Sony A900 Optics
The Sony A900 features a bayonet lens mount, which accommodates a wide range of Sony and Konica Minolta lenses. The Sony A900 is sold body-only, so no kit lens is offered. A small button on the front of the camera releases the lens from its mount, so it can be turned and removed.
All current Sony lenses may be used with the A900, but DT (sub-frame) lenses may require exposure adjustments due to the corner darkening produced by their sub-frame image circles. (The camera does support an APS-C exposure mode, that crops a sub-frame area from the larger sensor, but be prepared for possible exposure issues.) All Konica-Minolta lenses are compatible with the Sony A900 as well, but note that the 1x-3x Macro Zoom lens isn't compatible with the A900's Super SteadyShot image stabilization: Turn that feature off when using one of these lenses.
The Sony A900's CMOS sensor is the same size as 35mm film, so the angle of view at any given focal length will be the same as on a 35mm camera.
Sony Alpha 900: Upgraded Autofocus System
The Sony A900 marks a new level of autofocus sophistication for Sony. At first glance, the viewfinder might look like a step backwards, as there are fewer AF points visible in the viewfinder than we've seen in previous Sony SLRs like the A700. It turns out though, that there are now two classes of AF points: Nine Primary AF points are marked in the viewfinder and illuminate to show subject acquisition. In addition, 10 new "Supplemental" AF points are arranged between and/or around the Primary points,. As their name suggests, the Supplemental AF points assist with autofocus operation, but aren't selectable under user control, and don't illuminate in the viewfinder when they've locked onto a subject. The Sony A900's sensor pattern is shown in the illustration below.
In the diagram above, the blue rectangles with black marks in their centers are the Primary AF points, while the green rectangles mark the Supplemental points. Sony's been a little cryptic about the function of the Supplemental AF points, but the way they surround and fill in between the Primary ones suggest that they can provide additional position and velocity data for dynamic AF tracking.
It's not entirely clear, though, how the two types of AF points interact during autofocus operation: For instance, if the closest/most prominent subject detail falls under a secondary AF point, will it lock focus on it? Or, are the Primary AF points the only ones that can initially lock focus, with the Secondary AF points only helping with subject tracking during continuous AF operation? (Note that the Sony A900's manual points out that the Supplemental AF points are only active in wide-area AF mode.)
When it comes to subject tracking, more AF points are definitely a good thing, as they give the camera's autofocus system a much more comprehensive view of subject distance across the frame. On the other hand, as we've seen in the case of Nikon's D300 and D700 SLR models (which have no less than 51 AF points), a large number of AF points can actually slow shutter response in dynamic-area AF mode.
The cluster of AF points right at the center of the frame is typical of most SLRs, but Sony packs more in than many systems do. There's a "dual cross" array, a group of four sensors that create an expanded area of AF sensitivity, responding to both horizontally- and vertically-oriented subject detail. There's also a single, large, horizontally-oriented sensor that provides significantly greater AF accuracy, but only for lenses having maximum apertures of f/2.8 or larger.
As is often the case with full-frame SLRs though, the Sony A900's AF array covers a bit less of the total frame area than those of the A700 and other Sony sub-frame DSLRs, so subjects will need to be closer to the center to be covered by the AF system.
Sony A900 Autofocus Area Options
Autofocus Area also has three options available through the Function or Record menu: Wide, Spot, and Local (manual setting). The default option is a nine-point Wide Focus area, indicated on the Quick Menu display by an array of dashes inset within four widely-spaced brackets in the viewfinder image. You can override the chosen AF mode by pressing the Spot AF / OK button in the center of the Multi-controller, which defaults to the more accurate center AF point (the latter indicated by a target box in the center of the viewfinder). Wide AF bases its focus on the most prominent subject detail in the portion of the image that falls within the AF brackets. Spot mode bases its focus on the very center of the frame, where the square target resides. The Local setting is Sony's terminology for a manual AF area selection, and lets you manually set the main AF point by using the Multi-controller to highlight one of the nine Primary AF points. The active AF area is briefly illuminated in the viewfinder. New for the A900 is autofocus micro adjustment, which offers +/-20 step adjustments for up to 30 lenses to address front- and back-focus issues. (Big kudos on that feature. We're seeing it on more and more SLRs, the A900 marks the first time we've seen this in a Sony camera.)
