Canon 7D Optics
Canon EOS 7D Optics
The sub-frame sensor on the Canon 7D 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 Canon 7D 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 Canon 7D will show the same field of view as a 160mm lens on a camera with a 35mm frame size.
The Canon 7D also offers Lens Peripheral Illumination Correction, which corrects for lens shading (commonly called "vignetting"), producing uniform exposure across the frame by compensating for the light falloff seen with some lenses in the corners of the frame. Lens Peripheral Illumination Correction first appeared in the Canon 5D Mark II, and was also included in the midrange Canon 50D.
Canon 7D: All-New AF system
The Canon 7D incorporates an all-new AF system, with a new AF array, enhanced focus mode options, new servo tracking algorithms, and automatic correction to increase focus accuracy under artificial lighting. The AF system is now also more integrated with the exposure metering system, but we'll discuss that separately, in the Exposure section.
Canon 7D Autofocus System, AF Modes
The Canon 7D's AF system has a total of 19 points, arranged in a diamond-shaped pattern. All points in the main array are cross-type (sensitive to both horizontal and vertical detail), and usable with a minimum aperture of f/5.6. Additionally, there's an X-shaped AF point in the middle of the frame that provides increased AF accuracy for the center point with lenses having maximum apertures of f/2.8 or larger.
The Canon 7D's AF system includes a broader range of area selection modes than we've seen previously on any Canon SLR. Here's a breakdown:
That's a lot of AF options to choose from, but Canon's made it easy to switch back and forth between them via the default configuration of the M.Fn button: Pressing this button while rotating a command dial lets you quickly step between the five different AF modes.
Canon 7D AI Servo Autofocus Operation
Canon has apparently made considerable changes to its focus-tracking algorithms, as the 7D's AI Servo mode is more sophisticated than those of earlier models. The basic tracking speed of the Canon 7D is the same as that of the EOS 50D, but its handling of subject motion, lost focus, and integration of information coming from the lens have all taken a step forward, according to Canon representatives.
AI Servo is Canon's name for their predictive autofocus system, which tries to extrapolate future subject position from past subject motion so the AF system can move the lens elements smoothly to follow it. For instance, if a subject is moving toward the camera at some constant speed, AI Servo would determine that the lens needed to move a certain amount between each AF "look." By moving the lens elements more or less smoothly, the amount of additional correction in each AF cycle will be that much less, making it easier for the camera to follow rapidly-moving objects.
A subject moving at constant speed toward or away from the camera is one thing, but what about subjects that suddenly change direction? This is apparently one of the changes made in the Canon 7D's AF system, namely that it can better detect and cope with subjects that suddenly reverse course.
This would apply to something like a football player making a sudden course correction to avoid a tackle. That's certainly one example, but another might be dealing with the often significant changes in lens/subject distance in hand-held macro photography. That's where the greater integration of lens information comes into play: When the camera finds it's dealing with a subject at very close range, it will be expecting greater changes in relative focal setting then when shooting subjects at greater distances. Interestingly, Canon told us that the 7D can even adjust the sampling frequency of the AI Servo system in response to lens-derived information of this sort. Ultimately this could be seen as a third axis of the image stabilization system, adding ability to use the AF system to track a flower that's moving in the breeze.
Another difference in the Canon 7D's AI Servo operation has to do with how it deals with lost focus in a continuous-mode shot sequence: If the AF system detects a focus error for one of the shots in a series (which presumably means you had C.Fn III/2 set for shooting speed priority), the AI Servo system will disregard the focal setting associated with that shot in its motion-prediction calculations. By throwing out bad data, the AI Servo algorithm can greatly improve the accuracy of its predictions.
As noted earlier, a significant feature upgrade in the Canon 7D's AI Servo mode is that you can now specify which of the AF points you want the camera to begin its servo tracking from, when 19-point Auto AF Area mode is selected.
Finally, in a big step for photographer awareness, the Canon 7D now shows the currently-active AF point in the viewfinder, while in AI Servo mode. This always struck us as a critical piece of missing information in Canon's AF systems: When in AI Servo mode, we never had any idea what the camera was actually focusing on while we were looking through the viewfinder. With the 7D, you'll now be able to tell if the camera has locked onto something other than your primary subject. (Note that this tracking info is only displayed in 19-point auto-area mode when AI Servo is active.)
The Canon 7D uses the built-in flash head as its AF-assist illuminator, rather than a bright light built into the camera's body. In practice, this works well: the flash is quite bright, and probably has a longer range than the more typical on-body illuminator bulb. You can disable the 7D's internal flash (or external Speedlite) by going into the Flash Control menu, which still lets the AF-assist pulses fire; but note that you lose your flash capability until you turn it back on again.
If you attach a Canon Speedlite 550EX, 580EX, or 580EX II external flash unit to the Canon 7D, its internal AF-assist illuminator is used instead of the flash head itself, providing a useful working range of about 50 feet with a less obtrusive light source. For non-flash photography, Canon's ST-E2 wireless sync transmitter can apparently also be used for AF assist. The ST-E2's AF-assist light has a useful range of about 25 feet.
Dust Reduction Technology
First introduced on the Canon Rebel XTi, Canon's system-wide approach to reducing the impact of dust on the image sensor is also included on the Canon 7D. Every DSLR ever sold 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.
