Why can't I see bokeh in the viewfinder?

Question

I took this shot a few months ago, and something has been bugging me ever since. I was a bit surprised when I saw the image and saw the conspicuous "bokeh orbs". The thing is, I hadn't noticed anything of the sort when looking through the viewfinder. I hadn't noticed any particular highlights on the glass, and certainly didn't see such noticeable discs of light as you can see in the final image. They must have been there to a degree, but I certainly didn't expect the shot to turn out like it did.

I've been wondering why that is. I've searched for answers, and come up with a few possibilities of my own:

• the microprism filters the light such that a narrower beam of light is focused through the viewfinder, so you see more DOF than what goes through the shutter

• something to do with the human eye, in that your iris may provide more depth of field - your eye further focuses the light, certainly might be interpreted differently than light hitting a flat sensor

• the brightness and contrast of the final image may not match what was seen through the viewfinder. The room was fairly dark and the shot was at 1/60th second, so possibly the bokeh is brighter in the final image than I would have seen through the viewfinder. Maybe it was there, just not as prominent?

• I am imagining the whole thing, they were there all the time I just wasn't paying attention

Is it a combination of all of these (particularly the last one)?

Answer 1

It's probably a combination of two factors, firstly as you rightly suggest the focus screen plays a role - the view you see in the viewfinder effectively passes through a second aperture and so appears to be stopped down to about f/2.4 - f/2.8

So you can see bokeh through the viewfinder, it will just be much less pronounced due to the smaller effective aperture.

You can readily verify this, put on a fast lens stop down to f/5.6 and press the depth of field preview button (which closes the lens aperture, as if the camera were about to take a shot) you ought to see a darkening. Now stop down to f/2.5 and press the depth of field preview button, now the image in the viewfinder doesn't change as the lens is already stopped down that amount by the focus screen.

See also the accepted answer to this question:

Why is the depth-of-field preview in the optical viewfinder of my Canon 500D inaccurate?

Secondly you were probably focussing your attention on the bottle of merlot, the human eye only sees detail in a very narrow spot in the centre of your vision. The brain moves this spot around to create the illusion that you see the world in detail. If you were concentrating on the bottle you may simply have not actually 'seen' the bokeh in the background.