Canon Rebel XTi Optics

Lens Compatibility

Like other Canon DSLRs with sub-frame sensors (currently, the Canon Rebel XTi and Canon EOS-30D), the Canon Rebel XTi will work with pretty much any EF-mount lens ever made, as well as with the special EF-S lenses designed for cameras with APS-C size sensors. Designed with a smaller image circle (the area covered by the image on the film/sensor plane), EF-S lenses tend to be smaller and lighter than full-frame models with the same focal length and maximum apeture. EF-S lenses can't be used on full-frame Canon cameras (nor on their models with 1.3x crop factors, like the current EOS-1D Mark II N), but small-sensor cameras like the Rebel XTi can use any full-frame lenses in Canon's arsenal.

The sub-frame sensor on the Canon Rebel XTi means that it has a smaller angle of view (by a factor of 1/1.6x) than a full-frame camera with any given lens. While most properly called a "crop factor", the 1.6x ratio is most commonly referred to as the "focal length multiplier" since that's how it works in practice: Any lens used on the Rebel XTi will have the same field of view as one with a 1.6x greater focal length will when attached to a 35mm camera. (For example, a 100mm lens on the XTi will show the same field of view as a 160mm lens on a camera with a 35mm frame size.) This means that the 18-55mm kit lens for the XTi has a coverage roughly equivalent to that of a 29-88mm lens on a 35mm camera.

Like most kit lenses, the 18-55mm f/3.5-5.6 lens that ships with the Canon XTi is of only modest quality, being designed for low cost more than optical excellence. It will work well enough for average consumer-level shooting, and can actually be quite sharp when stopped down to f/8 or so, but a better lens is one of the first accessories new SLR owners typically buy. (See our sister site SLRgear.com for in-depth lab tests and user reviews of SLR lenses.)

Autofocus Improvements

It's clear that the Canon Rebel XTi has borrowed a lot of technology from the EOS-30D. One place where this is dramatically obvious is in its autofocus system: The Rebel XTi has the same AF system as used in the 30D. This means it sports 9 AF points vs the Rebel XT's 7, and also offers the "dual-precision" center AF point found on the 30D. (A shot of the AF sensor used in both the XTi and 30D appears at right, image courtesy Canon USA, Inc.)

What does "dual-precision" autofocus mean? SLR autofocus accuracy is governed in part by how far apart you can space the sensor elements for a single AF point from each other. The wider this "baseline," the more accurately the AF point can determine focus. What limits the AF baseline spacing in a camera system is the lens aperture. You can build sensors with wider baselines, but that also restricts the range of lenses they can be used with. By and large, camera manufacturers have set f/5.6 as the minimum aperture their AF systems will work with. Lenses with wide-open apertures smaller than f/5.6 just won't focus. But once the baseline spacing is set, using a faster lens doesn't improve focus accuracy any.

On their higher-end DSLRs, Canon uses special AF sensors for some of the AF points. These dual-precision AF points have two sets of focus sensors, one spaced to permit use with f/5.6 lenses, the other set to operate with f/2.8 lenses. Since an f/2.8 lens permits almost a 2x increase in baseline spacing, AF precision increases proportionately when using them.

As on the EOS-30D, the Canon Rebel XTi's central AF point uses a dual-precision focus sensor. Whenever you attach an f/2.8 or faster lens to the XTi, the central AF point will automatically focus more accurately: No user intervention is required.

The AF points on the Rebel XTi cover about 60% of the frame width, and about 50% of the frame height. The illustration above shows the relative spacing between the various AF points, with the distances between them marked in millimeters, at the focal plane. (Illustration courtesy Canon USA.)

AF-Assist Illuminator

Like the original Digital Rebel and Rebel XT before it, the Canon Rebel XTi uses the built-in flash head as its AF-assist illuminator, rather than a bright light built into the camera's body. This works well (as you'd expect, the flash is quite bright, and probably has a longer range than an on-body illuminator bulb), but the Rebel XTi shares the limitation with its predecessors that the AF-assist option is only available when a flash is in use (whether internal or external). That is, you can't use the AF-assist illuminator and also make an available-light exposure. If you attach a 550EX external flash unit to the Rebel XTi, its internal infrared AF-assist illuminator can be used instead of the flash head itself, providing a useful working range of about 50 feet with a less obtrusive light source. This AF-assist is still only available when the flash is enabled, though. For non-flash photography, Canon's ST-E2 wireless sync transmitter can apparently also be used for AF assist, a handy trick. The ST-E2's AF-assist light has a useful range of about 25 feet. (Note though, that we haven't tested the ST-E2 ourselves, so can't verify this performance independently.)

Dust Reduction Technology

One of the major innovations introduced in the Rebel XTi is Canon's system-wide approach to reducing the impact of dust on the image sensor. From the beginning, every DSLR has offered a sensor-cleaning mode, in which the mirror is locked up and the shutter opened to permit the sensor to be cleaned with compressed air, a solvent-carrying swab or other means. As the market has matured and more DSLRs have found their way into the hands of novice users, it has become clear that some automated way of dealing with sensor cleaning is needed.

