Why do similar focal-length lenses use different filter sizes?

Asked 8/24/2011

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My 18-55mm kit lens takes 52mm filters, but the Nikon 17-55mm f/2.8 uses 77mm filters. Why can two lenses with nearly the same focal range have such different filter thread sizes? Is it mainly because of better build/quality, the larger maximum aperture on the f/2.8 lens, the wider field of view, or other optical design factors?

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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As Nick mentioned, there are two reasons why a lens might need a large filter diameter:

  1. Your front element needs to be at least as large as your apparent aperture size.
  2. If your lens has a wide field of view, you may need a large front element to avoid vignetting.

In the particular case of the 17-55, I think it's more of the latter than the former -- the aperture on a 55mm f/2.8 is 19.6mm; far smaller than the 77mm filter size of the lens. Even the old Nikkor 55mm f/1.2 AI had a 52mm filter ring.

For visual proof, here's the lens at 55mm, f/2.8:

Nikkor 17-55 f/2.8G ED AF-S at 55mm, f/2.8, looking directly into the lens

As we can see, the apparent aperture is much smaller than the front element, even at f/2.8.

If we look at an angle at 55mm, f/2.8:

Nikkor 17-55 f/2.8G ED AF-S at 55mm, f/2.8, at an angle.

we see the edge of the image circle before the edge of the front element.

Considering 17mm,

Nikkor 17-55 f/2.8G ED AF-S at 17mm, f/2.8, looking directly into the lens

once again, the aperture is much smaller than the front element.

However, this time, if we tilt the lens,

Nikkor 17-55 f/2.8G ED AF-S at 55mm, f/2.8, at an angle.

we can still see through the lens at an extreme enough angle that our aperture appears adjacent to the edge of the front element. I'm fairly certain that the wide angle, in combination with the lens's long physical length, is the reason this lens needs such a large filter size.

Originally by user378. Source · Licensed CC BY-SA 4.0

user378

15y ago

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AI Answer

Generated from our catalog & community — verify before relying on it.

Filter size mostly follows the size of the front element, and that depends on lens design rather than “quality” alone.

The main reasons a lens may need a larger filter thread are:

  • a larger maximum aperture
  • a wider field of view that needs a larger front element to avoid vignetting

Aperture matters because the physical opening needed is related to focal length divided by f-number. Faster lenses often need to be physically wider. But that is not the whole story: wide-angle designs may also need a larger front element so off-axis light can reach the sensor without being cut off.

For your two examples, the 17-55mm f/2.8 is both faster and slightly wider, and its optical design is more demanding. The 77mm filter size is therefore not just about “letting more light in,” and it doesn’t automatically mean better image quality. It mainly reflects the lens’s required front element size to support its constant f/2.8 aperture and wide-angle coverage without vignetting.

UniqueBot

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15y ago

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