The 40D is much more durable, with its tougher magnesium alloy exterior, compared to the Rebel's plastic-like, polycarbonate exterior. There's a price for this toughness: The 40D is 10 ounces heavier than the Rebel XTi. However, the 40D has weather sealing throughout at key points of its body to prevent moisture and dust from affecting the inside of the camera. The 40D also has more robust processing power and a better prism (for a brighter viewfinder). There are many other differences between the two models, but those are some of the more obvious ones.

[Dec. 28, 2007 UPDATE: As one of our blog readers pointed out, the Canon EOS 40D isn't completely "ruggedized" and impervious to all elements. But weather sealing at the 40D's battery door, CF card door, and hot-shoe connection for the flash is still more protection than the previous 30D offered. —Ed.]

From the specs, you'll notice that the 40D also stands out from its predecessor, the 30D. The 40D is 10 megapixels vs. the 30D's 8 megapixels, and the LCD (Click on image at left) is 3 inches compared with 2.5 for the 30D. Another important improvement is that if you accidentally open the memory-card door, you won't lose your image. The 30D stops writing the data to the card once the door was open, while the 40D continues until the file is on the card. You'll also find an improved battery on the 40D, allowing for more shots on a single charge.

Lastly, in comparison with the 30D, there's another intriguing new feature—which may at first seem to be only of interest to professional shutter bugs: it's the 40D's new 14-bit A/D converter (up from 12-bits), which Canon says will produce images with finer tonal gradations and wider color ranges. But the converter may also benefit novices: Canon claims that since JPEGs (which are 8-bits images) are based on 14-bit RAW files (that are converted inside the camera), they will have fewer blown—or overexposed—highlights. That's important since you often can't fix an overexposed or blown out part of a photo using photo-editing software.

After a couple of days with the 40D, I was particularly impressed with its burst mode and live view LCD.

Burst Mode

A digital camera's burst mode allows you to fire off a number of shots in a short period. The exact number depends, in part, on what type of files you're shooting (JPEG, RAW or a combination of the two). Professional SLRs, like the 10-megapixel Canon EOS-1D Mark III, can typically fire off 9 to 10 frames per second for as many as 100 or more JPEGs.

Compared with such pro models, the 40D doesn't shoot quite as fast or for as long, but it's still very impressive. According to the specs, the 40D can fire off 6.5 frames per second for up to 75 JPEGs. When I used Canon's EF 24-105mm f/4L IS USM zoom lens (which costs about $1000) shooting in JPEG format, I fired off 57 frames in about 10 seconds—about 5.5 fames per second. (I happened to use a moderately slow shutter speed, 1/50 of a second, which may be the reason I didn't achieve 6.5 fps). This is a bit faster than Canon's claim for the 30D, which was 5 fps.

Live-View LCD

Another new feature that's starting to appear in SLRs is a Live View function, which lets you compose shots directly on the camera's LCD the way you would on a point-and-shoot digital camera. But unfortunately, when I used this function on the 40D, the autofocus system appeared to become disabled. At the very least, it doesn't work the same as it does when you're using the 40D's viewfinder.

So, how do you focus the camera? You have two options: The first is to manually focus. To do that, you must make sure your lens is set to manual (MF) and not autofocus (AF). When manually focusing, you can move a "focus frame" (a small rectangle that appears on the LCD) around the display until you've placed it over the focal point of your composition. Then, you magnify the picture by pressing a button to enlarge that section either 5x or 10x, and, you focus manually on this section of the image and shoot.

The second focusing option is to press an "AF-ON" button on the back of the camera. This uses the same AF features that the camera uses when you're not using live view. However, pressing the "AF-ON" button causes the LCD to black out (since the mirror drops down to engage autofocusing). When you let go of the button, the live image returns to the screen.

There are other interesting aspects to using this live view feature, such as being able to compose an image using a computer monitor and shooting in a silent mode. Live view shooting can also be quite useful when working with long exposures, such as shooting at night. When set on a tripod, the 40D's live view mode will allow you to minimize camera shake and avoid blurry photos, since the mirror will not be moving inside the camera.

Sensor Size and Lens Magnification Factor

Although there are many new or improved features on this SLR, one element that remains consistent with its predecessor, the 30D, is that it still has the same size (APS-like) sensor.

That's important because that sensor is smaller than a 35mm film slide (which is the size of a full-frame sensor). In the camera body, that smaller sensor format effectively "crops" the image produced by the lens. To you, the effect is to magnify the image or increase the telephoto capability of your lens.

So, on the 40D, which has an APS-like sensor and a lens magnification factor of 1.6x, a 100mm lens becomes equivalent to a 160mm lens.

This is good news for nature enthusiasts who love to photograph animals or sporting events at a distance. The down side is that this magnification factor dramatically curtails the wide-angle end of the spectrum. A nice 24mm wide-angle lens, when used on the 40D, effectively becomes a not-so-wide 38.4mm lens.

So, in order to help clarify just what this magnification factor means, I shot the same scene in a conference room here in our headquarters in Yonkers, New York, one that has a big window in it, using the new 40D, which has a magnification factor of 1.6x and the Canon EOS 5D, which has a full-frame-sized sensor, with a magnification factor of 1x.

In the Photo A (Click on the image at left), I used Canon's EF 24-105mm f/4L IS USM zoom lens (which costs about $1000) on both cameras. In Photo B, I used a 70-300mm f/4-5.6 IS USM lens (which lists for about $600) on both cameras.

In Photo A, I set the zoom lens at its widest, 24mm setting and snapped both photos. You can see how the 5D lets you view more of the interior setting than the 40D. Indeed, the sensor within the 40D is actually cropping the image produced by the lens and turning it into the equivalent of a 38.4mm lens. Notice how, in the 40D shot, the chair and the telephone are cropped by each edge of the picture frame. In the 5D shot in Photo A, there is lots of space to the left of the telephone and to the right of the chair.

In Photo B (Click on image at right), notice again how the 40D appears to zoom farther in on the scene, cropping most of the cars out of the bottom of the scene as well as some of the tree trunk. In the 40D image in Photo B, the 1.6x magnification factor turns the 300mm end of the zoom into the equivalent of 480mm. On the 5D, it remains a 300mm focal length.

However, it's important to remember that although lenses may appear to change when used on a camera that has an APS-like sized sensor, the lenses retain their original characteristics. For example, a 24mm lens may appear to look like a 38.4mm lens, but it will still have the distortion and other characteristics common in wide-angle lenses. Say you’re standing too near someone's face for a portrait and are using a 24mm lens. The nose will still appear much larger than if you used the longer focal length. And that will be true when using a wide-angle lens on both the 5D and the 40D.

It's worth noting an SLR's magnification factor when you're deciding on a camera body, especially if you shoot wide-angle compositions. If you own a number of compatible wide-angle lenses and can afford its higher price, consider purchasing a full-frame sensor SLR. Or, if you go for a camera like the 40D, you may need to purchase lenses that are wider than the ones you own. Aside from the magnification factor, the 40D has a very strong set of features that make it a very capable, high performing camera for many types of photography.

—Terry Sullivan