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It is my understanding that in regular conditions when using an SLR camera (No live-view) , the camera lifts the mirror to let the light hit the sensor. But while in Live-View, the mirror is already locked-up, so, at the time you press the shutter button, why the need to lower the mirror, and immediately lift it again just before exposure?

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which camera do you have? mine does not (40D) –  Michael Nielsen Dec 26 '13 at 18:27
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possible duplicate of Why does the D3100 mirror flip in live view? –  mattdm Dec 26 '13 at 19:36
    
@mattdm - I think that we may want to close in the other direction. This question is more generic than the suggested duplicate and there isn't really a solid answer on the existing question either. –  AJ Henderson Dec 26 '13 at 20:13
    
@AJHenderson Works for me, although I think the answer varies by camera so there might be room for both general and specific answers. –  mattdm Dec 26 '13 at 20:47
    
@mattdm - true, it could be that neither is really a duplicate. –  AJ Henderson Dec 26 '13 at 20:53

3 Answers 3

Two possibilities, focusing and metering.

Often when shooting in live view, the mirror snaps down to use the phase detect auto-focus, then snaps back up to take the image and remains up to keep live view going. If your camera supports using only contrast based auto-focus, then you can avoid that cycle.

Similarly, it may want to use the light metering capabilities that are available when the mirror is down, depending on how well the sensor can meter for itself as well.

In either case, it is often a setting that can be adjusted in the menus. I know my 5D Mark iii has a setting that will allow the photo to be taken directly in LiveView without the mirror flipping down.

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On a Canon 5D MK II one can set what type of AF is used - i.e. if the mirror stays up or whether it is lowered for AF. I would assume all later models will support both types of AF too. –  DetlevCM Dec 26 '13 at 18:14
    
@DetlevCM - yes, it is a setting on my 5D Mark III as well. I found out the hard way about it when I was at my sister's wedding. She told me I couldn't focus on taking photos, so I was trying to use LiveView to take them without having to hold up the camera to my face. Little did I realize that none of the photos actually took due to the stuff it had to do when the mirror flipped down. Oh well, after that it made me learn how the features work even if I only use LiveView once in a very blue moon. –  AJ Henderson Dec 26 '13 at 19:42
    
Metering is a reasonable possibility, but I've only encountered cameras that drop the mirror in LV for PDAF. Maybe @Itai can shed some light on any models that may do so for metering instead of AF reasons. –  Michael Clark Dec 27 '13 at 1:11

There is some interesting information about live view sequencing on Canon cameras here:

http://krebsmicro.com/Canon_EFSC/index.html

40D, 50D, 5d11, and 7D has the same operation that does not include the mirror movements. 450D, 500D and 1000D do include mirror operations because of their limited design where it is the same motor for mirror and shutter. You can see when this was written in the models on the list, but the general design strategy seems to be rebels vs semi pro and pro lines. So if you have a Canon camera I would guess it is a rebel and your answer is that it lowers the mirror due to this single motor design limitation.

It might be the same design principles in the other brands, if that's what you have.

If you are using flash ETTL2 it is another story. then it needs the mirror down for the initial preflash metering.

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Metering, not focusing. It's the final metering before the exposure is initiated.

Contrast focusing is used in live view, and phase detection is not engaged in the short period of time the mirror is dropped - watch your lens.

Its for metering, plain and simple.

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On all cameras? What if you are in a manual exposure mode? –  mattdm Dec 26 '13 at 19:36
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Good point about metering being another possibility. It is certainly not the only one though. I know that focus is the main factor in the mirror flipdown under certain settings on my 5D because I actually ended up with a problem where photos didn't get taken and did specific research about it after. If you don't believe it can be AF, look at page 212 of the 5D Mark iii manual. –  AJ Henderson Dec 26 '13 at 19:43
    
Many digital cameras meter using the sensor because they have no mirror or separate light meter. Most DSLRs that have a Live View mode that allows the mirror down just before the picture is taken do so to take advantage of PD AF. Most of those cameras also allow a setting that doesn't involve the mirror moving in which both metering and CD AF are done using the sensor. –  Michael Clark Dec 27 '13 at 0:58

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