Sony A900 Autofocus Modes
The Sony A900 has three main focusing modes, namely Single-shot, Continuous, and Automatic. In single shot mode, focus is set when you initially half-press the shutter button (or trigger focus via the AF/MF button, if you've configured it to act as a focus lock control via the AF/MF option on Custom Settings Menu 1). When focus lock is achieved, a viewfinder indicator illuminates, and the camera beeps to let you know that the subject is in focus. In Wide Area mode, the AF point selected illuminates briefly as well, so you tell what the camera has locked onto.
In single-shot mode, if the subject moves, the focus won't shift to track it. Conversely, in Continuous AF mode, when the shutter button is half-pressed and held, the camera automatically switches to Wide Area AF (if it wasn't already in that mode), and will continuously adjust both the focus setting and its choice of AF points to follow the subject. In this mode though, the selected AF points won't illuminate, and there's no audio signal to let you know when the subject is in focus.
Auto AF mode lets the camera start out in single-shot AF, but then switches to Continuous AF if it detects subject movement after the initial focus lock. When using the A900's continuous shooting option in conjunction with Auto AF mode, the first shot of a series will be captured in single-shot AF mode, but the camera will automatically switch to Continuous AF mode for the second and subsequent frames in the sequence.
Sony A900 AF Assist
Like the A700, the A900 has a very bright LED built into the body, that projects a pattern onto the subject for better focusing in dim light. The pattern projected by the LED is a significant help when dealing with low-contrast subjects. Thanks to the projected pattern, the Sony A900 can focus on a blank wall in total darkness. (Not that a blank wall would make a particularly interesting subject, but if it can handle that, it can probably handle most any normal subject.)
SteadyShot INSIDE: Sony A900 Anti-Shake
The Sony A900 also employs Sony's SteadyShot INSIDE (their latest branding; previously referred to as Super SteadyShot) anti-shake technology, which uses a highly sensitive angular rotation sensor and Smooth Impact Drive Mechanism (SIDM) to move the sensor assembly itself to counteract camera movement, rather than the conventional approach of moving an optical element inside the lens. This body-based anti-shake approach is based on technology Sony acquired from Konica Minolta, but Sony claims that it has been improved significantly since that time.
Sony claims that the SteadyShot INSIDE anti-shake system in the A900 provides a 2.5 to 4-stop reduction in the blurring produced by camera shake. Translating that into real-world shutter speeds, a two-stop improvement means that a shutter speed of 1/30 second would give you the same resistance to blur from camera shake that a speed of 1/120 would without anti-shake. A 4-stop improvement would mean you could shoot as slow as 1/8 second and get the same results (blur-wise) as when shooting at 1/120 second unaided. Even the lower end of the specified range of effectiveness means a pretty significant improvement in one's ability to hand-hold long exposures.
When SteadyShot INSIDE is activated, the SteadyShot scale on the right side of the viewfinder display indicates the degree of stabilization. A downside to Sony's body-based SteadyShot approach is that while you can see the results of stabilization on competing lens-based designs, you have only this scale to tell you how the A900's SteadyShot mechanism is doing. SteadyShot minimizes the effect of blurring caused by slight camera movement, which is more noticeable at long focal lengths.
Sony A900 Anti-Dust Technology
To help combat dust particles on the imaging sensor from changing lenses, Sony included both an anti-static coating on the sensor filter and an anti-dust vibration to automatically shake the sensor each time the camera is shut off. The anti-static coating may be of some benefit, but we frankly don't expect much from the A900's vibration cycle: It uses the same motion actuators as the image stabilization system. This means that the peak vibration frequency is probably on the order of a few hundred hertz (cycles per second). That's not nearly fast enough to produce strong enough g-forces to shake loose most dust particles; only the very largest pieces are likely to be dislodged by the SteadyShot system's relatively leisurely vibration. (Other vibration-based dust removal systems use frequencies in the tens of kilohertz: 20,000 to 50,000 cycles per second.)
The Sony Alpha A900 does not come with a kit lens, and since optical performance varies with the lens used, this section is intentionally left blank.