay the same size as I zoom, yet exposure does not get darker. On my Canon 18–55mm kit lens, I can see the aperture change while zooming.
So is the old lens “lying” about the aperture, or am I misunderstanding what f-number refers to on a zoom lens? Why can exposure stay the same even if the physical iris doesn’t appear to change size?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
5y ago
2 Answers
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f/4.5 doesn't mean that the hole in the iris is physically 1/4.5 times the focal length. It means that the size of the image of the hole, as viewed from the front of the lens through the lens elements in front of it, is 1/4.5 times the focal length. The image of the hole is the entrance pupil.
If you can, look at the image of the hole from the front as you zoom the lens. You should see the image get bigger, which is what matters for collecting light.
Originally by user48508. Source · Licensed CC BY-SA 4.0
user48508
5y ago
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You’re misunderstanding what the f-number is based on. It is not the raw physical diameter of the iris blades. It is based on the entrance pupil: the apparent size of the aperture as seen through the front elements of the lens.
On many zoom lenses, the front optical groups magnify that aperture image as you zoom. So even if the physical iris itself does not move or get wider, its apparent size can increase in proportion to focal length. That keeps the lens at the same effective f-number and exposure.
So your Granit is not necessarily lying. If it is truly an 80–200mm f/4.5 lens, the entrance pupil is what matters, not the visible metal opening by itself.
Your Canon kit lens is different because it is a variable-aperture zoom (f/3.5–5.6), and its design may also physically adjust the iris while zooming to maintain a selected setting when possible.
In short: f-number on a zoom lens refers to effective aperture, not simply the literal hole size you can see looking at the diaphragm.
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