Is it possible to buy a screen that lets a DSLR act like my dad's old 35mm Nikon (either a post WWII or post Korean war model), where you see two images until you get focus, then you see one image?

I'd really like that.

http://estore.usa.canon.com/Specification.asp?ITEM_ID=36053 seems like just what you're looking for (it'd for a 1Ds3, isn'it? If not, some 3rd party make some, sometimes at the expense of loosing the AF red dots).

Yes, there is.

http://www.katzeyeoptics.com/

Regards,
Luis

I use a Beattie that has a split prism and reference guidelines. Truth is the guidelines are useful, but with live view I don't use the split any more. I got it from (as I recall) Adorama.

Thanks for the ideas. I'll try them. I was wondering, is the split screen technique more precise than watching the live view screen, or autofocus, or at least as good?

I might add that what I'm talking abuot, since I may have the terminology wrong, is that there is a sort of "double vision" when out of focus. As the two images come together, you have focus.

Also, can you tell me more about the lgiht loss over f5 and usability problems, or if that has been overcome by katzeye?

Good screens: http://www.brightscreen.com/styles.html
I use a brigthscreen in my Conatx645 and it's perfectly accurate.
From my point of view a screen like this is much better as live view especially for wider shots when the subject distance is just in front of infinity. For close shots and/or macro live view might be okay.

Actually it's not a "double vision", it's a split of the image and it's just in the center of the image.
Acording to the higher density of these screens there is some loss of light, yes. But in the center frame the image is brighter. You get used to it.
On the other hand the higher density is good. Better to judge about DOF when you press the control button.
You should know that spot metering could suffer when you replace the original screen. But center weighted should be fine.

The only 'double vision' focusing that I'm aware of is on an old rangefinder camera. Don't think it's adaptable to any form of SLR, but that's definitely not my area of expertise. Older SLRs had split focus and microprism focusing as well as the groundglass.

Quite right, and I grew up with two of the most venerable: an Exakta Varex lla and also b. Funny to think that the Exakta was ´state of the art´ during its day...

Rob C

The "ghost" image or double image rangefinders were used on rangefinder camearas and on press cameras. The Crown and Speed Graphics used the Kalaart rangefinders (a box about 5"x2"x3/4" sitting on top of the 4x5 Graphic) where two entrance pupils spaced about three and one half inches apart were converged by a pivoting mirror linked to the focusing mechanism. Rangefinder cameras used a similar widely spaced pair of lenses linked internally.

The wide spacing (wider means more critical focusing), pivoting mirror and linkage to the camera's focusing mechanism add lots of bulk. I can't imagine reasonable way to accommodate it to DSLRs.