Canon 7D Exposure
Canon 7D: All-New Exposure System
Not content with coming up with an entirely new AF system for the 7D, Canon also developed an entirely new exposure system, including a new autoexposure sensor that incorporates color information for the first time in a Canon SLR. Canon has branded their new AE system IFCL, which stands for Intelligent Focus Color Luminosity metering, interesting because it incorporates both color and focus in a way we haven't seen previously. (Or at least in a way that hasn't previously been marketed.)
Hearing it described by Canon staff, the new AE sensor sounds like it's borrowed a page from Foveon, the erstwhile image sensor company renowned for its imaging chips that stacked red, green, and blue photodiodes on top of one another at each pixel position. Canon's new AE sensor doesn't go quite that far, but does have two separate planes, one each sensitive to Blue/Green and Green/Red light. That's enough color information to help with exposure accuracy for subjects at either end of the color spectrum.
Because silicon light sensors are significantly more sensitive to longer-wavelength light, unless an exposure sensor's response is substantially tweaked via a filter system of some sort (which naturally decreases sensitivity), it will tend to underexpose red-colored objects or scenes, and overexpose ones dominated by blue or green hues. Thanks to its new color-sensitive exposure sensor, the Canon 7D should be able to markedly improve exposure accuracy in situations where the subject is dominated by colors at one end or the other of the color spectrum.
One thing that struck us as new in the Canon 7D's AE system was the way it integrates distance information from the autofocus system into the exposure metering process. Competitor Nikon has for a while used their high-res AE sensor to help with AF tracking, but Canon's turned the idea on its head, and made very logical use of subject-distance information from the lens and AF system to make better exposure decisions.
Like most good ideas, the core of this one is very simple, but that's the nature of most good engineering: If you've identified the subject as being located a certain distance away from the camera, nearby objects that are close to the same distance are most likely part of the subject, too. Thus, rather than simply relying on a spot AE reading centered on the primary AF point, or blindly combining exposure information from a cluster of AE points in some arbitrary geometric grouping around the active AF point, the Canon 7D instead gives stronger weighting to exposure sensor segments that lie beneath adjacent AF points showing a similar distance reading.
Hearing it described, this sounds like a bit of a "Well, duh" conclusion, but this is the first time we've heard this particular exposure calculation approach described. Also, while it wasn't explicitly stated in the presentation we saw, we suspect that the color-sensitive capability of the new AE sensor plays a role here too: It seems highly likely that the 7D's exposure system takes color into account and considers contiguous areas of similar color to be an indication of the extent of the subject as well.
The Canon 7D's exposure metering options include 63-zone Evaluative, Center-weighted Average, Partial (9.4% of viewfinder at center), Spot (2.3% of viewfinder) options. Metering sensitivity range is specified at 1-20 EV (at 23°C/73°F, with EF 50mm f/1.4 USM lens, ISO 100).
The Canon 7D's Exposure Compensation setting allows the user to increase or decrease the metered exposure by up to five stops positively or negatively, in one-third or one-half EV increments. Although Evaluative metering is linked to the active AF point (whether automatically or manually selected), Spot metering is fixed to the center of the viewfinder (unlike Nikon SLRs, which can spot meter at any selected AF point).
Here you can see the coverage of the 7D's four metering modes. From left to right: 63-zone Evaluative, Center-weighted Average, 9.4% Partial, and 2.3% Spot metering.
In Live View mode, evaluative metering is always used, however Canon does not specify how many zones or sensitivity range. When Face Detection AF is enabled in Live View mode, the Canon 7D biases the exposure in an attempt to properly expose for a detected face.
ISO Sensitivity Options
An automatic exposure bracketing feature lets you set the Canon 7D's total exposure variation (across three shots) at anywhere from +/- one-third or one-half EV, up to +/- 3 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 0.7 EV, and then have the camera give you a variation of +/- 2/3 EV around that point. The number of shots in a sequence is fixed to three, however the sequence order can be selected changed from 0, -, + (default, under, over) to -, 0, + (under, default, over) in Custom Function setting I-5.
AE/FE Lock (" * " button)
White Balance Options
A White Balance bracketing option snaps only one image, but then writes three successive files from that single capture. Bracketing steps are from -/+ 3 steps. (Each step 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.)
Auto Lighting Optimizer (ALO)
Highlight Tone Priority (HTP)
HTP's action is pretty subtle, but the results are very evident when dealing with strong highlights under harsh lighting. The way it works is to set the camera's base ISO up one notch, to 200, so it's only half-filling the sensor's pixels with charge during the exposure. The Canon 7D then alters its tone curve, basically compressing the top half (that would normally be blown out) into a smaller range, thereby preserving the highlight detail. You can do this yourself when working from RAW files, you just need to significantly underexpose most of the scene, and then fiddle with the tone curve to drastically reduce the contrast, but only in the extreme highlights. If that sounds difficult, it is; it can be a real time-sink, and very difficult to make the end result look natural. Canon's HTP does this for you automatically, though, and the results look just great: You have no sense that the camera has been making radical adjustments to its tone curve; you just see all the detail in the highlights that otherwise would be missing. HTP is controlled via Custom Function setting II-3, giving you options to Disable (the default) or Enable it.
Noise Reduction Options
Continuous Shooting Mode and Self-Timer
The Canon 7D's Drive setting also accesses two Self-Timer modes, which open the shutter 10 or 2 seconds after the Shutter button is pressed. The 10-second setting gives you time to dash around in front of the camera, while the 2-second setting is useful for tripping the shutter without touching the camera, to minimize camera shake. A number of wired (RS-80N3, TC-80N3, LC-5) and wireless (RC-1, RC-5) remote control units are supported in Self-Timer modes as well, a first for this level of digital SLR from Canon.
Silent Shooting Mode
First a little explanation. Regardless of the camera, at high speeds, a mechanical shutter never fully exposes the sensor. On the Canon 7D, in order to get a fast exposure above 1/250 second (the X-sync on the 7D), the second curtain has to follow right behind the first, creating a slit that moves across the sensor. Well, it turns out that recent Canon sensors can simulate the first part of this mechanical slit by starting to scanning the pixels in a line from top to bottom. Then the second curtain does have to come into play to close off the slit and finish the exposure. That means you can open both mirror and shutter once to enter Live View mode, then fire off frames at 7 fps with only the sound of the second shutter, because the mirror and first curtain don't move.
Mode 1 is quite fast, with less noise and vibration, both because the mirror's not flapping around (it's locked up in Live View mode), and because of the electronic first curtain trick.
The other quiet mode, Mode 2, is more about spreading the sounds out. It's a single-shot mode, regardless of what Drive mode you have set before you enter Live View. Just press and hold the shutter down. All you hear is a quick "tick." That's the second curtain shutting. The image appears onscreen for two seconds, and then the screen goes black, because the second curtain is still closed. Hold the shutter for as long as you like. When you decide to release it, the rest of the camera functions will run, resetting for the next shot, and Live View will return to the LCD. These reset sounds are also pretty quiet, so I'm sure Mode 2 would be helpful when photographing wildlife.
The images above were taken from our standardized test shots. For a collection of more pictorial photos, see our Canon EOS 7D Photo Gallery .
Not sure which camera to buy? Let your eyes be the ultimate judge! Visit our Comparometer(tm) to compare images from the Canon EOS 7D with those from other cameras you may be considering. The proof is in the pictures, so let your own eyes decide which you like best!
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