Why is a lens front element often larger than focal length ÷ f-number?
Asked 1/25/2019
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I understand that the entrance pupil diameter for a lens is approximately focal length divided by f-number. For example, at 240mm and f/6.3, that works out to about 39mm. But many lenses have front elements and filter threads that are much larger than that. My Sony 24-240mm f/3.5-6.3, for instance, uses a 72mm filter size and its front element is not much smaller.
If the basic aperture calculation suggests a smaller diameter, why do real lenses often need a much larger front element? Is it just to accommodate angle of view, or are there other optical reasons?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
7y ago
2 Answers
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Lenses with very narrow angles of view require front elements that are roughly equivalent to the size of the entrance pupil. A typical telephoto prime lens will have a front element less than 10% larger than the entrance pupil at the lens' maximum aperture. This is because the light rays collected by the lens are almost perpendicular to the imaging plane and the entrance pupil will not be much larger than the diameter of the front element.
But with wider angles of view and their closer subject distances, the entrance pupil can be much larger than the front element:
A simple single element lens:
A multiple element compound lens:
If the front element of a wider angle lens were only large enough for the entrance pupil to be fully visible from subjects centered on the lens' optical axis, the lens would severely vignette the light coming from the off axis portions of the frame. Thus wide angle lenses tend to have much larger front elements than the size of the entrance pupil so that a larger portion of the entrance pupil is visible from the more peripheral parts of the field of view.
When portions of the lens' field of view are obstructed from a full view of the entrance pupil, it can result in dark corners and oddly shaped out of focus highlights. Consider wide aperture lenses with even a normal field of view:
Such a lens is said to have "cat's eye" bokeh:
Even when there is no mechanical vignetting caused by the lens barrel, from wider angles the entrance pupil appears to be an oblong shape, rather than a circle.
Compare these examples, all intended for full frame cameras:
- Canon EF 300mm f/4 has 77mm filter threads. 300mm/4 is 75mm
- Canon EF 100mm f/2 has 58mm filter threads. 100mm/2 is 50mm
- Canon EF 85mm f/1.8 has 58mm filter threads. 85/1.8 is 47mm
- Canon EF 50mm f/1.4 has 58mm filter threads. 50mm/1.4 is 36mm
- Canon EF 35mm f/2 has 67mm filter threads. 35/2 is 17 mm
- Canon EF 24mm f/1.4 has 77mm filter threads. 24mm/1.4 is 17mm
Your 24-240mm f/3.5-6.3 lens having a near 72mm wide front element probably is more about reducing vignetting at 24mm and f/3.5 than it is about the needed entrance pupil for 240mm and f/6.3.
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
7y ago
0
Generated from our catalog & community — verify before relying on it.
Yes. Focal length ÷ f-number gives the diameter of the entrance pupil, not necessarily the physical diameter of the front glass.
In a simple or long telephoto design with a narrow angle of view, the front element can be close in size to the entrance pupil. But in wider-angle and zoom lenses, the front element often must be larger so the aperture remains fully visible to off-axis rays from the edges of the frame. If it were only as large as the entrance pupil, the lens barrel or front optics could block those rays, causing mechanical vignetting.
Also, the entrance pupil is an image of the aperture stop as seen through the front lens groups. Because lens elements magnify or reduce that image, the physical front element can be quite different from the entrance pupil diameter.
For zooms, the design has to work across many focal lengths, focus distances, and field angles, so the front element is often sized for the most demanding case, not just the longest focal length at maximum aperture.
So the larger front element is mainly due to field coverage, pupil magnification, and avoiding vignetting—not just the nominal f-number calculation.
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