Why is the front element of a telephoto lens larger than a wide angle lens?

A wide angle lens has a wide angle of view, therefore it would make sense that the front of the lens would also be wide. A telephoto lens has a narrow angle of view, so it would make sense that the angle would be smaller. However, this is the opposite of what one can see on real lenses.

Why is this the case?

Answer

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.