Canon EOS 350D Digital RebelCanon makes an impressive update to their wildly popular "Digital Rebel."!<<Optics :(Previous) | (Next): Timing>> Page 8:Exposure and FlashReview First Posted: 3/23/2003, updated: 6/4/2005 |
Exposure
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The Aperture Priority and Shutter Priority modes work much the same as on any other camera, allowing you to adjust one exposure variable while the camera selects the other for the best exposure. Program mode keeps both variables under automatic control, while Manual mode gives you full control over everything. The Automatic Depth-of-Field mode (A-DEP) uses all seven autofocus zones to determine the depth of field in the active subject area. Once it has determined the range of focusing distances present across the seven zones, it automatically computes the combination of aperture and shutter speed needed to render the nearest and furthest points in sharp focus. This is a remarkably useful feature, even for professional photographers. In many situations, you want to keep several subjects in focus, while at the same time trying for the highest shutter speed (largest aperture) that will permit that. In practice, faced with such situations, I've usually resorted to just picking the smallest aperture feasible and hoping for the best. With the 350D's A-DEP mode, the camera takes the guesswork out of this process and gives you the fastest shutter speed it can manage while still keeping things in focus.
Exposure metering options are similar to the 300D, as the 350D offers Evaluative, Partial, and Center-Weighted options. In another improvement over the previous Digital Rebel 300D, the Rebel XT lets you choose the metering mode in any of the Creative Zone shooting modes. The EOS 350D offers variable light sensitivity, with ISO equivalents of 100, 200, 400, 800, and 1,600. For adjusting the exposure, the 350D's Exposure Compensation setting increases or decreases overall exposure from +/-2 EV in one-third or one-half EV increments. (You set the step size through the Custom menu.) An automatic exposure bracketing feature lets you set the total exposure variation (across three shots) at anywhere from +/- one-third or one-half EV all the way up to +/- 2 EV. The nice part is that the automatic variation is centered around whatever level of manual exposure compensation you have dialed in. Thus, you could manually set a positive exposure compensation of 0.7EV, and then have the camera give you a variation of +/- 2/3 EV around that point.
I really like the amount of information the 350D gives you about its exposure, not only in terms of the settings it's using, but in the form of feedback on how pictures you've captured turned out. You can select an "Info" display mode when viewing captured images on the rear-panel LCD screen. Notable here is that you not only can see all the exposure parameters, but you get excellent feedback on the tonal range of the image itself. One form of feedback is the histogram display at upper right, which shows how the tonal values are distributed within the image. Histogram displays are useful for directly seeing how the overall exposure turned out in an image, but I've found them to be of limited usefulness for making critical judgments about highlight exposure.
Digital cameras need to be exposed more or less like slide film, in that you need to zealously protect your highlight detail. Once you've hit the limit of what the sensor can handle, the image "clips" and all detail is lost in the highlight areas. The problem is that it's quite common for critical highlights to occupy only a very small percentage of the overall image area. Because they correspond to such a small percentage of the total image pixels, the peak at 100 percent brightness can be very hard to distinguish in the histogram display. To handle such situations, the Digital Rebel XT blinks any pixels that are 100 percent white on its screen, alternating them between black and white. This makes localized overexposure problems leap out at you, making it very easy to control the critical highlight exposure precisely.
Besides the above mentioned exposure information and feedback, the 350D's playback options include a thumbnail index display, normal full-frame viewing of captured images, and a zoomed view. There's also a "jump" mode, activated via the Jump button on the rear panel of the camera. Jump mode lets you very quickly move through images stored on the memory card, jumping 10 shots at a time. New is the ability to also jump by 100 images at a time, or by image shot date, making the Jump button even more useful than before. The 350D's image playback can be zoomed in very small steps anywhere from 1.5-10x. Once you've zoomed in at any level, you can scroll the zoomed window all around the image area, using the rear-panel arrow keys. Once you've dialed in the magnification and scrolling parameters, you can go to the next or previous image by turning the Main dial next to the Shutter button.
