New in the Canon EOS 7D is a feature that tweaks the behavior of the AF system based on color information from the new color-sensitive exposure sensor, reducing AF problems often encountered when shooting under artificial illumination. Canon staff told us that back-focus errors can relatively common under artificial lighting, particularly in stadiums or other sporting venues. Canon didn't go into details of why this might be the case, or what specifically they did to correct it, but it's not too hard to figure out: Without special correction, a lens system will bring different wavelengths of light into focus on different planes.
See the illustration image, above right. High-quality camera lenses are usually well-corrected apochromats, so this isn't a problem at the image sensor. The relatively simple optics lenses and prisms in AF systems are another matter, though, so it's very likely that long red wavelength light would produce a different focus point than would short blue wavelength illumination.
By taking the color of the light source into account, the camera can compensate for this tendency, in much the same way that it can handle micro focus adjustments for different lenses. 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. 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 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.
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 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 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 EX, EX, or EX 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. 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. 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.
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:. 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. 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.
With a fast burst rate, users will be able to capture sporting events, wildlife or active children with precision and sharpness. To cater to the needs of users who prefer low-shooting speeds and capturing slower moving subjects, the EOS 7D Mark II comes equipped with manually adjustable speeds — a feature previously only available in the top range EOS 1D series.
With a new mirror bounce reduction mechanism, users may opt for Silent Mode while shooting under 5 fps. With this new system, subjects will be constantly kept in focus even with erratic changes in their distance from the camera. AF speed can also be adjusted to suit more sophisticated shots, such as slower focus shifts to suit softer moments.
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