Filter Size (Lens Diameter) and Exposure

Discussion in 'Beyond the Basics' started by frXnz kafka, Jun 23, 2008.

  1. frXnz kafka

    frXnz kafka TPF Noob!

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    Been looking to upgrade my walk-about lens lately, and I've been trying to wrap my head around filter sizes. Google was no help, so I thought I'd bring it here.

    What is the difference, in terms of the amount of light coming in, between say a lens with a 58mm filter size and a lens with a 77mm filter size? Does the 77mm let in more light, allowing for a faster exposure? Or is there some other advantage I'm missing?
     
  2. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    You are sort of on the right track...but looking at the wrong thing and putting the cart before the horse.

    A lens with a bigger maximum aperture will let in more light...and that will be in the name of the lens as an F number.

    Now, in order to get that larger maximum aperture, lenses often have to be designed with a larger front element and therefore a larger filter diameter.
     
  3. brileyphotog

    brileyphotog TPF Noob!

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    Often is the key word...the Nikon 50mm f/1.8, for instance, has a 52mm filter size.

    Little number behind the "f/" = big aperture = faster lens...easy as pie
     
  4. AndrewG

    AndrewG TPF Noob!

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    The amount of light a lens will admit is governed by its maximum aperture given by the f number. The filter size (diameter across the filter thread) has no influence on exposure. Thus an exposure of, say, 125th and f8 will be the same whether the lens has a filter diameter of 39mm or 77mm.
     
  5. astrostu

    astrostu Guest

    Math-wise, the amount of light that comes through the lens goes as the diameter-squared. The f/number is directly related to the aperture (aperture/focal length or vice versa, can't remember off-hand).

    So, if you have an f/2.8 lens vs. an f/4.0 lens, then you could feasibly take photographs that are (2.8/4.0)^2 = (0.7)^2 = 0.49 => 49% as long with the f/2.8 lens vs. the f/4.0 lens and still get the same amount of light. This becomes very important when shooting in low-light situations where you need a faster shutter speed.

    As a rule of thumb, ever successively smaller aperture (so like 6.3 -> 5.4, or 1.6 -> 1.4) will allow you to capture the same amount of light in approximately 73% of the amount of time.
     
  6. frXnz kafka

    frXnz kafka TPF Noob!

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    I know all about aperture and F-stops, but surely there is a difference between an f/4 lens with a 58mm filter and an f/4 with a 77mm filter, right?
     
  7. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    Not in terms of light gathering ability or exposure. F4 is F4.

    Sometimes, the choice of filter size is more of a convenience. I believe that most of Nikon's pro level lenses are 77mm, even though some of them might not need to be that wide. This is good because the filters and lens caps are interchangeable between them.
     
  8. astrostu

    astrostu Guest

    What Big Mike said. I would also think that the smaller filter lens would be poorer quality, just 'cause if you're going to make big glass, usually it's higher quality.

    Do you have an example ofan f/4 58mm aperture vs. f/4 77mm aperture? If you have a specific example, we might be able to tell you better why that may be the case.
     
  9. christopher walrath

    christopher walrath No longer a newbie, moving up!

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    The 77 is probably on something like an UBER-zoom (28-200 or more). But beware, the front element sticks out far enough that you may need a spacer for a thread on lens. A filter threaded right on the front might make contact with the front glass causing scratching, a fault that the chap who sold me my 28-200 apparently didn't know before he shipped it out. Lesson learned and all . . .
     
  10. Mike_E

    Mike_E No longer a newbie, moving up!

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    There is also the matter of the sweet spot on a lens. The bit in the middle is the best for taking photos. So, the more in the middle the better the lens. ;)
     
  11. frXnz kafka

    frXnz kafka TPF Noob!

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    Huh? 77mm is the standard for most pro glass, it's not just for super-zooms.
     
  12. christopher walrath

    christopher walrath No longer a newbie, moving up!

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    Sorry. Film photographer here and never saw a 77mm on any lens I had for the first twenty years. 'Course I just had basic MD Rokkor's and Sekor C's.
     

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