Well, this is fairly unusual—it appears that yesterday, in the comments to the "Another Internet Canard" post, both Ctein and I were wrong at the same time—usually one or the other of us has the situation covered even when the other makes a mistake. (Okay, okay, usually it's me who makes the mistake and Ctein who covers me.)
With some viewfinders (or, more precisely, viewing screens), faster lenses do not result in a brighter viewfinder image. As Oren explains, "it's because the screen in an AF SLR isn't a randomly diffuse ground glass (or ground plastic), it's an engineered light pipe with microlenses designed to accept light from a narrow field of view."
Okay. Sorry. But...everyone who's saying that a good test of this is to take a fast lens and watch the viewfinder as you stop down has still designed a bad test. Slow lenses have different geometry, different nodal point positions, and collect light from different angles than fast lenses do. If you want to compare the finder image of a slow lens vs. a fast lens, the best way to do it is to look at the viewfinder image of a fast lens and a slow lens, not the image of one fast lens at different apertures. Test what you are testing, not something else.
As I mentioned yesterday, I'm starting to like the gamey taste of crow (although I still don't like the taste of my own foot in my mouth).
I have modified the post accordingly.
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Original contents copyright 2011 by Michael C. Johnston and/or the bylined author. All Rights Reserved.
Featured Comment by Gordon Lewis: "Having just finished writing a 'cheatsheet' for Canon's Digital Learning Center on EOS focusing screens, I knew that Mike's bold statement (now amended) about how ƒ/1.4 lenses make the viewfinder brighter and easier to focus would soon result in his picking bits of crow out of his teeth. I wasn't so sure about Ctein, if only because he could be a vegetarian.
"The only thing of value I can add at this point is that Live View (for cameras that have it) is an excellent way to see exactly what depth-of-field you'll get at any given aperture. Just remember that you will have to manually stop the lens down to the taking aperture to preview depth-of-field, otherwise all focusing is at maximum aperture.
"Also keep in mind that if the display is set to auto-gain it will maintain the same brightness as you stop down. Some cameras let you set the display so that it's linked with the exposure histogram and so if the image likes 'just right' on the display it should look the same way in the actual photograph.
"In either case, if you have your camera mounted on a tripod, use 10X magnification, and focus manually at the taking aperture, you're practically guaranteed tack-sharpness at the plane of focus. You can even see whether your ƒ/1.4 lens has focus-shift. As for whether it's a good idea to use an ƒ/1.4 lens at maximum aperture, well, that's purely up to you."
Featured Comment by Ctein: "Yeah, I wrote my mea culpa under the original thread, for those who want to see my flavor of the humble pie. Based on some of the comments here, though, I think there are some people who are still a little confused. The bottom line on this is that it depends on the design of the focusing screen (and possibly the design of the viewfinder optics and lens). Some focusing screens will not show a loss of brightness until the aperture is smaller than X, where X is frequently ƒ/2.8, but that's no hard and fast rule. Others will."
Featured Comment by Bernard Piechal: "As for the focusing screen brightness, I've just checked it. On the camera that Mike sold just recently—Olympus OM-4. The simple result is: on the now-obsolete, traditional screen, ƒ/1.4 gives brighter image than ƒ/2. On the contrary, if I changed the screen for the 'modern' (#2–4, for OM users), the screen at ƒ/1.4 is not any brighter than at ƒ/2. The explanation is simple: the modern focusing screen is not groundglass, but the series of microlenses (on OM screens the lenses are even hexagonally spaced, not randomly). And these microlenses have their own numerical aperture (which defines the angle of acceptance of light rays, the more general 'version' of the f-number). Any rays coming on such focussing screen at bigger angle than angle of acceptance doesn't make it to the eye. When one opens the aperture all the additional rays comes at bigger angle of incidence, because that's what numerical aperture in optics defines. Also, yes, the optical situation of the ƒ/1.4 lens set at ƒ/2 is the same, as ƒ/2 lens wide open (as for the checking the difference in finder brightness between the two)."