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Every time you take a picture on a DSLR, not only does the shutter actuate but also the mirror actuates.

If using the LCD preview, and using phase detect autofocus, the mirror actuates too: the phase detect autofocus works only when the mirror is directing the light to the viewfinder. This time, the mirror is actuated when focusing, so if you need to focus multiple times before taking a photograph, the mirror actuates multiple times.

My questions are:

  • Is the mirror actuation count limited like the shutter actuation count?
  • Can you read the mirror actuation count somehow in modern DSLRs?
  • Which is a more limiting factor: a limited mirror actuation count or a limited shutter actuation count?
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    \$\begingroup\$ There is no limit - they are tested to work for ~XYZ actuations. If your camera is tested for 100 000 actuations, it will not stop to work on actuation # 100 001. The shutter could even fail much earlier - or much later (which is the case most of the time, AFAIK). \$\endgroup\$
    – flolilo
    Commented Feb 9, 2019 at 14:16
  • \$\begingroup\$ @flolilolilo In practice, there is an approximate limit based on statistical averages and that's what I meant... For example, everybody agrees that SSDs have a write endurance limit, but people generally understand that the SSD will not immediately fail after reaching the limit. \$\endgroup\$
    – juhist
    Commented Feb 9, 2019 at 14:18
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    \$\begingroup\$ My assumption would be that the mirror is a bit beefier than a shutter and not in need of any more complex mechanics. It slaps at the same rate every time. Because of this (bigger parts and less complexity), I’d imagine it lasts much, much longer. Interested to see if anyone has some data on it though. \$\endgroup\$
    – OnBreak.
    Commented Feb 9, 2019 at 15:58
  • \$\begingroup\$ @Hueco Some Canon cameras with Silent Shooting modes do have multiple "mirror speeds" at which the mirror is moved. Please seeWhat's the difference between Canon's silent and non-silent shooting? \$\endgroup\$
    – Michael C
    Commented Feb 23, 2019 at 2:03

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Mirror assemblies only usually need to be repaired when someone who doesn't know what they are doing messes with them. Most of the time the mirror assembly will outlast the shutter assembly.

At one point in time Canon factory service used the following protocol:

  • A first shutter replacement could be done replacing only the shutter assembly.
  • A second shutter replacement would only be done if they were also allowed to replace the other components inside the mirror box.
  • A third shutter replacement could again be done replacing only the shutter assembly.
  • A fourth shutter replacement would only be done if they were also allowed to replace the other components in the mirror box.
  • And so on. At least every other shutter replacement was required to also include mirror assembly replacement. If you had the mirror assembly replaced with a first shutter replacement, then the second shutter replacement did not require replacing anything else.

This policy was in force back in the 1990s and early 2000's in the early EOS era. Film cameras, particularly the pro models, tended to be used for longer periods of time than digital bodies do.

I have no idea if that policy is still in place. I've never had a shutter fail in a digital body before I replaced the body for other reasons. (Mainly to upgrade to a newer, more capable model that allowed me to do something the older model did not.)

Regarding your camera's shutter actuation count:

Many EOS cameras that allow third party software to access the "shutter count" and report it to you only include shutter actuations that happened during viewfinder based still image shooting. "Live View" actuations are not included at all.

From this answer to Can't see the shutter count on my canon 5d mark ii

The shutter count from EOS cameras with DiG!C III and later processors up to cameras released by the end of 2014 make the shutter count available through the remote controlled interface that may be accessed on the camera via a USB connection. The remote control interface may also be accessed via WiFi connection for cameras so equipped with WiFi capability. This includes all DiG!C III, DiG!C 4, and DiG!C 5/5+ cameras. The EOS 7D Mark II is the only DiG!C 6/6+ camera that allows shutter count access via the remote controlled interface. None of the models released since early 2015 with DiG!C 6/6+ or later processors allow the shutter count to be accessed without the proprietary tools used by Canon service centers. For more detailed information about specific models, please see this page at dire studio's website.

Your EOS 5D Mark II falls into the group that allows the remote control application to access the camera's shutter count for still images taken with the camera. Live View actuations are not reported via the remote control interface. Only the Canon service centers can extract that info from EOS cameras.

Note: Dire Studios has since found a way to access the shutter counts of some more recent EOS cameras. For some of those cameras, Live View actuations can also be reported using Dire Studios Live View Pack add-on to Shutter Count.

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