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I have a Nikkor 18-55 kit lens that has a 52mm filter, and recently I was looking that the 17-55 lens has a 77mm filter.

Is it because the latter is a better quality lens? I assume a bigger circle lets more light in.

Or because it has a greater maximum aperture (F/2.8 for the 17-55 vs F/3.5 for the kit lens)?

Or something else?

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1  
Filter thread sizes do not vary. Filter diameters vary. –  Jay Lance Photography Aug 25 '11 at 1:54
    
Thanks for all the answers, everyone of them contributed to the subject. –  Luciano Aug 26 '11 at 12:19

4 Answers 4

up vote 9 down vote accepted

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.

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Nikon went to great lengths back in the day to standardize on one filter size: 52mm. Even their old 35mm f/1.4 has that filter size! This was a bigger deal in the days of film as filters were much more widely used to control contrast in B&W and control colour casts in colour photography. Nowadays they're a bit more lenient. –  gerikson Aug 25 '11 at 8:21
    
+1 - Nice way to illustrate the principle. –  Nick Miners Aug 25 '11 at 10:30

There are a combination of factors affecting the filter thread size. Aperture is one, certainly, and is the most likely factor in the two lenses you refer to. However the focal length can also affect the filter size; a wider angle lens would need a larger front element than, say, a 50mm lens with the same maximum aperture simply because of the wider field of view. Aperture becomes a factor again with telephoto lenses as the physical size of the aperture, which is (focal length/f number) increases in direct proportion to the focal length.

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The filter thread size is generally mandated by the size of the front lens element. In the case of 50mm lenses, there are usually f/2.8 or f/2.5, f/1.8 or f/1.7, f/1.4, and f/1.2 aperture lenses. For a given focal length, to get a greater amount of maximum light, the lens body needs to be made wider to accommodate a larger maximum aperture, and the front lens element needs to be made larger to gather more light. It may not seem like much, but the difference in light between f/1.4 and f/1.2 is quite a bit, and the logistics of getting that much light down a 50mm lens without losing quality are rather complex.

In the case of the two lenses you listed, one has a constant aperture, while the other has a variable aperture. The difference between f/2.8 and f/3.5 is not that significant. The 18-55 has a maximum aperture of f/5.6 at 55mm, while the 17-55 has a maximum aperture of f/2.8 at 55mm. That IS a significant difference, as 55/5.6 = 9.8mm physical aperture while 55/2.8 = 19.6mm physical aperture, about double the size. That would be why the f/2.8 lens requires a larger front lens element, which thereby dictates a larger filter thread.

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I see, but the 50mm f/1.8 also has a 52mm. Looking at Evan's answer, it seems that it's also a factor how "deep" the lens is, and the 50 prime is shallow. –  Luciano Aug 25 '11 at 15:08
    
I guess there is another factor as well, that I did not mention. Lot of lens manufacturers try to maintain consistency with their filter sizes, to minimize the need for their customers to buy new filters for every lens. In the case of Canon, I think they try to keep the filter sizes at 58mm and 77mm, with one of their lenses, by necessity, having an 82mm filter, and some (cheaper) having a 52mm. In that respect, its simply a matter of consistency, and less a matter of lens requirements. –  jrista Aug 25 '11 at 23:23

One thing affecting front element size, and thus filter size, is also sensor size. On so called "crop" sensors, sensor itself is narrower, which results in lenses specifically engineered for those lenses to be smaller.

For example on Canon camp, EF lenses are for full frame sensors and EF-S for crop sensors. EF lenses can be mounted on EF-S cameras and work perfectly, but because they support also the full frame sensors, their lens elements need to be larger, too.

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