Apaflo Loc: Anchorage, Alaska

I'm talking about looking through the viewfinder while composing a shot. The lens aperture is wide open.

But the lens is not the only aperture! There are two others that affect the brightness and DOF seen through the viewfinder with wide aperture lenses. The aperture of the viewfinder light path is roughly f2 or a little wider. If you have an f/1.0 lens the scene as viewed through the viewfinder will not get any brighter at lens apertures wider than about f/2. So f/1.2 is exactly the same, through the viewfinder, for both brightness and DOF as f/2.0.

But the above applies to older cameras designed for film. Today, with a digital SLR it is also affected by the use of a different kind of viewfinder screen material. It is not ground glass meant to make focusing easier, it is a plastic micro prism to enhance brightness. That is used because the AF system robs some light, and manual focusing is not necessary (supposedly anyway). With a modern DSLR the effective aperture through the viewfinder is probably between f/2 and f/2.8, and may in some cases even be f/4.

That makes it difficult to focus an f/1.2 lens manually wide open because you can't see where the DOF edges are for f/1.2, only for f/2.8.

jeep_daddy Loc: Prescott AZ

That about covers it Bert. What else would you want.

imagemeister Loc: mid east Florida

This is ONLY on latest slr digital cameras ......NOT on older film cameras

Apaflo Loc: Anchorage, Alaska

The structure of the viewfinder, including the size of the mirror and the entrance to the pentaprism, has always presented an aperture of roughly f/2 (it depends on the model). That was true of 1950's SLR cameras.

The addition of Auto Focus using a partial mirror to send some light to the AF sensor (introduced before digital) reduced the need for ability to manual focus and increased the need for brighter viewfinder design, hence the introduction of micro prism plastic to replace the ground glass. That restricts the effective aperture even more, and in modern entry level DSLRs with pentamirror viewfinders the total combination might actually restrict the viewfinder to f/4.

An easy way to see this is look at a nicely illuminated scene with the camera on a tripod to keep it all steady. Use the widest lens you can. If it is f/1.4, set it to that. The depress the "DOF Preview" button, which will stop the lens down from widest to whatever it is set at. Of course if it is set at f/1.4 nothing should change. Move it to f/1.8 and do that again. Then try f/2 and so on until you can actually see a significant change in brightness. That will probably not happen until f/2.8.

Some years ago there was a site that listed the measured effective apertures of a few camera models, but I can't find it anymore.

burkphoto Loc: High Point, NC

*Fresnel* focusing area. I miss those. I had a screen with central split-image rangefinder and a fresnel collar around that. Great stuff!

imagemeister Loc: mid east Florida

Despite your theories, my Pentax spotmatic's viewfinder gets brighter with a f1.4 lens VS an f2 lens .....

Peterff Loc: O'er The Hills and Far Away, in Themyscira.

This view finder discussion is somewhat irrelevant to the original question and also rather academic. Is there much visible difference in an optical viewfinder in terms of brightness in bright daylight? No until you get to smaller than f/2.0. Does depth of field change? I don't see how that is affected by the viewfinder. I do see a difference between my 55mm f/1.2 lens, 50mm f/1.8 lens and 60mm f/2.8 lens. Viewfinder, not significant, other things, sure...

Perhaps we should consider practical realities rather than theory....

BebuLamar

Most owners of those lenses won't admit it but one of the advantages is bragging right.

Gene51 Loc: Yonkers, NY, now in LSD (LowerSlowerDelaware)

Peterff Loc: O'er The Hills and Far Away, in Themyscira.

What's wrong with bragging rights? Owning a lens or other equipment doesn't make someone a better photographer, but everything has a value associated with it. I like having a few old Canon manual focus lenses working on my EOS camera. They are fun to use, and not too many people have them. When I get my FDn 85mm f/1.2 converted and working I'll be very happy. Will others care? Maybe not, but it will deliver some interesting results and reactions.

forjava Loc: Half Moon Bay, CA

I think the rocking technique has emerged into digital cameras. Saw it in a YouTube video.
A variant on rocking that I use with digital is the focus rail.

Apaflo Loc: Anchorage, Alaska

You need to understand the theory in order to use the practical realities.

This is not about the DOF or brightness of the images captured. You can't switch between a 55mm f/1.2 lens and a 60mm f/2.8 and compare either the DOF or brightness in the viewfinder with any precision at all.

Put the 55mm f/1.2 on the camera and set it at f/1.2. Mount it on a tripod, and focus on a yard stick where you can see the DOF. Press the DOF preview button. At f/1.2 nothing should change. But now set the lens aperture to f/1.4 and do that again. Still nothing will change! Set the aperture at f/1.8, and still nothing will change. At some point between f/2 and f/4 you will see a significant change in the displayed DOF (and brightness). Probably on an entry level camera that will be closer to f/4 and on a high end camera it will be closer to f/2.

That isn't theory, that is practical experience that you need to have. With the 60mm f/2.8 lens if you want to manually focus you adjust the focus to be at it's sharpest. With the 55mm f/1.2 you have to rock focus back and forth to see where the edges of the viewfinder DOF are, and then set the focus in the middle of that to be between the edges of DOF at f/1.2 on the sensor.

so it goes...

I tend to shoot handheld in a lot of low light situations, so I need that ability in a lens. Another thing, and maybe it's just me, but with say a 1.4 lens, when I put the aperture to 3.5 or f4.0 my image is better than with a lens that starts at 3.5 or f4.0. just my opinion...

RichieC Loc: Adirondacks

The "sweet spot" in aperture on any lens are the middle apertures for sharpness, with either extremes avoided unless you have to. Obviously, a bigger aperture moves this "sweet spot" another stop or two brighter/faster and introduces very desired special effects like bokeh.

Finally, they are typically a better build/more expensive and thus will out perform at any slower lens especially of the same make, or no-one would ever spend the money.