A key feature of any SLR 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. In truth, 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 or low-pass 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 material at the bottom of the sensor cavity or mirror box catches and holds them. This approach was pioneered by Olympus, but has since been adopted by Canon, Nikon, Pentax, Panasonic, and Sony.
In typical fashion, Canon's camera engineers took a comprehensive look at the issue of sensor contamination, and came 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 7D 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 due to sensor cleaning.
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 Canon 7D, 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 Canon 7D'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.
Previous models in this line of Canon cameras used an anti-static coating on the anti-alias filter to make it harder for dust to gain a foothold in the first place. Canon has developed a new fluorine-based coating for the 7D, which purportedly improves this dust resistance property. 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 7D 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, though. 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 picture 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.
Don't count on the anti-dust system to do everything, though.
Despite the many advances in Canon's anti-dust technology, though, we feel compelled to point out that we've thus far seen no anti-dust system that completely eliminates the need for sensor cleaning. Sooner or later, you're going to need to clean your sensor, so we strongly recommend purchasing a good-quality sensor cleaning kit right along with your DSLR. Automated anti-dust systems like Canon's will certainly help with some of the dust, typically the dust that the nylon brush-based cleaning systems can also handle. Inevitably, though, you'll encounter dust that sticks to the sensor's cover glass tenaciously; dust that only a wet/dry cleaning approach can remove. We ourselves use and recommend products from Copper Hill, which we've found to be both highly effective and among the most reasonably priced on the market.
Canon 7D Optical Test Results
The Canon EOS 7D is sold in the US as body only, or in a kit bundled with the Canon EF 28-135mm f/3.5-5.6 IS USM lens. Because of the Canon 7D's 1.6x crop factor (making the 28-135mm equivalent to 45-216mm on a 35mm camera), we don't recommend this lens for subframe bodies, especially since it isn't a stellar performer when coupled with higher-resolution sensors. See below for test results with the Canon 7D.
Kit Lens Test Results
Fair performance with the Canon 28-135mm IS (Image Stabilized) kit lens.
|28mm, f/8.0||135mm, f/8.0|
The Canon 7D is offered with an 28-135mm IS kit lens, with a fairly generous optical zoom range of ~4.8x, but an odd focal range for a subframe camera. The 35mm equivalent range is about 45-216mm, which isn't wide at all at the wide-angle end. Details are somewhat soft across the frame at 28mm, even at f/8.0. Corners are slightly softer, and some chromatic aberration is noticeable. Results at full telephoto are similar with softness across the frame and again, moderate levels of chromatic aberration. We liked this lens when we tested it for SLRgear a while back (see our Canon 28-135mm IS review for those details), but we think it's just not up to the challenge of an 18-megapixel sensor. All in all, the EF 28-135mm IS has about average performance for a kit lens, and the built-in Image Stabilization is very handy for low-light photography, but we really think most users will want something that starts a little wider.
Below average sized macro area for a modern SLR kit lens, with soft detail. Flash throttled down well.
28-135mm IS Kit Lens
|Macro with Flash|
As with zoom performance, the Canon 7D's macro performance will depend entirely on the lens in use. With the 28-135mm IS kit lens set to 135mm, the 7D captured a below average size minimum area measuring 3.62 x 2.42 inches (92 x 61 millimeters). Details were quite soft, with additional blurring in the corners from the lens. (Most lenses have some softening in the corners at macro distances.) The Canon 7D's flash throttled down for the macro area pretty well, and there was no detectable shadow from the lens barrel, resulting in a good exposure with the flash.
Moderately low geometric distortion with the 28-135mm IS kit lens.
|Barrel distortion at 28mm is 0.7 percent|
|Pincushion distortion at 135mm is 0.2 percent|
The Canon 28-135mm lens produced about 0.7 percent barrel distortion at wide-angle, which is slightly below average but still noticeable in some of its images. At the telephoto end, there was about 0.2% pincushion distortion, about average but still noticeable in some shots. Geometric Distortion is the tendency for the lens to bend straight lines outward (like a barrel -- usually at wide-angle) or inward (like a pincushion -- usually at telephoto).
Chromatic Aberration and Corner Sharpness
Moderate and bright chromatic aberration at both wide-angle and telephoto with the 28-135mm IS kit lens. Soft corners on the right-hand side at wide-angle.
Chromatic Aberration. Chromatic aberration in the corners with the Canon 7D's 28-135mm kit lens is pretty evident (we'd call it on the high side of "moderate," or "noticeable") at both wide-angle (28mm) and telephoto (135mm) settings. The color fringing gradually reduces in brightness and width as it approaches the center of the image, where it is almost nonexistent.
Corner Softness. The Canon 7D's 28-135mm IS kit lens produced some soft corners in a few shots. At full wide-angle, corners on the right side of our test targets were moderately soft compared to the center, and the blurring extended fairly far into the frame. The left side, however, showed almost no softening in the corners. The center of the image was a bit soft. At full telephoto, all four corners were only slightly soft, and not much softer than the center. An average performance here, especially considering this is a full-frame lens where much of the captured image should be in the sweet-spot of the image circle.
The images above were taken from our standardized test shots. For a collection of more pictorial photos, see our Canon EOS 7D Photo Gallery .
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