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Most professional cameras have a burst mode function. Furthermore, if the camera is SLR, the optical viewfinder shows what happens between the images in the burst, but when an image is taken, there is a viewfinder blackout.

I understand that cameras perform autofocusing at wide open aperture.

Now, if I take a burst of images on an SLR, do the aperture blades continuously alternately move between wide-open and partially-closed positions? Does the potential movement of the aperture blades depend on whether continuous servo mode AF is used?

If I look through the viewfinder to see what happens between the images, do I see the world through a wide-open aperture or a partially closed aperture?

What if I'm using a ridiculously small aperture, say f/22? I think I can autofocus in burst mode using said aperture. Does this indicate the aperture blades actually move?

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  • Voting to close as too broad because the answer is not the same for all interchangeable lens cameras. – Michael C Sep 14 '19 at 12:57
  • Please limit the question to a single system. Thanks! – OnBreak. Sep 19 '19 at 16:55
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For an SLR, moving the aperture blades will be a lot less effort than moving the mirror. So I doubt that there is a universal answer independent of camera model.

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Any kind of focus tracking ("AI focus" and the like) requires the camera to refocus between shots, and focusing requires the lens to be fully open.

Also, you can use your DoF test button with aperture set to something small (f/16..) to see how the image looks like when the diaphragm is closed, and you will observe that this is a lot dimmer than what you see in the viewfinder during the burst: the diaphragm is reopened between the shots.

Experiment:

  • Set the camera to AV mode with a small aperture
  • Look down the lens and press/release the DoF Test button, you will see the diaphragm close and reopen
  • Set the camera to burst mode and shoot a burst while looking down the lens: you will see the diaphragm close and reopen
  • Repeat experiment for any focus mode (on my 70D, this happens already for "one-shot" AF...)
  • At the end of the experiment, erase all the compromising pictures of your nostrils.
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I tested my cheap Canon EOS 2000D with a really slow burst mode, with continuous shooting & servo AF.

First, I selected aperture f/5.6. Then, I started shooting. The viewfinder was bright between the shots.

Then, I selected aperture f/22 and started shooting. The viewfinder was still equally bright, no change from f/5.6.

Thus, I assume this camera with its horrendously slow burst rate will actually move the aperture blades. However, this is an entry-level DSLR so there can be difference to better DSLRs. I'm interested in answers for better DSLRs; I don't have such a DSLR.

My main camera today is an EOS RP mirrorless, and with it I can't really test because I'm not seeing the world through an optical viewfinder but rather through a digital display integrated in the EVF. The camera is in any case correcting for the exposure in the EVF automatically.

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  • Turn the camera around and look at the aperture through the front of the lens. Set it to F22 and look at it under a bright lamp. – xiota Sep 14 '19 at 12:18
  • @xiota Oh, I perhaps could test the RP after all... – juhist Sep 14 '19 at 12:20

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