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I have two related questions regarding behavior of aperture area/diameter with respect to various focal length in a zoom lens, like, Canon EF-S 18-135mm f/3.5-5.6 (with variable aperture and variable focal length).

Please note this question is not for "constant aperture" zoom lens rather it is for a zoom lens with variable aperture.

  1. In case of zoom lens, for example, Canon EF-S 18-135mm f/3.5-5.6 (with variable aperture and variable focal length). At any particular F-number (F-stop), for example at f/5.6 , does the diameter of the aperture increase with increases in the focal length? I mean, will the diameter of the aperture at 18mm and f/5.6 would be smaller than that of at 135 mm and f/5.6?

  2. In the above case (i.e. Canon EF-S 18-135mm f/3.5-5.6 ), What is amount of light that would enter the camera at 18mm and f/5.6 compared to that of at 135 mm and f/5.6? Will the amount of lights be same or different?

D.Prasant
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    The only difference between constant aperture lenses and variable aperture lenses is whether the magnification due to the lens elements between the physical aperture stop and the front of the lens enlarge the entrance pupil enough to maintain a constant f-ratio or not. – Michael C Mar 20 '17 at 11:47

1 Answers1

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1) Yes, the diameter of the aperture increase with increases in the focal length. You can check this by going into aperture priority mode, setting it at 5.6, then pressing the DOF button while looking into the lens, you'll see it's size change at the different focal lengths.

2) There's a couple different 18-135's on DXOMark.com, but it looks like about T4 at 18mm vs T6.2 at 135mm.

Mike Dixon
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  • The actual diameter of the aperture diaphragm doesn't change. What changes is the diameter of the entrance pupil, that is the apparent size of the aperture as viewed through the front of the lens. The entrance pupil size is affected by the magnification provided by the lens elements between the front of the lens and the aperture stop. – Michael C Mar 20 '17 at 11:46