Olympus E-20Olympus updates their bargain-priced Pro SLR with a 5 megapixel sensor and improved electronics<<Viewfinder :(Previous) | (Next): Exposure & Flash>> Page 5:OpticsReview First Posted: 11/28/2001 |
Optics
Like the E-10, the E-20 has a unique fixed-mount lens design. It's a departure from other professional SLR digicams, in that it doesn't accept interchangeable lenses. For some photographers, this will undoubtedly be seen as a limitation. On the other hand, Olympus offers a pretty wide range of front-element auxiliary lenses, the 4x zoom range is probably sufficient for the bulk of normal studio and location shooting, and there are advantages to the fixed-lens design that I'll delve into shortly. Aside from the fixed-mount design, the E-20's optics are unique in their incorporation of several elements normally found only in very high-end lenses in the 35mm world.
The diagram above shows the E-20's lens optical system in cross-section, with several key parts of it called out. The first noteworthy feature is that there are two aspheric elements, and no fewer than three low-dispersion or extra-low-dispersion elements, both of which are costly to manufacture, and a hallmark of high-end optics. The second design element is the "Gauss Type Lens Group" appearing in the middle of the diagram. Gauss groups are normally found only in high-end wide-aperture telephoto lenses, and significantly reduce chromatic aberration and other optical defects in such designs. Another consequence of the arrangement of elements in the E-20's lens is that the light arriving at the CCD will be pretty highly collimated, with all the light rays hitting the CCD surface more or less at a right angle. Olympus feels that this is important for digital imaging systems, due to the strong three-dimensional structure of the CCD surface. (I didn't have this confirmed by Olympus, but we strongly suspect that the "purple fringe" problem found on many consumer-level digicams is due to a too-high angle of incidence of the light falling on the CCD surface.)
Another aspect of the E-20's lens system that Olympus calls particular attention to is that it is designed to have a "circle of confusion," matching the dimensions of the CCD pixels. This is a bit of a technical area, but lens systems are generally designed to be able to deliver a particular maximum resolving power, measured by how tightly they can focus a hypothetical point source of light. Most film-camera lenses are designed with the resolution limits of film in mind (no surprise), which apparently results in "circles of confusion" (sounds like a planning meeting at The Imaging Resource ;) or "blur spot" size of six microns or more. The problem with applying such lenses to digital imaging is that the lens ends up being the limiting element in the overall optical path. Olympus' contention is that lenses need to be designed to match the requirements of the new medium. We don't have any way of verifying the impact of all this optical technology, but can say that the lens on our E-20 evaluation sample looked exceptionally sharp.
Turning to the more mundane aspects of the E-20's lens, its 4x, professional ED (Extra-low Dispersion), glass, 9- 36mm lens (equivalent to a 35-140mm lens on a 35mm camera) design. With a very fast minimum aperture of f/2.0 ~ f/2.4 (depending on zoom setting), the lens features 14 elements in 11 groups. As noted above, these groups can be broken down into one extra-low dispersion glass element, two aspherical glass elements, two aspherical elements, and two high index low dispersion elements. Aperture can be manually or automatically adjusted, with a range from f/2.0 to f/11.0, depending on the zoom setting, in 1/3 EV steps. Focus ranges from 1.97 feet (0.6 meters) to infinity in normal mode and from 8.0 to 30.0 inches (0.2 to 0.8 meters) in macro mode. Macro mode is accessed by pressing the Macro button and turning the Command dial until the traditional macro flower symbol appears in the status display panel. Zoom is controlled by turning a textured-rubber bezel around the lens, rather than by pressing zoom control buttons. I like this method, as it gives you the same feeling as zooming a traditional 35mm lens, and provides more precise and positive-acting zoom control. The E20 captured a tiny macro area in our testing, approximately 2.92 x 2.19 inches (74.29 x 55.72 millimeters).
Focus can be automatically or manually controlled, with an AF/MF switch on the side of the camera to designate the mode. The E-20's autofocus system uses a Dual AF-Active (IR and Passive TTL) contrast detection method to determine focus, basing focus on the center of the subject. What this means is that the camera uses two methods to measure focus. First, the Active Triangulation Reflective AF bounces an infrared beam off of the subject to judge the distance between the subject and the lens. The subject is then roughly focused by moving the lens, followed by a more precise focus based on what the CCD is seeing. Once the subject is in focus, a green circle in the optical viewfinder lights solid. When shooting with manual focus, the focus is adjusted by turning the focus ring on the outside of the lens. When using manual focus, the camera-to-subject distance is also optionally reported in the bottom of the viewfinder screen, in both feet and meters. Like most other digicams with manual focus options that I've tested, the E-20's manual focus is a "fly by wire" system, in which the focus ring isn't coupled to the lens elements directly, but rather simply commands the camera's stepper motors to adjust the focus. It still does a pretty decent job of giving you the feel of actually adjusting the lens, because the direction and distance the ring is turned translates pretty directly into focus adjustment. The one downside of this system I see though, is that the maximum focus-adjust rate ("slew rate") is limited by the camera electronics, rather than by how fast you can crank the control ring. If you're used to the fairly loose focus adjustments on modern autofocus 35mm film camera lenses, you may be frustrated by the focusing speed of the E-20. On the other hand, it is one of the better-feeling manual focus adjustments I've seen on a digicam.
As first mentioned in the "design section, although the E-20 does not accommodate interchangeable lenses, its built-in lens does feature 62mm filter threads for attaching accessory lenses. Available lenses are a 28mm f/2.0 wide angle lens, 200mm f/2.4 and 420mm f/2.8 telephoto lenses, and a macro lens. Accessory grip and support kits are also available, and necessary, since the added lenses increase the weight and the length of the lens unit.
The optional 420mm-equivalent, f/2.8 lens (shown above attached to the E-10 camera) makes the camera quite long and heavy, requiring a lengthy support bar to hold the camera and lens together. This lens extension evolves the camera into a rather large, somewhat awkward device. Still, I applaud the availability of such a long, fast accessory lens, and found the resulting images to be very sharp. (Take a look at a 400mm f/2.8 lens on a 35mm film camera though: If your wallet can get past the several thousand dollar sticker price, you'll find the resulting lens probably weighs more than the E-20 and it's long telephoto combined.) The 420mm accessory lens can only be used with the camera's own lens set at maximum telephoto: Any excursion toward wide-angle focal lengths will result in vignetting. "Street" price for the 420mm lens is less around $600 pretty cheap by 35mm standards for a lens of that length and aperture.
A 200mm, f/2.4 extension lens can also be attached to the camera. Though no support device is required, the longer lens definitely requires a second hand to hold the camera steady. As with the 420mm lens, the 200mm unit can only be used with the main lens set to telephoto focal lengths.
A 28mm wide angle lens attachment is also available, with a maximum aperture setting of f/2.0. This shorter lens doesn't require any additional support, and does not vignette at any focal length setting of the primary lens.
As mentioned earlier, the E-20 accommodates both an infrared and wired remote control. What I really appreciated with the wired device is the ability to halfway press the shutter release to set exposure and focus. Most external remotes don't provide this capability. The wired remote is also necessary to take full advantage of the bulb exposure capability: Pressing on the shutter during a 30 second exposure is likely to introduce camera shake, even with a sturdy tripod. The infrared remote control is the same unit that Olympus includes with many of its other digicams, allowing you to trip the shutter and access several playback functions, but not half-press the shutter button to lock focus, nor press and hold it for bulb exposures.
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