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I have read when we check the image from viewfinder, we can set the aperture, shutter speed and ISO based on the light coming through the shutter.

But, only when we click the shutter release is the image captured, when shutter opens and shuts for the decided "speed".

How can the camera meter when the shutter is actually shut?

I must be missing something. Can anyone please clarify?

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In SLRs, there is (often) a semi transparent mirror to direct the image to the view finder. Part of the image (and light) passes through, and is reflected in a different direction to go towards the focusing and metering sensors

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  • semi transparent (pellicle) mirrors are not "typical" in SLRs en.wikipedia.org/wiki/Pellicle_mirror – dav1dsm1th Jun 4 '14 at 9:00
  • @dav1dsm1th Added a source for you there. – Rowland Shaw Jun 4 '14 at 9:16
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    Thanks @Rowland Shaw for the link - today I learned several things, most significantly, I don't know as much as I think I do (and I shouldn't jump to conclusions about what I think someone is describing). – dav1dsm1th Jun 4 '14 at 9:52
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    For the vast majority of SLRs the light for metering does not pass through the semitransparent mirror. Rather it is reflected up through the focusing screen to the meter sensor housed in the prism. SLRs with PDAF have a mirror with a semitransparent section in the middle the allows light through to be reflected by a second mirror to the focusing array. – Michael C Jun 5 '14 at 5:25
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Compact and "mirrorless" cameras meter and focus directly off the image sensor. SLR's do things differently, however.

The usual setup is that somewhere up in the prism housing, on top of the camera body, there is a sensor or a set of sensors which handle metering. These skim a portion of the light off the viewfinder - in other terms, when you take the picture and the mirror flips up these sensors are actually blinded just as you are. No matter, they have already done their job by then.

(Similarly, as Rowland Shaw said, focusing is done by sending the light that has passed through the semitransparent center of the main mirror via a secondary mirror down into the bottom of the camera body to reach a set of autofocus sensors which live down there.)

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