The A900’s headline feature is of course its full-frame sensor, a chip first announced in January 2008 and making its debut here. Full-frame sensors measure the same size as 35mm film, which allows them to dispense with the field-reduction factor of other DSLR sensors – so mount a lens like the new Carl Zeiss 16-35mm and you’ll really get 16-35mm coverage as oppose to the 24-53mm coverage when it's mounted on other Alpha DSLRs.
The larger surface area of full-frame sensors also allows physically bigger photosites which in turn means greater sensitivity and dynamic range. So while the A900 boasts double the total pixel count of the earlier A700, its individual photosites actually remain a little larger, which should (in theory at least) allow it to perform better at higher sensitivities
The 24.6 Megapixels captured in each image is a great deal of information to number-crunch, so to handle the figures, Sony’s equipped the A900 with dual Bionz image processors. These allow the camera to impressively fire at 5fps, and while that rate’s matched by the 1Ds Mark III, the Sony model is again much cheaper.
Like all Alpha DSLRs, the new A900 features in-camera image stabilisation which physically shifts the sensor to counteract camera shake. Like those earlier models you won’t see the stabilising effect though the viewfinder, but the great benefit is it works with any lens you attach, be it wide or tele, prime or zoom. The really impressive part is the A900’s SteadyShot system is shifting a considerably larger sensor than earlier Alpha DSLRs – indeed it’s the first full-frame DSLR with sensor-shift anti-shake – but Sony’s still managed to squeeze the mechanism into a relatively compact body, at least for a high-end DSLR.
Another benefit of full-frame sensors are the large optical viewfinders which go with them. Sony’s excelled itself here by delivering 100% coverage which compares favourably against its similarly-priced rivals. Completing the specification are a VGA screen, HDMI port, accurate battery feedback, support for UDMA cards, AF micro-adjustment of lenses and Sony's Dynamic Range Optimizer which applies adjustments to the tonal range in real-time. Even though the A900 doesn't have Live View (or a movie mode), that's still a very impressive specification for the money with some unique aspects.
The big question of course is how well the A900 performs in practice. Most obviously, does the 24.6 Megapixel sensor really deliver a significantly higher degree of detail than anything that's come before it, and are there any compromises with noise at higher sensitivities? And while Sony's track record is proven with consumer and mid-range DSLRs, how does the A900 measure-up against established brands in the semi-pro and pro markets?
In our full review we'll answer these questions, putting the A900's features to the test while directly comparing its image quality against key rivals. At the time of writing, Canon's EOS 5D Mark II wasn't yet available, so we've taken the next best thing and compared the A900 against the flagship EOS 1Ds Mark III which shares the same resolution as Canon's new model. Completing the lineup in our comparisons is the A900's other major rival, the full-frame Nikon D700, which may 'only' sport 12.1 Megapixels, but is packed with features and handles beautifully.
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