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I understand what aperture is and how it applies to photography. Larger F number = less light but more depth of focus, smaller F number = more light but less depth of focus. On an older Nikon lens that I have, there is a little tang on the lens mount that I can move back and forth and will open up the iris. When the lens is mounted to the camera, it is opened up all the way. I understand that this is to aid in focusing through the viewfinder. However, when the camera takes the picture, the iris doesn't move. I am able to adjust the aperture and take photos, but how is it controlling the "iris".

A similar thing happens with modern lenes, but I can't see any iris at all.

So how does a modern DSLR camera and lens actually adjust it's aperture size?

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Modern automatic SLRs / DSLRs adjust the aperture in exactly the same way as manual SLRs, by closing the iris blades in the lens. The only difference is that it is closed/opened using an electric motor and this happens extremely quickly when firing the shutter, so you are probably unable to see it with the naked eye.

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    Most(?) SLRs have a "depth of field" preview button which you can use to see the iris - put your camera into aperture priority mode, set an aperture a couple of stops down from the maximum aperture of your lens and then, while looking into the lens, press the DoF preview button - you should see the iris blades in place. – Philip Kendall Oct 23 '14 at 14:58
  • Awesome! My Nikon D50 didn't have that. I'll have to try it on my wifes D90 when I get home. We have a Rebel XS at the office and found an unlabeled button on the left side under the lens release... pushed it and my mind is blown. Thank you – Flying Trashcan Oct 23 '14 at 15:30
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    Lacking a DoF preview, you can also just set a manual exposure that's long enough to observe the lens stopping down, e.g. 1 second at f/16. – coneslayer Oct 24 '14 at 0:59

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