Consumer point and shoot digital cameras have no trouble with dust because their lenses and sensors are assembled together in a cleanroom environment at the factory. Once assembled, there's no path for dust from the outside world to make its way onto the sensor surface. A key feature of Digital SLRs however, is the ability for the user to easily swap lenses. This expands creative options enormously, but every time the lens is removed, dust from the environment is free to enter the camera body. From there, it's only a matter of time before some of it makes its way to the surface of the sensor where it casts shadows that appear as dark blobs in your images. (Properly speaking, it's the anti-aliasing filter that collects dust, rather than the sensor itself, but common parlance refers to "sensor cleaning." For the sake of familiarity, we'll generally refer to sensor cleaning here, but will make mention of the anti-alias filter as seems appropriate.)

The principal approach other manufacturers have used to deal with dust has been to make the system self-cleaning, by rapidly vibrating either the anti-aliasing filter itself or a protective cover glass lying above it, to shake loose adhering dust particles. Once dislodged, a strip of sticky materia at the bottom of the sensor cavity or mirror box catches and holds them. This approach was pioneered by Olympus, but has recently been adopted by Sony as well, in their DSLR-A100 model.

In typical fashion, Canon's camera engineers took a deeper look at the issue of sensor contamination, and have come up with a multifaceted, system-wide approach to reducing the impact of dust on users' photographs.

Canon's approach uses a vibratory cleaning method as a primary part of the overall strategy, but they've introduced several refinements as well. Here's what they've done:

Shooting Priority: Shutter Button Overrides Cleaning Cycle

It's critical for a digital camera to power up and be able to capture the first shot quickly, to avoid missing the action when first starting up. Some cameras with integrated cleaning systems can take a second or two for the cleaning cycle to complete before they're ready to capture an image. To avoid this problem, the Canon XTi aborts its normal power-on cleaning cycle as soon as the user touches the shutter button. This insures that you won't miss a critical shot

Special Shutter Coating/Construction

It turns out that only some of the dust that appears in DSLR images comes from outside the camera: As they age, normal wear and tear can make shutter curtains shed microscopic particles that eventually end up on the sensor. In the Rebel XTi, Canon has introduced a special shutter-curtain coating designed to greatly reduce the shedding of particles.

Along with the specially treated shutter, Canon has also begun using a different plastic in their DSLR body caps, one less prone to creating shavings that can drop into the mirror box.

Split Anti-Aliasing Filter

Rather than introducing a separate cover glass into the optical chain, Canon has split the XTi's anti-aliasing filter into two parts. One element sits right above the sensor, while the other is positioned further away. It's this outer element that's vibrated during the cleaning cycle.

In describing the technology, Canon notes that the outer anti-aliasing element is positioned further from the sensor surface (a millimeter or so) than is normally the case. This greater distance reduces the effect of any dust that does adhere, by making the shadow cast by each dust particle larger and softer-edged.

Anti-Static Coating

To make it harder for dust to gain a foothold in the first place, the outer anti-alias filter element bears an anti-static coating. Dust particles frequently carry static charges, so the anti-static coating avoids one of the key mechanisms by which dust particles adhere.

Dust Delete Image Processing

No matter how good an automatic cleaning system, there are going to be some stubborn dust particles that it can't dislodge. To deal with these, the Canon XTi has the ability to shoot a dust reference photo, and then transfer that information to Canon's Digital Photo Professional software, which can use it to eliminate the shadows cast by dust particles on the images.

This sort of image processing to eliminate dust isn't an entirely new development, it's been a feature of Nikon's Capture software for some time. Canon's implementation has some additional wrinkles that make it somewhat more useful. Primary among these is that Canon's "Dust Delete" processing works for both JPEG and RAW format files. (Nikon's works only with RAW images.) This really expands the utility of the anti-dust processing, and makes it more accessible to amateur users who shoot exclusively or primarily in JPEG format.

Canon's anti-dust approach is also different in that the dust map ("Dust Delete data") that the software uses to perform its magic is stored in the headers of the JPEG or RAW files created by the camera. There's thus no need to keep track of a separate dust image file, the information is always available in the file headers, assuming you've actually performed the dust-mapping process. You can update the Dust Delete data any time you think the camera might have been exposed to dust, or after you've manually cleaned the sensor. The latest dust map is automatically incorporated into the EXIF headers of all JPEG images, or the headers of any RAW files. While we don't have any technical details on how the dust map is stored, Canon claims that the encoding scheme used for it is very efficient, so the dust map information adds very little to the file size.

The screenshots above show the steps in capturing a Dust Delete reference image. Starting in the upper left from Shooting Menu Screen 2, selecting Dust Delete Data produces a screen that shows when the last Dust Delete reference image was captured. Selecting OK on this screen initiates a cleaning cycle, after which the camera prompts you to take a picure of a blank white surface. (The camera automatically defocuses the lens and sets the aperture appropriately, to produce the best possible dust image.) The camera then processes this data, and reports whether it was successful or not. If the surface you used to capture the image wasn't sufficiently uniform, you'll get an error message, but if the image was good, you'll see the confirmation screen as shown above in step 6. The Dust Delete Data just generated will now be included in the headers of any JPEG or RAW images captured, until you decided to capture a new dust reference image.

We don't have any quantitative way of evaluating dust-removal systems, but based on Canon's description of it, their anti-dust technology does appear to go a step or two beyond anything else currently on the market, providing a more comprehensive solution to the problem of dust in DSLR images than we've seen to date.

 

The images above were taken from our standardized test shots. For a collection of more pictorial photos, see our Canon EOS 400D Rebel XTi Photo Gallery .