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I bought the new Sony A77 last month and I am interested to learn about how its translucent mirror technology works but couldn't find enough information on it.

My question is: does it have a shutter? If it does, is that shutter really mechanical and moving 12 times in 1 second?

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2 Answers

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Note that although the A77 does have a mechanical shutter, it also has an adjustment to pre-open the mechanical shutter, and just use an electronic shutter to start a picture. This reduces shutter lag (by a large percentage, but it's already normally short enough to be mostly imperceptible) and noise, which can be handy if you're shooting things like weddings.

Edit: as to why you have both electronic and mechanical shutters: you need an electronic shutter to do video. The mechanical shutter is way too slow and noisy to be suitable for shooting video.

For fast action you just about need the mechanical shutter though. An electronic shutter (on a CMOS sensor) takes around 1/50th to 1/60th of a second to read the data from all the lines in the sensor -- enough to visibly distort fast-moving objects.

The mechanical shutter can do somewhat the same thing, but reduces it from 1/50th of a second to 1/250th. That's enough shorter to eliminate most visible problems.

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so Jerry what I understand from you that the electronic shutter is really the one take the picture, and the mechanical works before and after it? – shobani Feb 17 '12 at 9:25
@shobani: not exactly. You can take pictures strictly with the mechanical shutter. You can also have it start the picture electronically, but stop mechanically -- and if I recall correctly, that's the default, at least a shutter speeds up to/including the X-sync speed. – Jerry Coffin Feb 17 '12 at 9:45
interesting :) .. this fires a couple of questions in mind.. why does it has two shutters? if the electronic shutter is capable of taking the picture why we still need the mechanical? and can I force which one the camera use, like forcing mechanical or electronic only? – shobani Feb 17 '12 at 9:55
@shobani: see edit in answer. – Jerry Coffin Feb 17 '12 at 10:57
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Yes, the A77 has a mechanical shutter and it does move at 12 FPS per second. There are a number of cameras with similar shutter-speeds including some ultra-zooms but indeed this is very fast.

What it does not have is a motor to move the mirror. This is more problematic for speed then the shutter itself since a mirror is heavy and has to move out of the way completely and back that fast.

Instead it has semi-transparent mirror (transluscent is not the right word technically). The mirror is made of flexible material that reflects about 30% of the light and lets the rest straight through. This makes the imaging sensor have to work a little harder but the autofocus sensor more so.

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Good answer Itai, thanks alot So is the sound that I hear when I take the shot is a mechanical sound or just a fake one? – shobani Feb 10 '12 at 22:32
Mostly the shutter. Make sure you have 'Audio Signals' disabled in the Setup menu since I do not remember if it adds a fake sound. – Itai Feb 10 '12 at 22:45
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I doubled check it .. I have it disabled and I still hear the sound, so I guess its the mechanical shutter sound. Many thanks again :) – shobani Feb 10 '12 at 22:49

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