Viewfinder brightness limit

[KeithB]

Since I just got spanked* on another forum, I thought I would post the question here.

In a discussion on F/1.4 lenses, the brightness "limit" at f 2.8 came up.  I quoted Ken Rockwell from here:
http://www.kenrockwell.com/nikon/d70perf.htm#finder

And was told this was hogwash:
"Stuff and nonsense! How would the viewfinders do that, by magic?

The only way would be if open-aperture metering was "limited" to the f/2.8 stop, like a rev-limiter on an engine. And for all I know, maybe some cameras do this.

But in the spirit of empiricism, I actually DID perform my own experiments...I happen to have two identical camera bodies here, and two lenses of very close to the same focal length but of more than a stop difference in maximum aperture (31mm f/1.8 and 35mm f/2..

And, of course, the faster lens shows a brighter viewfinder image and less d.o.f. in a careful comparison. (Although I felt a little sheepish actually running the experiment, like when your friend is looking up and says, "how about that, someone wrote 'gullible' on the ceiling," and you look up at the ceiling.)

I do NOT want to pick a fight with your expert, which is why I elided his name, but that's just another eye-rolling internet myth as far as I'm concerned. Of course, maybe he's using a Whapoflex and his camera works differently than mine--I can't speak to that."

Any thoughts, Bob?

*Here is where I was spanked:
http://theonlinephotographer.typepad.com/the_online_photographer/2011/04/internet-canard.html#comments

[Bob Atkins]

While I'd be reluctant to cite KR as a reference for anything, in this case he's at least on the right track.

The standard Canon screens are indeed designed to work well with slower lenses. This includes cheap consumer zooms as well as excellent lenses like the 400/5.6L USM prime. A consequence of optimizing the lens to work well up to f5.6 is reduced performance at faster apertures  - e.g. f1.0 to f2.8

Canon make a screen (Super Precision Matte) specificaly designed for use with fast lenses, at the expense of a darker view with slow lenses.

See http://www.learn.usa.canon.com/app/pdfs/quickguides/CDLC_FocusingScreens_QuickGuide.pdf

I'm sure the standard screen will show some difference between a 50/1.2 and a 50/2.8, but the super precision matte screen will show more difference and give a brighter image

[KeithB]

Thanks Bob, Mike admitted he was wrong:
http://theonlinephotographer.typepad.com/the_online_photographer/2011/04/mmm-crow-tastes-good.html

If you don't like KR, (Though he is usually right about the engineering stuff) how about Chuck Westfall:
http://digitaljournalist.org/issue0804/tech-tips.html

[KeithB]

Addendum:
I did not know that I could not use my vewfinder to evaluate critical focus at less than F/2.8!