A stupid question on DSLR viewfinder

[martti]

The theme for my morning walk was DOF and bokeh. I was armed with my dear 5D III and my favorite lense the 35mm f/1.4 L.

To my surprise, the pictures that I took with the lense full open, seemed to show much less DOF than the image I saw in the viewfinder. I repeated the test a couple of times and really, the image through the viewfinder does not really give a realistic picture of what you get at f/1.4.

Is this because of the focusing screen that is in use or does the lense stop down to 2.8 or something before the shot is taken? This is just a question that came to my mind and I will not take any answers personally.
Does somebody know?

 

[Loswr]

The stock focusing screen is laser microetched for increased brightness with slow lenses. Consequence of that is it shows the DoF and VF brightness of slightly narrower than f/2.8 even with faster lenses.

You can see the effect on brightness indirectly - start wide open and hold the DoF preview button as you stop down the lens. The scene won't get dimmer at first, but the transmissive LCD will get progressively less washed out.

There are 3rd party focus screens that show the true DoF of fast lenses (as does the Eg/Ec-S screen for bodies with what Canon considers user-replacable focus screens.

 

[martti]

Thank you for confirming my observation. In fact, I ordered a full-matte focusing screen for my 5DIII but it was scratched already out of the package and the seller did not respond to any emails. Of course, they would have said that it was OK when they sent it and it was me, the stupid savage, who had scratched it....I lost about 80 dollars and two hours of my precious time in this experiment.