It's the lowest-priced interchangeable-lens digital SLR as we write this. How good is its revolutionary image sensor? Sigma's SD9 is a full-featured interchangeable-lens AF digital SLR that utilizes a unique image sensor: the Foveon X3 CMOS chip. Where the CCD and CMOS sensors in other digital SLRs read only one color (red, green or blue) per pixel, each of the X3 chip's 3.54 million pixels (3.43 million effective) reads all three colors. The accompanying sidebar explains how it works; the significance of this is that makes for the sharpest 3.5-megapixel images around.
Aimed mainly at the serious-amateur market, at $1800 street price the SD9 is the lowest-priced interchangeable-lens digital SLR as of this writing. It's based on Sigma's top-of-its-line SA9 AF 35mm SLR, with similar performance and feature set, although there are some differences (besides the digital aspects): the digital camera lacks the film version's multiple-exposure mode, its AF system requires a light level of EV 2 compared to EV -1 for the film camera (but actually focuses faster than the SA9), the digital version has a top shutter speed of 1/6000 compared to the film camera's 1/8000 (and the shutter-speed dial is just marked with arrows indicating "slow" and "fast"—shutter speeds are displayed on the LCD panel and in the finder with the digital camera, although they are set by rotating the shutter-speed dial as with the film camera), and the film camera has a built-in electronic flash unit. Shared with the SA9 are such handy features as a depth-of-field preview button and a mirror pre-lock (set the drive dial to up to activate it).
Focusing
Like the SA9, the SD9 utilizes phase-detection autofocusing. The AF motors are in the lenses rather than in the camera body, to optimize performance. Two AF modes are provided: single AF for stationary subjects, and continuous AF for moving subjects. Continuous AF is predictive: the camera calculates (from successive AF measurements) the subject's direction and speed, and adjusts focus to compensate for the distance it travels during the (very brief) lag between the moment you fully depress the shutter button to make the shot and the moment the exposure is actually made. To select either AF mode, just press the AF button and rotate the command dial (which encircles the shutter button) until the desired icon (S or C) appears on the LCD panel atop the camera. Of course, you can also focus manually at any time, by setting the focus-mode switch on the lens to M, and rotating the focusing ring until the image appears sharp in the viewfinder. With Sigma HSM lenses, you don't have to switch the lens to M to focus manually; just rotate the focusing ring with the lens in AF mode—a convenient feature that nicely complements HSM lenses' quick and quiet AF operation.
Exposure
Like its film counterpart, the SD9 offers four exposure modes: shiftable program AE, shutter- and aperture-priority AE, and metered manual. In program AE, the camera automatically sets both the shutter speed and the aperture for correct exposure; the shift feature allows you to change the camera-set shutter-speed/aperture combination at a twist of the command dial to select a desired shutter speed or aperture if you wish. In shutter-priority AE, you set the shutter speed you wish to use for action-freezing (or blurring) considerations, and the camera will quickly and near-instantly set the corresponding aperture to maintain proper exposure; in aperture-priority AE, you set the aperture you wish to use for depth-of-field considerations, and the camera will immediately set the corresponding shutter speed for correct exposure at that aperture. In manual mode, you set both shutter speed (via the shutter-speed dial) and aperture (via the command dial). The camera will tell you when it thinks you've set the right exposure via icons at the bottom right of the viewfinder display: 0 indicates correct exposure, - indicates underexposure, and + indicates overexposure (both followed by a digital readout up to 3.0 to show the degree the set exposure varies from the camera meter's recommendation). Of course, you can ignore the camera meter and base exposure on a hand-held meter reading, or your own experience.
Three metering systems cover just about any exposure situation you're likely to encounter. Most shooting is best done in 8-segment mode (selected by pressing the metering mode button and rotating the command dial until its icon is displayed on the LCD panel); this mode provides the greatest percentage of good exposures in a wide range of shooting situations. You can also select center-area metering, which confines the reading to the area indicated by a circle in the middle of the viewfinder (handy when you want to base exposure on a specific subject or area of a scene); and traditional average metering, which measures the entire image area, but places more emphasis on the central portion.
Features to enhance the exposure experience include AE lock (press the AEL button), exposure compensation (press the exposure-compensation button and rotate the command dial until the desired amount of compensation—you can choose from +3.0 stops to -3.0 stops in 0.5-stop increments—appears on the LCD panel) and three-frame automatic exposure bracketing (set the drive dial to AB, and rotate the command dial to set the desired bracket amount). You can also set ISO equivalents of 100, 200 and 400, by pressing the ISO button and rotating the command dial until the desired ISO appears on the LCD panel—but remember that, as with film, higher ISOs result in reduced sharpness and more grain (digital grain is called "noise").
More Camera Features
One especially notable feature of the SD9 is its built-in dust protector. Getting dust on the image sensor during lens changes is a big concern for interchangeable-lens digicam users, and this device eliminates the problem.
The SD9's metal vertical-travel focal-plane shutter provides electronically controlled speeds from 15 seconds to 1/6000 at ISO 100, and from 1 second to 1/6000 at ISO 200 and 400. Maximum flash-sync shutter speed is 1/180 (up to 1/6000 with the optional Sigma EF-500 DG Super SA flash unit in FP mode).
There's no built-in flash unit, but optional Sigma DG-series flash units provide TTL flash via the hot-shoe atop the viewfinder. Studio photographers can add a PC terminal for studio flash via the accessory PS Synchro Terminal Adapter ST-11.
Set the drive dial to the continuous-shooting icon, and you can shoot up to 6 frames at 1.9 frames per second at maximum resolution (up to 30 frames at 2.5 fps at lowest resolution). The drive dial also lets you select single-shot operation, 2- or 10-second self-timer, mirror lock, auto bracketing and power off.
Optional accessories include a wireless remote controller (especially useful for mirror-lock use), the Power Pack SD (which doubles battery capacity and provides a vertical grip/shutter button) and dioptric correction lenses to complement the -3 to +1 adjustable diopter correction built into the finder.
The Digital Side
Of course, the SD9 is a digital camera, so it has features not found on its film counterparts. Most obvious is the 1.8-inch color LCD monitor on the camera back. As is the case with other digital SLRs, the SD9's monitor is used for checking images after shooting them, and for making settings, but not for previewing images (you do that through the SLR viewfinder, just as with a film SLR).
You can review images in-camera by pressing the view button (which displays the last image recorded), then using the left and right arrows on the 4-way controller next to the LCD monitor to scroll through the stored images. (The up and down arrows jump three images back and three images ahead, respectively.) Press the + button at the top right of the camera's back to zoom in on an image (up to 400%, in five stages), or the - button to zoom out. Pressing the - button while in single-view mode activates the contact sheet (nine-image) display. There's also a slide-show feature, which automatically plays back all images on the card, or selected images, at a rate you choose.
One very nice feature is that you can shoot more images (assuming there's space for them on the card) at any time, even while displaying already-shot images—handy if something cool occurs while you're reviewing your images.
Press the info button, and you bring up the info screen for the displayed image, providing access to such data as shutter speed used, aperture, metering mode, ISO setting, resolution, exposure mode, focus mode, focal length, white balance setting, image dimensions, file size, exposure compensation and more for that image.
