One great feature of the A900 is that you can change the focusing screen without having to send your camera to an authorized repair center.
If you own an A900, this video guides you step by step on how to change the focusing screen yourself.
The focusing screen is that piece of glass that goes over the mirror that has the lines you end up seeing in the viewfinder. It can be changed to one that has a grid and the points of composition in compliance with the rule of thirds, or a plain but bright one for manual focusing.
You can find the Focusing Screen Type L and the Focusing Screen Type M in SonyStyle.
Bear in mind that the A900 is the only Alpha DSLR that allows the owner to change the focusing screens, the rest of the Alphas can have the screen changed, but only by authorized personnel, otherwise it may void your warranty.
If you attempt this kind of procedure, I suggest you watch the video a few times before proceeding. Get a good idea of what you need to do, practicing the movements first is also recommended.