The A900 has so many features that I don't have room here to cover more than the highlights without this review turning into a sales brochure, but I really must make special mention of the viewfinder. Other reviewers have raved about it, and I find myself in complete agreement. I've been using the A900 a lot over the past few weeks, and going back to my own Alpha A100 is like watching TV after a night out at an IMAX cinema. The viewfinder uses a high-quality optical glass pentaprism and has 100 percent frame view. It also has dioptre adjustment, a flip-up shutter to prevent light entry on long exposures, a large well-cushioned surround and a proximity sensor that turns off the monitor when the camera is raised to your eye. The designers have had to do some clever mechanics with the larger-than-normal reflex mirror to get it to fit inside the body, but the result is unquestionably one of the best viewfinders I've ever used.
Like the EOS 5D MkII the A900 has no built-in flash. It has a hot-shoe for mounting an external flashgun, but it's worth remembering that Sony uses Minolta's old hot-shoe mount, which is a different shape to the ones used by every other camera manufacturer, so if you have a third-party flash you'll need an adaptor to use it on this camera. Also included is a wireless flash trigger which is compatible with Sony's accessory F36, F42, F56 and F58 flashguns.
One of Sony's stand-out features is the Dynamic Range Optimiser, first introduced on the A100, and considerably refined over the past few years. The DRO system automatically adjusts the tone curve of the image in high-contrast situations to help retain shadow detail and avoid burned-out highlights. The DRO on the A900 is the most sophisticated version yet, with two automatic settings and an advanced manual option that offers five levels of adjustment. There are many similarly-named systems on other manufacturer's cameras, but in my experience Sony's is generally the most effective. Shooting in my usual dimly-lit car park the higher manual settings produced a result that matched closely to the naked-eye appearance without adversely affecting picture quality, while in real-world high-contrast shots the automatic settings produced outstanding dynamic range comparable to high-quality slide film. The A900 has by far the best dynamic range response of any digital SLR that I've yet seen.
Sony's other key feature is the Super Steady Shot sensor-shift image stabilisation system. Sony inherited this technology from Minolta, and has included it on all the other Alpha system DSLRs as well as some of its compact cameras. However the big full-frame sensor is much heavier and consequently harder to move around than and APS-C sensor, which might have caused some technical problems, but these seem to have been overcome, because the IS system works very well, allowing shake-free hand-held shooting at 1/15th of a second with the 24-70mm f/2.8 zoom lens, and around 1/200th of a second with the 500mm f/8 reflex lens that I was also testing.
One slight problem I did encounter with the big 500mm lens was a significant amount of mirror vibration, camera shake caused by that big reflex mirror moving up and down. The A900 has a mirror-up feature on the two-second self timer, and this combined with a cable release provided a solution to this problem. The A900 also comes with an infrared remote control as standard.