Another feature deserving comment is the 350D's separation of the autoexposure and autofocus lock functions. In consumer-level digital cameras, half-pressing the Shutter button locks exposure and focus simultaneously. You can use this to deal with an off-center subject by pointing the camera at the subject, locking exposure and focus, and then reframing the picture before finally snapping the shutter. The only problem is that you sometimes need to perform a more radical recomposition of the subject in order to determine the proper exposure. For instance, you may want to zoom in on the subject, grab an exposure setting, and then zoom back out before taking the picture. Situations like that require locking the exposure independently of the focusing, and the 350D provides for just such eventualities by way of a separate AE lock button on the back of the camera, right under your right thumb. Through the camera's Custom menu, you can specify the operation of the AE Lock button, as well as the AF/AE locking function of the Shutter button. A very handy feature indeed.
The EOS 350D offers a full range of White Balance settings, including six presets, an Auto setting, and a Custom setting. The six presets include Daylight, Shade, Cloudy, Tungsten, Fluorescent, and Flash. The Custom setting bases color balance on a previous exposure, meaning you can snap an image of a white card and then base the color temperature on that image. A White Balance bracketing option snaps only one image, then writes three successive files from that single image. Bracketing steps are from -/+ 3 stops in whole-stop increments. (Each stop corresponds to five mireds of a color conversion filter, for a total range of +/- 15 mireds. This corresponds to about a +/- 500K shift at a normal daylight color temperature of 5,500K.) The WB Bracketing is set on the same grid as the White Balance correction grid. Fairly sophisticated, the white balance correction tool lets you shift the color balance toward more or less green, amber, magenta, or blue, using a +/-9 step grid format. You move a highlighted square through the grid to adjust the color balance. It's a slightly more advanced interface than I'm used to seeing on digital cameras, but a useful one that greatly extends the camera's color corrective abilities. The EOS 350D also offers a Parameters option through the LCD menu, which lets you select one of two default settings, select a Black and White mode, or configure as many as three custom Parameters setups. Each setup lets you adjust Contrast, Sharpness, Saturation, and Color Tone (red/blue hue balance). The two preset parameters are Parameter 1 and Parameter 2. Parameter 1 sets contrast, saturation, and sharpness to +1 for a brighter, sharper image, more akin to what digicam owners expect from their cameras. Parameter 2 sets up the camera to perform much like the EOS 20D at its default settings, with contrast, sharpness, saturation, and color tone controls all at their neutral positions, allowing the more savvy photographer to adjust these parameters in post processing if desired. Finally, you can set the camera's color space to sRGB or Adobe RGB.
Metering accuracy and bias
Where the original Digital Rebel frequently underexposed shots relative to what my personal preferences would have been for a given scene, the exposure system in the Rebel XT seemed to be much closer to correct more of the time. What appeared to be going on with the original Rebel was that the camera responded very strongly to high-key subjects and strong highlights in otherwise well-balanced scenes. This was technically the most correct approach to take, and is the one preferred by most professional photographers, but I felt it was poorly suited to the needs and desires of most amateur shooters. The Digital Rebel XT will be more prone to let strong highlights blow out, but the midtone values will be closer to what amateur shooters are expecting to see, more of the time. (For what it's worth, the XT's default tone curve also seems a bit less contrasty than that of the Digital Rebel, so it's somewhat less prone to losing highlight detail in the first place. To my mind, this is a very significant advantage of the Canon Digital Rebel XT over its predecessor. The idea is that it's most important to preserve detail in the highlights of the images, since once detail is lost to overexposure there, it's gone forever. Dark midtones and shadows can always be fixed on a computer after the fact, albeit at the cost of somewhat elevated image noise levels.
Given that Canon is aiming the Digital Rebel XT at a more consumer-oriented audience though, and particularly given that they're pitching direct-from-camera printing as a primary feature, I think they should really reconsider the Digital Rebel XT's exposure system settings. In my experience, the average consumer is much more concerned with how their midtones look, as opposed to detail in the highlights. Even many "enthusiast" shooters prefer images in which the overall level of brightness matches what they saw in the scene, regardless of whether that means losing some detail in the highlights. On the 350D, this problem is exacerbated by Canon's decision to boost the default contrast level relative to that of the 10D. With the resulting steep tone curve, underexposing to save highlight detail results in even darker midtones and shadows than would otherwise be the case.
