Considering the Nikon lenses:

Prime lenses:

Zoom lenses:

I don't see any relation between the size of the front glass and the focal length, focal range or image quality.

If we take only zoom lenses, there would be a link between the maximum aperture and the size of the glass, larger aperture requiring a larger glass. Actually, this is not true, since AF-S Nikkor 17-35mm f/2.8D IF-ED has a large maximum aperture, but a small front glass. Also, this doesn't work at all for prime lenses, where the lens with the largest aperture has the smallest front glass.

The quality of the lens doesn't seem to influence the size of the front glass neither, at least not for the prime lenses.

So what forces to make larger lenses with larger front elements?


3 Answers 3


Generally speaking, a larger front element is necessary to achieve a wider maximum aperture. More specifically, a larger front element helps achieve the necessary "entrance pupil" diameter required for a given lens, provides the necessary primary light-gathering power of a lens, and helps achieve the necessary angle of view of the lens. (The entrance pupil is the diameter of the physical aperture as viewed through the front of the lens.)

The physical diameter of a lens generally must increase as the maximum aperture increases, and once you pass f/2.8, each additional stop greatly increases the physical size of the lens. Additionally, once you pass f/2.8, each additional stop requires a considerably greater amount of light, and larger front lens elements are a key factor in gathering that additional light.

For ultra-wide angle lenses, such as the 14mm f/2.8, a larger lens element is often necessary to assist in capturing light rays from a wide enough angle of incidence, more so than for achieving a wide aperture (14/2.8 = 5mm physical aperture, quite small.)

For wider-aperture telephoto lenses, the physical aperture tends to be much larger, which tends to dictate the size of the front lens element more than the necessity of gathering wide-angle incident light rays. The 70-200mm f/2.8 lenses have a physical aperture of 71.4mm, some 14 times larger than the 14mm f/2.8 lens.

Lenses like the 70-300f/4.5-5.6 and 24-120 f/3.5-5.6 have much smaller maximum apertures for their focal lengths. 300/5.6 = 53mm, some 1.5 times smaller for 100mm greater focal length. A 300mm f/2.8 lens would require a 107mm aperture, which is twice as large as a 300 f/5.6, and would require a much larger front lens element to gather enough light to accommodate such a large aperture. The 80-400mm again has a fairly small maximum aperture at its longest focal length...400/5.6 is 71.4mm again, vs. 100mm for the 200/2 and 107mm for the 300/2.8. The 80-400mm lens has a larger front element than say the 14/2.8 or even a 50/1.4 due to the physical size of its aperture...which even at f/5.6 is considerably larger than any wide angle lens. A 50mm f/1.0 lens would have a physical aperture of 50mm, which over 20mm smaller than the 71.4mm of a 400/5.6 lens.


I think you are comparing way to wide of a range of lenses. Take a simple comparison, such as the Nikkor 85mm f/1.8D AF and 85mm f/1.4D AF. The 1.8 uses a 62mm filter, and the 1.4 uses a 77mm filter.

Another great comparison is the following set of lenses, where we have three apertures all at the same focal length. The Canon 50mm f/1.8, 50mm f/1.4, 50mm f/1.2L. The 1.8 uses a 52mm filter, the 1.4 a 58mm filter, and the 1.2 a 72mm filter.

The biggest factor will be the maximum aperture that the lens can achieve. A larger aperture requires a bigger lens, and thus a larger front element and filter.

Both Canon and Nikon prefer to keep a few common filter sizes throughout the range for comparability of filters and accessories. 77mm is common in the professional series for both, but depending on the lens a smaller filter may also be used in a professional lens.

Other factors do play a part, such as focal length - but none as big as a part as aperture to the front element/filter size.

Full line examples of filter sizes:


The size of the front glass of a lens is determined by the following:

  1. the bigger the sensor, the bigger the front glass. Canon EF lenses, made for full-frame cameras, generally have bigger front glasses than EF-S lenses, made for cropped sensor cameras.

  2. the bigger the max aperture, the bigger the front glass: everything being equal, a 50mm f1.2 lens is definitely having much bigger front glass than a 50mm f1.8 or even f1.4 lenses.

  3. the wider the focal length, the bigger front glass: everything else being equal, a 10mm wide-angle lens definitely have bigger front glass then a 50mm lens.

  4. the longer the physically length (not the focal length) of the lens, the bigger the front glass: The Canon pancake 40mm and the nifty-fifty primes have big apertures even though their front glasses are so small, is because they are physically so thin, so close to the sensor.

Normally a long focal length lens is also physically longer, and they would have bigger front glasses, just look at those 500mm and longer monster primes.

If the lens has long focal length but short physical length, the front glass would be smaller but its curvature would have to be larger to bend the light more, and it's better be made of super-duper LD glass to reduce aberrations and distortions as much as possible. Your example of the Nikkor lens with ED glass is a good example.

The reverse example would be those cheapo 2x tele-converters you can buy from ebay for $20, they are very short physically, have very small front glasses, and their IQ are crappier than toilet paper.


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