Sony DSLR-A100 Optics
Sony A100 Optics
The Sony A100 features a bayonet lens mount, which accommodates a range of Sony and Konica Minolta lenses. The A100 comes with a Sony 18-70mm f/3.5 kit lens. A small button on the front of the camera releases the lens from its mount, so it can be turned and removed. The A100's CCD is smaller than a 35mm frame, 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 100mm lens will provide about the same view as a 150mm lens on a 35mm camera.)
The A100 provides both manual and automatic focus control modes, set by the Focus Mode switch on the left side of the camera. You can select between Auto and Manual focus modes. But the Function dial and button provide access to additional Auto Focus Area and Auto Focus Mode options.
Auto Focus Area has three options: Wide Focus Area, Spot AF Area, and Focus Area Selection. The default option is a nine-point Wide Focus area, indicated 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 Area AF bases its focus on the most prominent subject detail in the portion of the image that falls within the AF brackets. Spot Focus bases its focus on the very center of the frame, where the square target resides. The Focus Area Selection 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. Once you've set the AF area, sliding the switch to the Focus Area Lock position locks the AF point.
Auto Focus Mode offers four options: Single Shot AF, Automatic AF, Continuous AF, and Direct Manual Focus. Single Shot AF is a general purpose autofocusing mode that sets focus when you half press the Shutter button. Automatic AF automatically switches between single and continuous AF depending on the subject's motion. Continuous AF is designed for subjects in motion, continuously focusing on the subject even when the Shutter button is half-pressed. Direct Manual Focus operates like Automatic AF, but after focus is automatically found, it can be adjusted manually. The focus signal in the viewfinder tells you when you can manually adjust focus.
Sony A100 AF Assist
The Sony A100 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. This is a real limitation for available-light photography, as the camera can expose at light levels below those it can focus at. (Although its low-light focusing ability is much better than average.) If the camera is fixed on a tripod, you can work around this limitation, but it's somewhat awkward: With the flash head up, half-press the shutter button to make the camera focus. Then switch the focus mode to manual focus, being careful not to touch the focus ring on the lens. Stow the flash head, and then take your picture. (But don't forget to switch back to AF mode for the rest of your shooting!)
Sony A100 Anti-Shake
The A100 also employs Sony's Super SteadyShot anti-shake technology, which uses a highly sensitive angular rotation sensor and Smooth Impact Drive Mechanism (SIDM) to move the CCD assembly itself to counteract camera movement, rather than the more common 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 a more powerful processor in the A100 increases the system's effectiveness beyond that of similar systems in previous Konica Minolta SLR models. (Theoretically, a faster CPU could let the system respond to and compensate for higher-frequency vibrations.)
Sony claims that the Super SteadyShot anti-shake system in the A100 provides a 2 to 3.5 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 3.5 stop improvement would mean you could shoot as slow as 1/11 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 Super 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 A100's SteadyShot mechanism is doing. SteadyShot minimizes the effect of blurring caused by slight camera movement, which is more noticeable at long focal lengths.
We don't have a quantitative way of measuring anti-shake performance, but the system on the Sony A100 seemed to be pretty effective. In the past, we've generally found that body-based anti-shake systems were more effective at compensating for lower-frequency, larger-amplitude camera shake than were lens-based systems, but less effective with higher-frequency vibration. Body-based systems have also generally seemed to be more effective with shorter focal length lenses than long telephotos, perhaps because body-based sensors may be less sensitive to angular movement than lens-based sensors are to the corresponding linear motion out along the lens barrel. Finally, lens-based anti-shake systems do have a practical advantage in that they also stabilize the view in the optical viewfinder, while sensor-based ones do not. We'll report more on our experience with the A100's anti-shake and make some quantitative comparisons with the performance of some other systems soon, hopefully in the near future. There's no question, though, that even with telephoto lenses, the A100's Super SteadyShot anti-shake system produces a dramatic improvement in the sharpness of handheld photos at slow shutter speeds.
In an interesting wrinkle, Sony also appears to be using the same actuators used in the anti-shake system to deliberately vibrate the sensor at startup and shut down to dislodge dust particles that may have landed on its surface. This is reminiscent of the hypersonic dust-removal system pioneered by Olympus and also recently adopted by Canon, but at this point we don't know whether the vibrational frequencies used by the two systems are similar or not, nor how the effectiveness of the two compare.
Sony A100 Optical Test Results
Below are the results of our optical tests on the Sony A100. We used the 18-70mm kit lens as the benchmark, since most people will buy and use the camera in this configuration.
Good performance from the 18-70mm kit lens.
The Sony Alpha A100 features a bayonet lens mount (the old Minolta mount) to accommodate a wide range of lenses, and comes bundled with an 18-70mm lens. Detail is good at full wide angle, with good definition throughout the frame, though with slightly soft corners. There's also a small amount of coma distortion in the top corners. Results are also very good at full telephoto, though again, with slight softening in the corners of the frame.
An average size macro area with the kit lens, though good detail and resolution. Flash performs well also.
|Standard Macro||Macro with Flash|
The Sony Alpha A100's macro performance will of course vary with the lens in use. With the 18-70mm kit lens attached, the A100 captured a minimum area of 3.12 x 2.09 inches (79 x 53 millimeters), which is about average, more than adequate for photographing typical small objects for eBay, etc. If you're really into super macro shots, you may want to either add some front-element closeup lenses, or buy a dedicated macro lens. Detail is strong and resolution high, though details are a bit soft throughout the frame. The flash throttled down very well, with fairly even coverage, though the shot is slightly underexposed overall.
Slightly higher than average barrel distortion, but very low pincushion.
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 Alpha A100 kit lens' 0.9% barrel distortion at wide angle is a little higher than I personally like to see, and is slightly higher than average among the cameras we've tested. At the telephoto end, the A100's 0.03% pincushion is very low, and translates to about one pixel's worth of distortion.
|Barrel distortion at 18mm is 0.9%|
|Pincushion at 70mm is 0.03%|
Moderately high at 18mm, extends a ways into the frame. Very little aberration at 70mm.
|Wide: moderate, top left @ 200%||Wide: fairly bright, top right @ 200%|
|Tele: quite low, top left @200%||Tele: quite low, top right @200%|
Chromatic aberration is moderately high at full wide angle on the 18-70mm kit lens, showing about 6-7 pixels of bright coloration on either side of the target lines, with the effect . However, at full telephoto, pixels are fewer and much less bright. (This distortion is visible as a very slight colored fringe around the objects at the edges of the field of view on the resolution target.)
Slight softening in the corners of the frame with the kit lens, though not too strong. More noticeable at telephoto.
Slightly soft in the lower left corner.
Sharp at center.
Stronger blurring in lower left corner.
Sharper at center, though still a hair soft.
The Sony Alpha A100's 18-70mm kit lens produced slightly soft corners in a few shots, with the strongest effect at full telephoto. While the telephoto example above from the lower left-hand corner of our test unit looks pretty soft, that was by far the worst corner, and even there the effect didn't extend too far into the frame. Other corners were much better, overall lens performance was quite good in this area.
Good accuracy from the optical viewfinder.
|18mm, optical viewfinder||70mm, optical viewfinder|
The Alpha A100's optical viewfinder showed about 97% frame accuracy at wide angle, and about 95% at telephoto (the difference most likely due to were we decided to set the edges of the frame, given the barrel distortion at wide angle. Good results, equal to or slightly better than most consumer DSLRs on the market. (Although we'd personally like to see true 100% viewfinders on all SLRs.)
The images above were taken from our standardized test shots. For a collection of more pictorial photos, see our Sony Alpha DSLR-A100 Photo Gallery .
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