Low Light Capability
When operating the camera in full-manual exposure mode, the EOS 350D offers a Bulb exposure setting for very long exposures. Normally, exposure times are limited to a maximum of 30 seconds in Aperture- or Shutter-Priority modes, but in Manual mode, you can expose for as long as the batteries will last or the Shutter button is pressed by selecting Bulb mode and holding down the Shutter button for as long as you want the shutter to remain open. Obviously, hour-long exposures and more aren't a practical reality, as sensor noise will totally swamp the signal long before that point is reached. Still though, the 350D seems quite able to take very long exposures with very little image noise resulting. Like the 10D and 300D, the 350D employs noise reduction algorithms that automatically reduce excess image noise from long exposures.
A full discussion of image sensor noise is beyond the scope of this review, but the simple story is that the most obvious and objectionable noise you'll see in long digital camera exposures is so-called "fixed pattern" noise, caused by variations in "dark current" between sensor pixels. "Dark current" is just what it sounds like. Current (a signal) appears even when the sensor isn't being exposed to light. When you look at a long time exposure shot with a digital camera, you'll often see very bright pixels, where minor manufacturing defects have resulted in unusually high "dark current" levels. Often called "hot pixels," these flecks of color are very distracting visually.
The normal way to deal with hot pixels is to take an exposure with the camera's shutter closed, immediately after shooting the subject. If this "dark frame" is exposed for the same time as the subject was, you can largely eliminate the hot pixel problem by subtracting the dark frame information from the actual exposure. In practice, this works fairly well, but has the disadvantage that you have to wait for the dark frame exposure to be taken, requiring an appreciable amount of time in the case of long time exposures. (If you shot a one-minute exposure for the photo itself, you'll have to wait another minute for the dark frame exposure to be made.)
While most other high-end digital cameras on the market use a dark frame subtraction method to deal with image noise, the 350D (like Canon's earlier CMOS-based cameras before it) appears to be doing something very different, as there's very little delay between the end of the primary exposure and the writing of the image file to the memory card. There's clearly no "dark frame" exposure involved. I suspect that this advanced noise reduction processing in the 350D is another consequence of the "active pixel" CMOS technology Canon developed internally. Having active circuitry associated with each pixel in the sensor array allows lots of fancy processing that would be impossible otherwise, and it looks like Canon's noise reduction system takes advantage of this. However it's done, the 350D's noise reduction approach appears to be very effective.
As noted earlier in the "Optics" section of this review, the 350D uses its flash head as an autofocus-assist illuminator. This approach provides great working range, but has the unfortunate limitation that its AF-assist isn't usable for available-light photography.
Flash
The Rebel XT's built-in flash has a guide number rating of 43 feet (13 meters) at ISO 100, translating to a range of about 15 feet at ISO 100 with an f/2.8 lens. (Reasonably powerful, but not dramatically so.) The new flash pops up 5.5mm higher than the flash on the Digital Rebel, offering more clearance over lenses and also somewhat reducing the likelihood of red-eye at closer ranges. It's also designed to illuminate the area covered by a 17mm lens. Just like the Digital Rebel, which has a similar pop-up mechanism, the new mechanism still rattles noticeably when the camera is moved, something that was quieted on the 20D. The Digital Rebel XT gives you a great deal of control over flash exposure, allowing you to adjust flash and ambient exposure independently of each other, in one-half or one-third EV increments. This makes it very easy to balance flash and ambient lighting for more natural-looking pictures. The XT also uses E-TTL II control for both the built-in and compatible external flashes (according to Canon this includes the current 550EX flash, as well as the new 580EX), a new standard that promises, and seems to deliver better, more balanced exposures. Custom Function 08 turns this mode off and returns to an average metering system. E-TTL II is only available with the built-in flash or when the camera is paired with either the 550EX or the new 580EX flash.
Another nice touch is the Flash Exposure Lock button, which fires the flash under manual control before the actual exposure, to determine the proper exposure setting. This struck me as very handy, akin to the more conventional autoexposure lock function for handling difficult ambient lighting conditions. A Flash Exposure Compensation feature controls the flash exposure +/- 2 stops in 1/2 or 1/3-stop increments.
