I have noticed that in my Canon 500D, the depth-of-field preview in the optical viewfinder is inaccurate with large aperture settings.
If I press the DoF preview button, there is very little difference between, say, f/1.8 and f/3.5. In particular, pressing the DoF preview button with f/1.8 vs. f/2.8 seems to make no difference at all.
Obviously, there is a huge difference in the photo, and certainly I can also see the same difference if I use live view (LCD screen) and the DoF preview button. And even with the optical viewfinder, the DoF preview button seems to work as expected with smaller apertures (say, the difference between f/4.0 and f/8.0 is clear and what I see in the viewfinder matches what I see in the photos).
What is going on? Exactly what limits the performance of the DoF preview button with the optical viewfinder, and what is the largest aperture with which it still produces "correct" results? Are there differences between different camera models regarding this aspect?
After a lot of googling, I was able to find this page which suggests that the focusing screen in the optical viewfinder might be the limiting factor:
"Oddly, these modern screens get no brighter when you're using a lens faster than f/2.8. Try it: put on an f/1.8 or other fast fixed lens and flick the depth of field button. You'll see no change in anything until you stop down to about f/2.5!"
Sounds familiar – but the above quotation is about Canon 5D, which is obviously a very different thing from my 500D.
I also found this page which is specifically about 500D, but the discussion thread seems to give few conclusive answers.