f-stop and faster shutter speed

Discussion in 'Photography Beginners' Forum' started by Geno, Nov 4, 2008.

  1. Geno

    Geno TPF Noob!

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    Ok, so I did a quick search and couldn't find the answer I was looking for so here goes:

    If there are two lenses with identical focal lengths, both set at the same f-stop and sensor ISO, but one has a larger front element, will that lens allow faster shutter speeds?

    I am familiar with the fact that binoculars and rifle scopes with larger objective lenses produce brighter images in low light conditions, but I'm not sure how this translates when f-stop is introduced.

    What I am interested in is faster shutter speeds without changing ISO or focal length, and maintaining DOF.

    I hope this makes sense.

    Thanks
     
  2. goodoneian

    goodoneian TPF Noob!

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    that's a good question, i've been wondering the same thing as well
     
  3. Helen B

    Helen B TPF Noob!

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    No, it is solely a function of the f-number. Different lenses of the same focal length and the same f-number may have different size front elements, but they will have the same size of 'entrance pupil'. It is the size of the entrance pupil that is important, and you could consider the entrance pupil to be something like the equivalent of the objective of a simple lens in terms of light gathering power.

    The entrance pupil is the hole you see when you look into the lens - it is the image of the hole in the aperture blades created by the lens elements in front of the blades.

    Here are two lenses of the same focal length and f-number, but different front element diameter. You can see that the 'hole' in them (the entrance pupil) is the same size.

    [​IMG]

    Best,
    Helen
     
  4. Dubious Drewski

    Dubious Drewski TPF Noob!

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    And once again, Helen swoops into the thread and saves the day! Why are you always so helpful? And what are you doing to your tongue?
     
  5. Do'Urden's Eyes

    Do'Urden's Eyes TPF Noob!

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    I always though it looked like she was picking an eye lash or hair off of it. :lol:
     
  6. Geno

    Geno TPF Noob!

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    Thanks for the replies, that answers my question. Just to expand a little, why then do the long lenses of sports photographers have such large glass? If light transmission in unchanged, wouldn't a smaller glass be less expensive?

    Again, sorry if this seems like a dumb question, but I always appreciate the expertise of other members. :hail:

    Thanks.
     
  7. Big Mike

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

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    The longer the focal length, the larger the entrance pupil needs to be...to maintain the F number/aperture rating.
    Remember that the F numbers are actually a ratio. F4 is actually 1/4 or 1:4. meaning that the focal length is 4 times the diameter of the entrance pupil. So if you have a 100mm lens, the pupil at F4 is 25mm...but if it's a 400mm lens, the pupil at F4 is 100mm...so you need a very large (wide) lens body and a lot of glass.

    That's the way I understand it, I'm sure Helen can be more specific/accurate.
     
  8. Overread

    Overread has a hat around here somewhere Staff Member Supporting Member

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    also remember that when you use auto focus - the lens auto focuses with the light let in at the max aperture of the lens - so a lens with a larger max aperture will let in more light - thus will have advantages in a brighter image in the viewfinder and better lower light auto focusing abilities.
    The aperture blades only close when you press the shutter button
     
  9. Geno

    Geno TPF Noob!

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    Thank for the quick responses. That all makes sense now that it has been explained. Much appreciated.
     

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