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Professional DSLRs can lock the mirror up before doing a shot — to prevent mirror-induced vibration. But as I can see here, cameras such as Nikon D4, while being capable of mirror lock-up, still bounce the mirror up-down on each shot in the series. This doesn't seem useful: the operator doesn't look into the optical viewfinder during the process anyway, so the mirror could be simply locked up, allowing for even more shots per second in the series.

Unlike the very basic DSLR cameras (e.g. Canon EOS 1100D), which are limited in a mechanical way to require the bounce on each shot so that MLU is not available, the more expensive cameras do provide this feature, so they shouldn't have the limitation.

So I guess there's some other technological reason to need this bounce. What is this reason?

  • why would you take burst shots without aiming the camera? – ths Oct 19 '19 at 11:47
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    @ths aim once, shoot multiple frames — with exposure bracketing, or to catch a moment when no one is blinking their eyes, etc.. – Ruslan Oct 19 '19 at 11:52
  • If the mirror is up, the you lose the view finder and camera loses AF and AE. I don't know you but I do look at the view finder when I take bursts, especially if its a sport scene that requires continuous tracking. – user3528438 Oct 21 '19 at 18:32
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Not all do.

The Canon¹ EOS 1D X and EOS 1D X Mark II, for instance, have limited options that allow for shooting bursts at their maximum frame rate only with the mirror locked up.

  • The EOS 1D X has a maximum frame rate of 12 fps using the viewfinder with the mirror flipping between each frame. It can do 14 fps with the mirror remaining locked up between each frame during viewfinder shooting.
  • The EOS 1D X Mark II has a maximum frame rate of 14 fps using the viewfinder with the mirror flipping between each frame. It can do 16 fps with the mirror remaining locked up between each frame during viewfinder shooting.

Even at longer exposure times, when the length of time the shutter remains open reduces the maximum number of fps², if the camera is set to shoot at the maximum "locked up" frame rate, the mirror will not cycle between frames.

Of course there is no autofocus or tracking done between frames: the exposure and AF position is set before the first image is taken and stays the same for the entire series.

There are many other DSLRs that can take multiple images without flipping the mirror down. The camera just needs to be in "Live View" mode with any autofocusing mode that flips the mirror down to use the dedicated PDAF system turned off.

Most Canon¹ DSLRs with Live view capability include this option of using imaging sensor based contrast detection AF or Dual Pixel CMOS AF on some models.

¹ I use examples from Canon's lineup because it is what I am most familiar with. The makers of other DSLRs offer similar options with at least some of their models.
² For example, if an exposure time of 1/4 second is selected the camera will be limited to less than 4 fps because it takes one second, plus the shutter cycle time, to do four frames at 1/4 second each. However, if 16 fps is selected in the maximum frame rate menu option for the 1D X Mark II, the mirror will not cycle between each 1/4 second exposure.

  • Your last paragraphs are they key, I think: DSLRs generally use phase detection autofocus, which requires the mirror up focus again in-between shots. – Roel Schroeven Oct 20 '19 at 10:15
  • @RoelSchroeven It was once the case that OVF AF used PDAF while imaging sensor based AF used CDAF, but that is no longer strictly the case. Many manufacturers have developed imaging sensors that do different "flavors" of PDAF using the main imaging sensor, such as Canon's Dual Pixel CMOS AF. Many MILCs also use similar or, in the case of Canon, identical forms of PDAF that are based on the imaging sensor. – Michael C Oct 20 '19 at 23:28
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This doesn't seem useful: the operator doesn't look into the optical viewfinder during the process anyway

You are mistaken... There are plenty of cases where you shoot bursts on mobile objects (sports, wildlife) and you need to be able to check that you are panning correctly and that they stay in the frame.

On earlier bridge cameras, extracting the data from the sensor would take time and during a burst (at 2 images/second...) the viewfinder would be dark most of the time, with just short glimpses of the subject between frames. This was enough to make them unusable.

  • This doesn't answer the question of why the user is not given an option to lock the mirror up or let it cycle between each shot in a burst or bracketing series. – Michael C Oct 19 '19 at 21:40
  • @MichaelC As I understand the question and escially its last sentence, it is not about an optional MLU, but why mirror bouncing exists at all. – xenoid Oct 19 '19 at 22:28
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    As I understand the question, it asks, "Why do DSLRs have to bounce their mirror when taking series photos?" – Michael C Oct 20 '19 at 8:09
  • I don't think the have part is important -- the OP incorrectly assumes that it's necessary, xenoid's answer explains why it's useful, and Michael's answer explains that it's not necessary. Between those two, we've got a decent answer. – Caleb Oct 21 '19 at 11:57

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