Several of the more impressive features of the Canon flash system depend on the dedicated 550EX or 580EX speedlight. Among these are true FP (focal plane) flash sync, flash exposure bracketing with external flash units, flash modeling, and E-TTL II exposure control. FP sync requires a flash unit to provide uniform light output for a relatively long period of time, long enough for the focal plane shutter curtain to fully traverse the sensor plane. On the Rebel XT, this requires a flash duration of 1/200-second. Uniform, long-duration flash pulses like this permit use of shutter speeds as high as the 1/4,000-second maximum that the Rebel XT is capable of. This can be invaluable when you want to exclude ambient light from the exposure. (FP sync mode is referred to as "high speed" mode on the Canon 550 and 580 flash units.)
Here's the rundown on Canon Speedlights and their compatibility with the 350D:
| Speedlight Model | On-Camera Capability | E-TTL Wireless Compatibility |
| 580EX | All, E-TTL II | Master or Slave |
| 550EX | All, E-TTL II | Master or Slave |
| 480EG | External auto plus manual operation | None |
| 540EZ | Manual operation only | None |
| 430EZ | Manual operation only | None |
| 420EX | All | Slave Only |
| 420EZ | Manual operation only | None |
| 380EX | All | None |
| 220EX | All | None |
| 200E | Not Compatible | None |
| 160E | Not Compatible | None |
| MR-14EX Macro Ring | All | Master Only |
| MT-24EX | All | Master Only |
| ST-E2 transmitter | E-TTL, attach to camera | Master Only |
| Non-dedicated shoe-mount units | Manual operation only | n/a |
| Studio strobe packs | Manual operation only, connect via threaded PC sync socket on camera body | n/a |
You'll note the references to "E-TTL remote" capabilities in the table above. Canon's Speedlight system permits TTL flash metering with multiple remote units, and even allows you to set differential power ratios between the slaved units, over a six-stop flash exposure range.
The "Flash Modeling" feature of the 550EX speedlight is quite useful. With a F550EX connected to the 350D, pressing the camera's Depth of Field Preview button causes the speedlight to fire at 70 flashes per second for about one second. This creates the illusion of a constant light source for your eyes, letting you preview the lighting on your subject when the flash fires. VERY handy, and likely to save lots of shoot/check/reshoot time!
As alluded to above, the "X-sync" speed of the 350D is 1/200-second. (This is the maximum shutter speed that can be used on the 350D when working with a non-dedicated, FP-capable speedlight.) When used with higher-powered studio strobe systems, Canon recommends a maximum shutter speed of 1/60-second or slower, to accommodate the time/intensity profile of such units.
A final benefit of the dedicated Canon speedlights is that they carry powerful autofocus assist illuminators that can extend the range of the built-in AF assist light of the Rebel XT. The AF assist beam on the 550EX is rated as good to about 50 feet, versus the roughly 13 feet of the lamp on the 350D itself. (As mentioned above though, note that the ST-E2 wireless sync transmitter can also be used for AF assist during non-flash photography, a handy trick.)
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20D with E-TTL II |
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Digital Rebel without E-TTL II |
Canon's E-TTL II flash exposure system also works with certain lenses to include object distance data into its calculations so it can adjust the flash power accordingly. A preflash is fired and the resulting readings compared to the ambient light reading for each of the camera's 35 metering zones from just prior to the flash, to identify and compensate for specular objects (that is, very reflective surfaces). In instances where most cameras would underexpose an image because of a reflective object in the frame, the Rebel XT will ignore the brighter areas and expose the subject correctly in most instances. This is designed to help shooters like event photographers--especially wedding photographers, whose cameras are constantly forced to balance a bright white dress against all manner of reflective materials on the clothing of others, in addition to the usually black tuxedos of the groomsmen. The two shots at right show the action of E-TTL II on the 20D, vs that of the less-sophisticated flash exposure system on the original Digital Rebel. Both shots were captured with the respective camera's default flash exposure settings, the same 550EX flash unit, and the same 18-55mm lens.
Continuous Shooting Mode and Self-Timer
The EOS 350D's Continuous Shooting mode is rated by Canon at three frames per second, for a maximum of 14 frames. Do note though, that the number of consecutive shots could be limited by Compact Flash space, if your memory card is nearly full. Our worst case test target captured 13 images before the buffer filled, while less complicated subjects have yielded 40 frames at full JPEG resolution.
The camera's Drive setting also accesses a Self-Timer mode, which opens the shutter 10 seconds after the Shutter button is pressed, giving you time to dash around in front of the camera. A Remote Control mode works with the dedicated and wireless remote units as well.
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