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On my D7000 in live view mode, taking a picture performs these steps:

  1. (Mirror is up, aperture is stopped down.)
  2. Shutter closes.
  3. Aperture opens all the way.
  4. Aperture closes back down.
  5. Shutter opens and closes.
  6. Aperture opens all the way back up.
    • There's a pause of about half a second between these steps.
  7. Aperture closes back down.
  8. Shutter opens, live view resumes.

Why does the aperture need to move at all? Couldn't it simply stay stopped down?

  • I'm in manual focus, so it's not autofocus.
  • I'm in manual exposure mode, so it's not metering.
  • I've set white balance manually, so it's not white balance
  • The high dynamic range mode (Active D-lighting) is off, so it's not that.
  • Flash is off.

I have noticed that if I change the aperture setting during live view, the aperture doesn't change right away - when you take the picture, it opens all the way up, then closes down to the new aperture setting. This doesn't explain why it opens and closes after the picture, though, since it stays at the new aperture.

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2  
How do you know the aperture opens and closes? –  ElendilTheTall Apr 6 '11 at 9:17
    
Does autofocus occur between steps 3 and 4? –  mattdm Apr 6 '11 at 12:02
    
@mattdm I believe autofocus occurs during step 1. –  Evan Krall Apr 6 '11 at 22:28
    
@ElendilTheTall I can see it through the front of my lens. –  Evan Krall Apr 7 '11 at 5:44

2 Answers 2

I would be willing to bet this is simply due to the camera changing modes back and forth between live view, actually exposing a photograph, and returning to live view.

  1. In live view, it sounds like it stops the aperture down to where it should be.
  2. When you take the shot, live view "exits", so the camera is set back to normal. That would reset the aperture to normal and close the shutter.
  3. However, since you are actually taking a photograph, it then has to restop the aperture, expose the sensor, then restore the camera back to its default settings again.
  4. Finally, since you were using live view, it starts up live view again, which will open the shutter and set the aperture appropriately for your live preview.

I've done some basic embedded systems programming in the past, and you often run into limitations that are tough to overcome given the nature of the hardware you are working with (which is usually very, very limited.) Its fairly common and pretty easy to simply use a basic state machine and workflow to track what "mode" the system is in, and have a reset action take place when you transition between states. In the case of a camera, you would want to make sure that everything is set back to "zero" or "default" state before exposing, so the logic that actually performs an exposure is working from known good starting values. It requires more registers, more memory, more constants, and more logic to share 'state data' between one 'machine state' and the next, when generally state machines are supposed to be globally data stateless in the purest sense of the word.

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This is certainly a possibility, though I doubt the lack-of-resources argument --- the camera has a zillion settings in the menu. –  Evan Krall Apr 13 '11 at 4:00
5  
I'm leaning towards this in general. It really fits from a programming point of view - once the camera is in a known state it can execute the same piece of code to take a picture. Less bugs that way. –  rfusca Apr 13 '11 at 4:14
3  
@Evan: Sure, there ARE zillions of settings, which is probably what the bulk of the existing storage and logic are dedicated to. The expensive processor in most DSLR's is the image processor, not the camera control IC, since that is the most intensive processing the camera does. IC yield is the biggest source of cost in electronic devices, so keeping the IC's you need for a camera as small as possible helps keep cost down. I would imagine camera manufacturers try to keep yield on the camera control chip as high as possible so they can invest in image processing and sensor chips. –  jrista Apr 13 '11 at 5:23
2  
I would expect the mirror to flip up and down as well if the camera were resetting to "normal" before taking the picture, but it doesn't. If they can bother to program the camera to not flip the mirror, it seems like they could avoid moving the aperture, too. Plus, the aperture movements take time; this adds delay to the shot. I'm still leaning towards there being a reason besides programmer laziness/lack of resources here. –  Evan Krall Apr 22 '11 at 9:10
2  
Old joke on the value of getting back to a known state: A mathematician is asked: "Suppose you walked by a burning house and saw a hydrant and a hose not connected to the hydrant. What would you do?" He answers: "I would attach the hose to the hydrant, turn on the water, and put out the fire." Then he was asked: "Suppose you walked by a house and saw a hose connected to a hydrant. What would you do?" He responded: "I would disconnect the hose from the hydrant and set the house on fire, reducing the problem to a previously solved form." –  coneslayer Apr 22 '11 at 11:23

The aperture opens as wide as it can to let in a maximum amount of light so the camera will have the best chance of focusing. When taking a photo without using live view the same process occurs. The D7000 does include a "Depth-of-field preview" button you can use to preview what the depth-of-field will look like.

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I'm not sure if that's entirely correct - the aperture is wide open while you're looking through the viewfinder, to aid focusing as you say, but it stops down when you press the shutter button to take the shot and stays stopped throughout the exposure. Evan Krall describes something that sounds odd to me: I can't think why the aperture should open up after the shutter closes, because focusing should have occurred before then. –  ElendilTheTall Apr 6 '11 at 12:12
    
@ElendilTheTall It sounded odd to me too; hence the question. –  Evan Krall Apr 7 '11 at 1:43

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