Sony DSLR-A550 Optics
Sony A550 Optics
The Sony Alpha 550 is available bundled with a Sony 18-55mm f/3.5-5.6 DT SAM kit lens. (Tested below.) The lens features a built-in focus motor for fast, quiet autofocus, and is designed to be used with subframe Sony SLRs. (More on that later.) For additional test results, see our review of this lens on SLRgear.com.
Alpha lens mount. The Sony A550's Alpha lens mount is compatible with the full array of Alpha lenses, both screw-drive and electronic autofocus models.
The Sony A550 features a bayonet lens mount, which accommodates a wide range of Sony and Konica Minolta AF lenses. A small button on the front of the camera releases the lens from its mount, so it can be turned and removed. The A550's APS-C sized CMOS sensor is smaller than a 35mm frame (commonly called a subframe sensor), so the angle of view at any given focal length will not be the same as on a 35mm camera. To find the approximate 35mm equivalent focal length, multiply the focal length of the lens by 1.5. (Thus, a 50mm lens will provide about the same view as a 75mm lens on a 35mm camera.)
Sony A550 Autofocus
The Sony A550 provides both manual and automatic focus control modes, set by the Focus Mode switch on the left side of the camera body, or on the lens. The Function button provides access to additional AF modes and AF Area options. The Autofocus Mode option under the Function menu offers Single-shot AF (AF-S), Automatic AF (AF-A), and Continuous AF (AF-C) settings. Single-shot AF acquires and locks focus when the shutter button is half-pressed, while Continuous AF mode constantly adjusts focus while the shutter button is half-pressed. The Automatic AF setting will lock focus on a still subject, but will switch to Continuous AF mode if the subject moves.
Autofocus Area has three options available through the Function menu: Wide, Spot, and Local (manual setting). The default option is a nine-point Wide Focus area, where the camera selects which of the nine focus points is used. (Note that only the center point utilized a cross-type sensor sensitive to detail in both the horizontal and vertical axis. The other 8 sensors are line-type, sensitive to detail in one direction only, although the four line sensors at the corners of the AF array are angled, so they'll respond to both horizontal and vertical detail.)
You can override the chosen AF mode by pressing the AF button in the center of the Multi-controller on the camera's rear panel, which will select the center AF point (the latter indicated by the 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 total AF area. Spot mode bases its focus on the AF point at very center of the frame. 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 AF points. The active AF area is briefly illuminated in the viewfinder during autofocus.
Unique to Sony SLRs, the company's "Quick AF" Live View mode uses the same phase-detection AF sensors employed when the optical viewfinder is used, resulting in autofocus speeds that are just as fast. This is because the second image sensor located in the A550's viewfinder housing alleviates the need for the additional mirror flips required by most other phase-detect AF Live View implementations. Other systems need to drop the mirror, focus, and raise it again to before taking a shot in Live View mode, or employ a slower contrast-detect autofocus method using data from the main image sensor. Since the image sensor feeding the A550's live preview is located above the mirror, the mirror stays down until the final exposure, exactly as it does in optical viewfinder mode. The downside though is less accuracy, as the secondary sensor only offers around 92% frame coverage. Also, because the optical path for focusing is different than that for capture, there can be issues with front- or back-focusng, just like when using the optical viewfinder.
New for the A550 (and A500) are Face Detection and Smile Shutter options in Quick AF Live View mode. Face Detection can detect up to eight faces, and adjusts focus, exposure, image processing, and flash output. Smile Shutter can be used to trip the shutter automatically when a smile is detected. You can adjust the "sensitivity" of Smile Shutter from "Slight", "Normal", or "Big" smile.
Also new for the A550/A500 is a Manual Focus Check Live View mode, which derives its image from the main sensor. As the name implies, autofocus is not supported in this mode. The preview image in this mode can be magnified by 7x or 14x for critical manual focusing, and unlike Quick AF mode, frame coverage is 100%. Luke (our lab technician) was a little frustrated when using this mode to frame our viewfinder accuracy target, as it automatically shuts off after a few seconds. And, if you turn it back on a few times, the camera makes you wait before it will come on again. It seems to be limited by either a heat sensor, or a cumulative timer to prevent heat issues from extended use. It really is meant only for a quick manual focus check.
Sony didn't include a depth-of-field preview button on the A550, nor on any of its newer consumer models.
Sony A550 AF Assist
The Sony A550 uses its built-in flash head as a very bright AF-assist light for better focusing in dim lighting. This has the advantage that the light from the flash is very bright, but the downside is that you can only get AF assist when the flash head is raised. On some previous Alphas, this was a real limitation for available-light photography, as the camera can expose at light levels well below those it can focus at. The Sony A550 however has a Flash Off mode which allows the flash to be raised and used as an AF illuminator only, not contributing to the exposure itself.
Sony A550 Anti-Shake
The Sony A550 also employs Sony's SteadyShot INSIDE anti-shake technology, which uses a highly sensitive accelerometer and Smooth Impact Drive Mechanism (SIDM) to move the sensor assembly itself to counteract camera movement, rather than the more common approach of moving an optical element inside the lens, so it works with just about any lens attached.
Sony claims that the SteadyShot anti-shake system in the A550 provides up to a 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 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 A550's SteadyShot mechanism is doing. The upside, though, is that the SteadyShot scale does give you a very good idea of how hard the SteadyShot mechanism is working, so you can choose a moment when the camera is moving less to snap the shutter, thus maximizing your chances for a sharp image.
