Same aperture, different focal lengths

Discussion in 'Photography Beginners' Forum' started by jamesino, Jan 29, 2008.

  1. jamesino

    jamesino TPF Noob!

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    Suppose if I have two identical camera bodies. One is mounted to a 100mm f/4 prime and the other a 50mm f/4 prime lens. Despite them having the same f -stop (40), the 100mm will have a larger aperture opening: 100/4=25mm, while 50/4=12.4mm. So does this mean that the camera with the 100mm lens will have more light hitting its sensor? If so, does this mean I can choose a faster shutter speed with the 100mm lens, while maintaining all other factors constant?
     
  2. Big Mike

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

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    No. While the aperture would be physically bigger on the 100mm at F4, than the 50mm at F4...the exposure of F4 remains the same.
    I believe this can be attributed to the wider FOV of the shorter lens...which means that it has more light gathering ability. (I'm not certain if that's the right explanation though).
     
  3. Helen B

    Helen B TPF Noob!

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    You are right that the 100 mm f/4 will let in more light from a single point (really a tiny area of an object) than the 50 mm f/4 - it will let in four times as much light from a single point, because the area of the entrance pupil of the 100 mm is four times that of the 50 mm (the diameter is double, as you have said).

    In the image, the light will be spread out more - in fact four times more - for the 100 mm vs the 50 mm, so both factors cancel out exactly, and the image brightness remains the same. At the same distance, the image of an object is twice the linear size with the 100 mm than with the 50 mm, which is four times the area.

    Does that make sense to you?

    Best,
    Helen

    PS
    There is a case where the image from the 100 mm at f/4 will be brighter than from the 50 mm at f/4: when the light is imaged to effectively a true point - ie a point with no real area, or a single photosite. This can happen with sharp images of stars.
     
  4. jamesino

    jamesino TPF Noob!

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    How come the light will be spread out more in the 100mm? Aren't both hitting the same sized sensors?
     
  5. Helen B

    Helen B TPF Noob!

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    The image of an object is larger with the 100 mm than with the 50 mm, at the same distance. More light arrives from the object to the image, but it has to cover a larger area. Sensor size has nothing to do with it.

    Best,
    Helen
     

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