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?

1 Answer 1


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.

  • 3
    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, 2014 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 Oct 23, 2014 at 15:30
  • 3
    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, 2014 at 0:59

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.