Sony A550 Anti-Dust Technology
To help combat dust particles on the sensor from changing lenses, Sony included both an anti-static coating on the sensor filter and anti-dust vibrations to automatically shake the sensor with the anti-shake mechanism each time the camera is shut off. There is also a manual cleaning mode, where the camera first shakes the sensor, then lifts the mirror and opens the shutter, allowing access to the sensor for use with a blower or other cleaning device.
We've generally found dust-removal systems based on cameras' anti-shake systems less effective than those that use an vibrate the sensor ultrasonically, but it bears noting that no dust removal system completely eliminates the need for occasional manual sensor cleaning. Copper Hill Images is an advertiser of ours, but we'd recommend their wet/dry cleaning system even if they weren't (it's what we use in our own lab): See the Copper Hill website for details.
Sony A550 Optical Test Results
Below are the results of our optical tests with the Sony A550 and the bundled 18-55mm kit lens.
Lens Test Results
Good performance with the 18-55mm kit lens.
|18mm @ f/8||55mm @ f/8|
|1.4x Digital Zoom||2x Digital Zoom|
The Sony A550 is available bundled with a 18-55mm f/3.5-5.6 lens. This lens possesses a very typical optical zoom ratio of about 3x, with a 35mm equivalent focal range of about 27-83mm because of the A550's 1.5x "crop factor". Results were good at 18mm with strong detail across most of the frame, though there was some minor softening in the corners, even at f/8. Coma distortion in the trees was low in the corners, but chromatic aberration was moderately high along high contrast elements near the edges of the image. Results were also good at the 55mm setting, with sharp results in the center, but corners were somewhat soft. Chromatic aberration at full telephoto was negligible. Overall, good results for a kit lens.
The Sony A550 also offers what the company calls "Smart Teleconverter" in Live View mode. Smart Teleconverter is simply a digital zoom without interpolation, so the image is simply cropped. Two settings are available, 1.4x and 2.0x. 1.4x mode results in a 7.4-megapixel image cropped from center giving a equivalent focal length field-of-view of 116mm with the kit lens, while 2.0x mode results in a 3.5-megapixel image with an effective focal length of 165mm. Because the images are not interpolated, there is no apparent loss of quality when viewed on the screen, though keep in mind maximum print sizes will be limited by the smaller image sizes.
An average minimum coverage area (for an SLR zoom kit lens), with slightly soft detail overall. Flash did an okay job throttling down.
|Macro with 18-55mm
kit lens (55mm @ f.5.6)
|Macro with Flash|
As with zoom performance, the Sony A550's macro performance will depend entirely on the lens in use. However with the 18-55mm kit lens set to 55mm, the Sony A550 captured an average sized minimum area measuring 2.37 x 1.58 inches (60 x 40 millimeters). Detail was slightly soft in the center, and the corners were moderately soft. (Most lenses have some softening in the corners at macro distances.) The flash was just a bit on the bright side, resulting in just a slightly overexposed image. The flash had no trouble clearing the lens as there is no detectable shadow, though the bottom of the image is slightly darker than the top.
Higher than average geometric distortion at wide-angle, very low at telephoto.
|Barrel distortion at 18mm is 1.1 percent|
|Pincushion distortion at 55mm is less than 0.1 percent|
The Sony A550's 18-55mm kit lens produced about 1.1 percent barrel distortion at wide-angle, which is higher than average and noticeable in some of its images. At the telephoto end, there's less than 0.1 percent pincushion distortion, which is practically non-existent. This is the tendency for the lens to bend straight lines outward (like a barrel -- usually at wide-angle) or inward (like a pincushion -- usually at telephoto).
The Sony A550 does not appear to be applying any geometric distortion correction to its JPEGs, as uncorrected RAW files show the same amount of distortion.
Chromatic Aberration and Corner Sharpness
Moderately high chromatic aberration at wide-angle; lower levels at full telephoto. The lens produced some soft corners, especially at telephoto.
|Wide: Upper right
C.A.: Moderately high and bright
Softness: Minor blurring
|Tele: Upper left
C.A.: Moderate, but very dull
Softness: Strong blurring
C.A.: Very low
Softness: Sharp, but lower contrast
Chromatic Aberration. Chromatic aberration in the corners with the A550's 18-55mm kit lens at wide-angle (18mm) is moderate in terms of the number of pixels, but quite bright, so the effect is noticeable in some shots. At full telephoto (55mm), C.A. is also moderate in terms of pixels, but the colors in the fringes are quite muted, partially due to a loss of contrast. Color fringing gradually reduces in brightness and width as it approaches the center of the image, where it is very low at wide-angle and telephoto.
Corner Softness. The Sony A550's 18-55mm kit lens produced some soft corners in a few shots. At full wide-angle the corners had only minor blurring, with the top right corner being the softest; really nothing to be concerned about. The center of the image was sharp with good contrast. Some vignetting (corner shading) is also visible at full wide-angle. At full telephoto, the top left corner was quite soft and the softness extended pretty far into the frame. The other corners were better and the center was fairly sharp, though telephoto images suffered from a loss of contrast across the frame. An average performance overall for a kit lens here. (Note that the lens was "wide-open" for these shots, and corner sharpness generally improves when a lens is "stopped-down" a couple of f-stops below full aperture.)
The Sony A550 doesn't appear to be applying any chromatic aberration in its JPEGs, as uncorrected RAW files show similar amounts.
The images above were taken from our standardized test shots. For a collection of more pictorial photos, see our Sony Alpha DSLR-A550 Photo Gallery .
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