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These are exciting times for camera development.

http://www.dpreview.com/reviews/sonyslta55/

The company has rejected the traditional DSLR design and instead created a hybrid that, like a compact camera, is from the ground up built around live view, but one that is also capable of offering full-time DSLR-style phase-detection autofocus. The combination means they can offer features such as phase-detection AF during movie recording and extremely fast continuous shooting rates (10 frames per second on the A55), previously unthinkable at this price.

This is made possible by adopting an approach that has more in common with a mirrorless camera (like the Panasonic G2, for example) than an SLR by removing the bits that pretty much define such cameras: the optical viewfinder and moving mirror.

The designation 'SLT' stands for single lens translucent and it's the 'translucent' bit that's the key to what differentiates these new models both from conventional DSLRs and mirrorless interchangeable lens cameras. The SLTs do have mirrors, but they're mirrors that let the majority of the light pass straight through to the sensor, rather than having to swing out of the way to allow exposure. As a result they are fixed in position, always reflecting a portion of the light emerging from the back of the lens onto a phase-detection AF array housed in the top of the camera. (A newly-developed 15-point array in the case of these two cameras).
 

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There's a lot of potential with this design, but these initial cameras don't look like they quite have the horsepower to really take advantage of it.

You can shoot at 10 fps with full autofocus like this, not having to rely on predictive AF - but, the electronic viewfinder doesn't show you the "live" image of what you're shooting when you're clicking at 10 fps, rather it basically gives you a slide show of the images you've just shot - so you're looking at a notable delay between what's happening and what you're seeing.
So if the subject moves away from the AF point, or you're moving the camera too fast/slow to keep them in sight, it's plausible you're going to focus on something else and miss the shot.

Now it's hard to complain about that when they're giving you 10 fps with AF for less than $1000, but still, it's not replacing the AF/fps performance for the D3s/1dIV - at least not with these cameras.

The regular 6 fps (or whatever it is) mode, works normally I believe, with the live image visible in the viewfinder.

Some other disadvantages include a shorter battery life than we're used to with modern dSLRs, the long-term durability of the "mirror" is a potential issue if it's anything like the pellix cameras of old and any damage to the mirror has the potential to affect image quality, and finally, when the buffer is full on these, it can take longer than you'd expect to clear (reports are around 45 seconds in some cases) - again, needs more horsepower ;)

If Sony works some of these things out, they may have a real winning design on their hands. As it stands these are very nice initial offerings for the price, but there are some caveats.
 

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The regular 6 fps (or whatever it is) mode, works normally I believe, with the live image visible in the viewfinder.
According to the conclusion page (page 17) from dpreview:

"No live view in 10fps/6fps shooting makes panning almost impossible"

Agree with everything else you said, OldScotch. Even in 3fps and static shoot modes, I suspect live view may pause momentarily during AF like many (if not all) live-view-based cameras do. That's exactly why I'm not giving up my DSLR's optical viewfinder any time soon.
 
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