i've never been entirely sure of how a split image focusing dealey-bopper in an slr works -- is it dependent on separation like a rangefinder? Something else?

I do know that there's no substitute for a good rangefinder, especially in dim light. Leica said its rangefinder was so accurate that if you put two pins, one inch in front of the other, a meter away and focused on the one in front you would notice a separation of the images on the one in back.

With an slr both pins would look the same, but I've tried that with a rangefinder and it works. With a very narrow depth of field, that error can be critical.

So, long way of saying -- if I were you I'd assume the ground glass is accurate, not the split image.

ps...i see some people already tried this with an slr and say they can see the difference there, too -- ok fine but i bet its a lot quicker in dim light with a rangefinder