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Question about Apertures

Discussion in 'Lens Lust' started by rlacy, Aug 15, 2008.

  1. rlacy


    Apr 22, 2007
    San Diego
    I am reading Bryan Peterson's book "Understanding Exposure" and came to the section on apertures. Bryan talks about the relationship between focal length of the lens and aperture and the resulting diameter of the aperture blades when open.

    His example is the 50 mm f/1.4 lens. If you divide 50 mm by 1.4 the aperture blade opening diameter is 37.4 mm. That got me wondering about other lens'. That would mean that the 85 mm f/1.4 has an aperture blade opening diameter of 60.7 mm.

    So here is my question: If these two lens' were mounted on the same type of camera, using 1.4 as the aperture, with the same ISO and took an image of the same scene, would the shutter speed be less for the 85 because the aperture opening is larger--therefore allowing more light to enter the lens and strike the sensor more quickly?

    Just a question I have never considered before and I am not sure if I understand this correctly or not. Thoughts?


  2. loudtiger


    Sep 6, 2007
  3. rlacy


    Apr 22, 2007
    San Diego
    Thanks, but "no" what? No thoughts, or my conclusion is wrong and the shutter speed is not faster on the 85. If that is what you mean, can you tell me why?

    I know that the light meter reading is not dependent on the lens mounted on the camera. But, if the aperture blade openings are different--one is larger and lets in more light--why are the shutter speeds to achieve a good exposure the same? That is what I am struggling with and I can't get my mind around this yet. Any help would be appreciated.

    Last edited by a moderator: Aug 15, 2008
  4. pforsell


    Jan 15, 2008

    the 85/1.4 has narrower field of view and needs larger aperture opening diameter to be able to "suck" in the same amount of light as the 50/1.4.

    Furthermore the physical aperture opening diameter is not focal length divided by f-number. With that calculation you get the entrance pupil diameter but the entrance pupil is a virtual entity not a physical "hole".

    The lenses and the entrance pupil diameters are tuned so, that you get (almost) the same amount of light with the same f-stop. Lenses with more glass elements have more losses and that efficiency is expressed as T-stops.

    The only Nikkor lens that I know of that has both the F-stop and T-stop values expressed by the manufacturer is the AIS 300 F/2 (T 2.2). This lens has geometrical F-stop (which affects DOF) of F/2 but internal losses make it only a T 2.2 lens. You need to compensate 1/3 stop with the shutter speed or ISO and the camera meter naturally makes that compensation automatically.
  5. The light will be manipulated by each lens to give the same end result at the sensor level. If you keep it on Manual and shoot @ 14mm vs 200mm it will give identical results.
  6. ^ or what peter said :smile:
  7. rlacy


    Apr 22, 2007
    San Diego

    Thanks for the in-depth reply. I have read your post several times and I think the "gist" of it is about to come clear to me. You certainly seem to know a lot about optics--especially camera optics. I appreciate your answer and the time it took to write it.

  8. Edward,

    Peter is correct that the F/# controls the DOF while the T/# relates to the amount of light that gets to the film/detector. The shutter speed will be the same if the T/# of each lens is the same. The aperture computed using the focal lenght and F/# is the entrance pupil diameter. This pupil is the image of the physical aperture stop formed by all of the lens elements between the physical aperture stop and the object space.

  9. cotdt


    Jul 14, 2007
    Bay Area, USA
    I'm curious, does this also apply to retrofocus lenses?
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