How to simulate a photo being taken from the photographer's POV in a film?
Drawing out a viewfinder should not be a big deal. You can do this quicker than searching for one. Do it in photoshop or another decent app. Draw the lines with anti-aliasing on a transparent layer and export it as a PNG with transparency. You may want to set the layer compositing mode in FCP to be something other than the default, play around a bit.
Simulating closing of the iris is just a quick fade to black and back; the sound effect sells it.
Desaturation, the addition of grain. Don't over do it.


I would try 'pillar boxing' the viewfinder shot (4:3 inset in the 16:9 frame of the film), possibly reducing the size so it is inset from the outside frame completely (black all around in addition to being 4:3) as this will give more of the feeling of being 'inside' the camera viewfinder. Then when the taken photo is shown that is full frame. 

You may find that desaturating the viewfinder view and showing the taken photo with normal saturation does a better job of achieving what you want; this will depend on the narrative purpose of this effect.