I've been doing a little research on how/why Sony built these cameras the way they did and the more I find out, the more I'm impressed and stoked.
The bulk of this is a cut and paste job from somewhere else, but I figured it was worthwhile enough to repost here.
The Pellix was the first camera to use a pellicle mirror. It was a manual focus camera. It did allow for fast frame rates, and that was it's primary use. The technology progressed up to the Canon EOS RT which allowed for AF with the pellicule mirror.
Sony is using a semi-transparent mirror to allow for full time phase detect AF even in video mode. Something that had never been done before. High frame rates are an important side benefit, which I'll get back to in a second.
Here is a great image on how a traditional DSLR works...
As you can see a "regular" DSLR uses a "transparent" mirror as well. At least in part. The main mirror is transparent and has a second mirror attached that directs light into the AF unit located in base of the camera (which gets back to my point that splitting the light isn't in itself an innovation, rather the way it's used is). When you have to flip the first mirror up to take the exposure, you lose your ability to autofocus, for a split second if taking a still, or indefinitely if taking a video.
If you look, you'll also notice that your auto exposure meter is dependent on your shutter mirror as well, but this is less important as our digital sensors meter the amount of light that hit them during exposure.
The translucent mirror gets around this problem and never breaks the autofocus light path during exposure. The allows for, at least in theory, very fast tracking of moving subjects. This will be a boon for sports shooters. There are also the side benefits of less shutter lag, less vibration and no viewfinder blackout.
Now all of these processes need light to work, and everytime light passes thru a piece of glass/plastic, or bounces off a mirror there is light loss and dispersion. With the original Pellix, this was a big issue because of the relatively slow film speeds we used at the time (400 was a "fast" iso). A little light loss isn't as much of an issue today with our superfast iso. The dispersion is a bit of an issue for both SLT's and traditional DSLRs. The AF unit on the A55/A33 is where the penta-prism used to be on a camera. This means on an SLT, the light only has to bounce 1 time to hit the (two) autofoucs ccd(s). On a traditional DSLR, it has two bounce off two mirrors before it hits the ccd. It's two (mirror) planes times 3 dimensions that have to stay in alignment for AF to remain spot on. Twice the error you have to worry about if your mirror comes out of alignment (which could lead to front/back focus errors).
I also mentioned high frame rates an how the relate to other "innovations". High frame rates allow the software in the camera do all the cool "tricks" like HDR, Anti Motion Blur, stitched panos, 3D panos, twilight mulitshot and 3D 16:9 stills. All of those things can be done with lower frame rates, but it's are harder to get them to "look right".
As a side note, the original pellicle mirrors darkened over time (which lead to obvious problems). If it did, you had to send them back to cannon for replacement. Now I don't know if this will be a problem with sony's translucent mirrors, but they are designed to be replaced by the user if needed.
Much has fuss has been made over the lack of an optical viewfinder in these types of cameras. However, the new a77 will have a 3mp display EVF. That's approximately 2100x1400 pixles on the display. To give you an idea, it's better than the 1080p display of a HDTV, and that's impressive.
Here's a little video. A bit dry, but it does a good job explaining how AF works...
[video=youtube;HXA0mTZCQbI]
http://www.youtube.com/watch?v=HXA0mTZC ... r_embedded[/video]
So the more I find out, the more I'm impressed. I will say it now, the A77 will be my next camera...
