Sony A33 Optics
Sony A33 Optics
Alpha lens mount. Despite the new translucent mirror design, the Sony A33's Alpha lens mount is compatible with the full array of Sony Alpha and most Konica Minolta AF lenses, both screw-drive and electronic autofocus models.
The Sony A33 features a bayonet lens mount, which accommodates a range of Sony and Konica Minolta AF lenses. The Sony A33 is available in two versions -- body only, and bundled with a Sony DT 18-55mm f/3.5-5.6 SAM kit lens. For additional test results, see our review of this lens on SLRgear.com. A small button on the front of the camera releases the lens from its mount, so it can be turned and removed. The A33's CMOS sensor 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 50mm lens will provide about the same view as a 75mm lens on a 35mm camera.)
Sony A33 Autofocus
The Sony A33 provides both manual and automatic focus control modes, set by the Focus Mode switch on the left side of the camera body, unless the lens itself has a Focus Mode switch. If there's a duplicate switch on the lens, it takes over this function, and the one on the body serves no purpose. With whichever switch is applicable, you can select between Auto and Manual focus modes. 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, Automatic AF and Continuous AF settings. Single-shot sets focus with each half-press of the Shutter button, while Continuous mode is constantly adjusting the focus, whether the Shutter button is pressed or not. The Automatic setting will lock focus on a still subject or continually adjust focus on a moving subject, for as long as the Shutter button is halfway pressed.
Autofocus Area also has three options available through the Function menu: Wide, Spot, and Local (manual setting). The default option is a fifteen-point Wide Focus area, indicated by an array of square focus areas inset within four widely-spaced brackets in the viewfinder image. (Note that only three points at the horizontal center of the frame utilize a cross-type sensor, sensitive to detail in both the horizontal and vertical axis. The other 12 sensors are line-type, sensitive to detail in one direction only.) 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 fifteen AF points. The selected AF area is indicated with an orange frame during selection, and with a black frame in the viewfinder while framing images, while unselected points have a light gray frame.
A depth-of-field preview button can be found adjacent to the grip, at the base of the lens mount. When held in, the A33 stops down its aperture to the selected value, and adjusts preview sensitivity to attempt to show the image with correct brightness.
Sony A33 AF Assist
The Sony A33 uses its built-in flash head as an 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 it's rather distracting, and 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 well 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 A33 Anti-Shake
The Sony A33 also employs Sony's Super SteadyShot anti-shake technology, which uses a highly sensitive accelerometer and image sensor shift mechanism to move the sensor assembly itself to counteract camera movement, rather than the more common approach of moving an optical element inside the lens. Sony claims that the Super SteadyShot anti-shake system in the A33 provides a 2.5 to 4-stop reduction in the blurring produced by camera shake. 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. What's more, because the A33's LCD and EVF display the image as seen by the imaging sensor, the effect of SteadyShot stabilization at work can be seen, unlike models that use a traditional optical viewfinder. The SteadyShot scale is still useful even with the stabilized view though, as it gives you a 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 A33 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 opens the shutter, allowing access to the sensor for use with a blower or other cleaning device. The Translucent Mirror must be manually raised to access the image sensor, and it's important not to touch the mirror in the process. There's no user method for cleaning the mirror itself, with the only option if this is required being to return the camera to Sony for service.
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 A33 Optical Test Results
Below are the results of our optical tests with the Sony A33 and the bundled 18-55mm kit lens.
Lens Test Results
Decent performance with the 18-55mm kit lens.
|18mm @ f/8||55mm @ f/8|
The Sony SLT-A33 is available bundled with a 18-55mm f/3.5-5.6 SAM 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 A33'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 at the 55mm setting were sharp in the center and bottom of the frame, but the top and left hand-side were soft. Chromatic aberration at full telephoto was negligible. Overall, decent results for a kit lens.
An average sized minimum coverage area, with good detail. Flash throttled down well.
|Macro with 18-55mm
kit lens (55mm @ f.5.6)
|Macro with Flash|
As with zoom performance, the Sony A33's macro performance will depend entirely on the lens in use. However with the 18-55mm kit lens set to 55mm, the Sony A33 captured an average sized minimum area measuring 2.33 x 1.55 inches (59 x 39 millimeters). Detail was quite good in the center, though the corners were moderately soft. (Most lenses have some softening in the corners at macro distances.) The flash throttled down well, resulting in a good exposure if slightly bright. The flash also 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 A33'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 A33 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 wide-angle.
Chromatic Aberration. Chromatic aberration in the corners with the A33'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. At full wide-angle, our copy of the 18-55mm lens was quite soft on the right-hand side, and the softness extended pretty far into the frame. The left side wasn't nearly as soft, with only minor to moderate blurring in the extreme corners. The center of the image was sharp with good contrast. Some vignetting (corner shading) is also noticeable at full wide-angle. At full telephoto, the ride side again was softest, but not nearly as soft as wide-angle. The corners in the left hand-side were quite sharp, and the center was also sharp, but the lens had lower contrast overall at full telephoto. A slightly below 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 A33 doesn't appear to be applying any chromatic aberration in its JPEGs, as uncorrected RAW files show similar amounts.
Internal Mirror Reflections
|Sony SLT-A33, 18-55mm kit lens||Sony DSLR-A560, 18-55mm kit lens|
When additional surfaces are added to an optical path, especially ones not parallel with the image plane, there is always a chance that light will travel an unintended path. In the case of the A33 and A55V, it appears that the rear surface of the translucent mirror film can reflect light back to the front surface of the film, which reflects it back to the rear, causing a "ghost" reflection or multiple reflections to appear in the final image under certain conditions. The above left crop shows this phenomenon in one of our Sony A33 flash test shots, where a very strong reflection from the plastic edging of our flash-range/uniformity target also has a small ghost image in the form of a horizontal white line below it. (Thanks to IR reader Erick E for pointing this out in one of our A55 flash range images!) The crop on the right is from a similar flash shot, taken with the same lens, but using the Sony A560 which is a traditional SLR without a translucent mirror in the optical path. As you can see, under nearly identical conditions (the A560's flash is a bit stronger), no ghosting is present.
We only found ghosting in a small subset of our test images, as it seems to require fairly specific conditions to be noticeable. Ghosts only appear in a couple of our flash shots similar to the one shown above, as it appears that camera/subject angle needed to be just right, to reflect enough light from the flash head back into the lens to show the ghost. Dave also saw this phenomenon in some night shots he has taken with the Sony A55V, which we hope to share some crops from shortly. From Dave's experience, the ghosting seems to require a very concentrated light source, sharply focused, and ideally a relatively small light source with darker areas surrounding it. The ghosting did not appear with larger, less sharply-defined light sources, or when light spilled from the source on surrounding areas (thereby making the lighting broader and more diffuse). Under conditions of really severe light overloads ghosts seem to be masked by lens flare. Based on our tests, we can confirm that this is a genuine phenomena with the Sony A33/55. Should you be concerned about it? Perhaps. Some people would certainly find it objectionable, especially if they did a lot of night photography of things like cityscapes. Personally, it wouldn't deter us from buying a Sony A33, because we don't do much cityscape-type photography: Our night shots tend to be ones where the subjects are people or areas lit by nearby light sources. With these sorts of shots, ghosting of the type shown above wouldn't be an issue. Bottom line, you'll need to decide for yourself if this would impact your personal style of shooting enough to outweigh the benefits of the Sony A33's pellicle mirror design.
The images above were taken from our standardized test shots. For a collection of more pictorial photos, see our Sony Alpha SLT-A33 Photo Gallery .