Why does a zoom lens’s maximum aperture often get smaller at longer focal lengths?
Asked 7/31/2011
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Many zoom lenses have a variable maximum aperture, such as f/3.5–5.6, where the widest available aperture decreases as you zoom in. Yet some zooms maintain a constant maximum aperture across the whole zoom range, usually at higher cost.
What is the technical reason for this behavior? If the aperture blades simply open wider or narrower, why can’t every zoom keep the same f-number at all focal lengths? Is the limitation mainly about image quality, or is there a more fundamental optical reason that requires more complex lens designs to overcome?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
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The notation f/4.0 is usually taken to mean the size of the opening left by the aperture blades is equal to the focal length divided by 4, so for a 400mm f/4.0 you'd expect to find a 100mm opening. So given the maximum size of the opening is clearly fixed, if your lens has a variable focal length you would expect f ratio to change as you zoom.
This is not quite correct however, f/4.0 only means that the image of the opening as seen through the front of the lens appears to be 100mm - if you look at the barrel of such a lens you'll see there's no space for a 100mm iris. In fact the aperture is smaller but is "magnified" by the lens elements.
So via clever (and expensive) optics its possible to make the aperture appear to get larger as you zoom, giving you a constant f number.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
15y ago
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The key reason is that the f-number depends on both focal length and the apparent size of the aperture: f-number = focal length ÷ entrance pupil diameter.
As a zoom lens gets longer in focal length, keeping the same f-number would require the entrance pupil to get larger too. For example, at 400mm and f/4, the entrance pupil must appear to be 100mm wide. In many zooms, the physical aperture mechanism cannot provide a large enough apparent opening throughout the zoom range, so the maximum f-number rises as you zoom in.
The aperture blades themselves are not the whole story. What matters is the entrance pupil — the image of the aperture as seen through the front of the lens. Lens elements can make that aperture appear larger, but designing a zoom so the entrance pupil scales appropriately across the whole zoom range requires more complex optics, larger elements, and higher cost.
So this is mainly a fundamental optical/design limitation, though constant-aperture zooms also tend to be bigger, heavier, and more expensive to maintain performance.
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