I've looked far and wide but i cannot find the answer for this question.

I know some older cameras and lenses used to have aperture rings on them, which allowed the user to set how open or closed the aperture is at any given moment. Now, on modern DSLR lenses, the aperture only closes when the shutter release button is pressed, and to view what your shot would be like with the aperture closed you must hold the Depth of Field preview button.

I know video lenses always have a ring to control the iris- really the same function as the aperture on a DSLR camera. Why don't modern DSLR lenses have aperture rings that allow the user to set an aperture and leave it? Why does the aperture only close to the stop that it is set for when the shutter release is pressed?

  • \$\begingroup\$ When the lens is at the final aperture the whole time (instead of only while exposing the frame), which sometimes happens when using adapted lenses for a different system, it's called "stop-down metering" because the TTL auto-exposure (assuming you have it) is taking a reading through a stopped down lens. \$\endgroup\$
    – hobbs
    Commented Feb 6, 2016 at 4:20

2 Answers 2


Why is the Depth of Field Preview button necessary?

With the lens wide open, as it normally is before you take the shot, you can't tell how much depth of field you'll get in the photograph. When you press the button, the lens is stopped down to the selected aperture letting you see the shot as it will be recorded, depth of field and all.

For both digital and film cameras, keeping the aperture wide open until the shutter is tripped allows maximum brightness in the viewfinder, which you normally want. It also allows enough light for autofocus systems, which often need an aperture of at least f/4 or even f/2.8 to function correctly. Older cameras that lack autofocus generally have some other focusing aid, such as a split prism and/or microprism ring, and these also work better at wider apertures.

I know some older cameras and lenses used to have aperture rings on them, which allowed the user to set how open or closed the aperture is at any given moment.

That's not really true, or at least it's not right for any of the SLR systems I know of. Many (if not most) older SLR's also had a depth of field preview button. You're right that they had an aperture ring to select the aperture, but the iris didn't close to the selected aperture until you pressed the shutter release button.

For example, I have an Olympus OM-1, and the lenses for that camera all have DOF preview buttons on the lens itself. Most of the SLR systems I can think of had a similar button or lever, although it was often located on the body rather than the lens. The main exception (in my memory) is the Pentax K1000, which lacked the DOF preview button, but still didn't stop down the lens until the shutter was triggered.

  • \$\begingroup\$ Things like auto-focus want as much light as possible, too. Even manual focus arrangements benefit from lots of light. \$\endgroup\$
    – user31502
    Commented Feb 5, 2016 at 22:27
  • \$\begingroup\$ @jdv You're absolutely right. Guess I should incorporate that. \$\endgroup\$
    – Caleb
    Commented Feb 5, 2016 at 22:28
  • 1
    \$\begingroup\$ Aside from more light, doesn't the shallower DOF also generally make it easier to manually focus with more speed/accuracy? I don't have much experience with rapid manual focus seeking, but always assumed that was the case. \$\endgroup\$ Commented Feb 5, 2016 at 23:54
  • \$\begingroup\$ @junkyardsparkle Probably so, but focussing screens generally had aids like split prisms and/or microprisms in the center of the viewfinder. These made it fairly easy to tell if you had focus or not. \$\endgroup\$
    – Caleb
    Commented Feb 6, 2016 at 1:07
  • 3
    \$\begingroup\$ Split prism focusing screens don't work if you stop down too far. Try it, at smaller apertures the split prism will actually go black. \$\endgroup\$ Commented Feb 6, 2016 at 2:05

why a lens's aperture only shuts to its specified stop when the shutter release is pressed, instead of staying static at that stop constantly

That's because a closed aperture would reduce the amount of light coming to your eye.

The viewfinder would be dim. Try the preview button on a lens with a wide open aperture like f1.4 and an aperture setting of f22 or even more: the viewfinder will become significantly darker.

The autofocus of (D)SLR cameras works better with an open aperture, too. With a closed aperture, the amount of usable AF points is reduced.

On a rangefinder or a TLR camera, this is not a Problem, because you are not looking through the lens.


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