Why are telephoto lenses often physically wider at the front than wide-angle lenses?
Asked 6/22/2011
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Wide-angle lenses cover a larger field of view, so it seems like they should need a wider front element. Telephoto lenses cover a narrower field of view, yet many of them have much larger front elements. Why is that? How do focal length, field of view, and maximum aperture affect the size of the front of the lens?
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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That is a generalization that is not always true.
The front element is not only sized to accommodate the field-of-view but also for the maximum aperture. Since aperture is measured as a fraction of focal-length, a longer lens requires a physically larger aperture to reach the same F-stop.
For example:
- The Nikkor 200mm F/2 lens requires an aperture 100mm across and you can see from the spec that the lens diameter is 124mm.
- The Nikkor 35mm F/2 lens requires an aperture 17.5mm across and you see that the diameter of lens is much smaller.
- The Nikkor 300mm F/4 lens requires an aperture 75mm across, so although it is longer than the 200mm F/2, does not need to have such a large front element and its diameter is 90mm.
- The Nikkor 14-24mm F/2.8 is an ultra-wide lens with a diameter of 98mm although its aperture only needs to be 8.6mm across (24/2.8) but it ends up much wider to accommodate the field-of-view.
Originally by user1620. Source · Licensed CC BY-SA 4.0
user1620
15y ago
0
Generated from our catalog & community — verify before relying on it.
The front element size is not determined mainly by angle of view. It is often driven more by the lens’s maximum aperture.
F-number is focal length divided by the diameter of the entrance pupil. So for the same f-stop, a longer focal length needs a physically larger opening:
- 35mm f/2 needs about a 17.5mm pupil
- 200mm f/2 needs about a 100mm pupil
- 300mm f/4 needs about a 75mm pupil
That’s why many telephoto lenses have large front elements: to support a relatively wide maximum aperture at a long focal length.
Field of view does matter, but more to the lens design and element curvature than to front diameter alone. A wide-angle lens can have a very wide view without needing a huge front element if its maximum aperture is modest. On the other hand, some ultra-wide, fast lenses also have very large front elements.
So the basic answer is: telephotos are often bigger at the front because maintaining a given f-number at a long focal length requires a larger physical aperture, not because they see a wider scene.
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