To bluntly answer the question: "Why not enable it by default?" The obvious answer is "because it has drawbacks". Otherwise it would be enabled by default :). Looking at this article from Canon directly explaining the option for the 1DMkIII and 1DSMkIII, then at this one for the 7D, I think we can safely say that the effect will vary depending on the camera ...


In order to allow the camera to make less noise, the speed the mirror is moved out of the light box and/or the speed the shutter is reset for each shot is slowed down. This increases shutter lag and decreases the maximum number of frames per second. Sometime the maximum frame rates is more important, regardless of the noise produced by a camera. I doubt ...


It's slower in terms of frame rate, and there's a noticeably longer shutter lag.


The Canon 5D Mark III has a silent shutter mode. This reduces the mirror slap sound significantly, and is great in most situations. But it comes at the cost of a couple of frames per second if you're shooting in continuous mode.

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