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.)
Color-Sensitive AE Sensor
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.
The Canon 7D's new AE sensor divides the frame into 63 separate zones, the data from which can be evaluated in a variety of ways, depending on the AE mode you're operating in. AE and AF zones are aligned, allowing exposure information to be associated with specific AF sensors and the area around them.
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
The Canon 7D offers regular ISO equivalents of 100, 200, 400, 800, 1,600, 3,200 and 6,400, with the option to use fractional settings in 1/3-EV increments (100, 125, 160...). The EOS 7D also offers a high-range ISO setting enabled when ISO Expansion is turned on, which is equivalent to ISO 12,800. An Auto ISO mode adjusts the ISO as the camera deems necessary, from 100 to 3,200, however the Auto ISO range is not programmable, and neither is the minimum shutter speed the camera should use before raising the ISO.
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)
The Canon 7D has the simplified AE Lock button from more recent models, which unbundles the AF Lock feature from the old button on the older EOS cameras like the 20D and 30D. Marked with an asterisk (*) symbol, the AE Lock button simply holds the exposure at one setting while you recompose the image. It's very useful when spot metering, but also when dealing with subjects where you want to draw your exposure from one place, while autofocusing on another. You may hold the AE Lock button down while depressing the shutter release multiple times to maintain the same exposure across multiple shots. Pressing the button with the pop-up flash activated or with an external flash mounted activates the FE Lock (Flash Exposure) function, which fires the flash and locks the proper exposure for the following frame.
White Balance Options
As you'd expect, the Canon 7D offers a full range of White Balance settings, including six presets, an Auto (3,000-7,000K) setting, a Kelvin temperature setting (2,500-10,000K, in 100K increments) and a Custom setting (2,000-10,000K). The six presets include Daylight (5,200K), Shade (7,000K), Cloudy/Twilight/Sunset (6,000K), Tungsten (3,200K), White Fluorescent (4,000K), and Flash (6,000K). The Custom setting bases color balance on a previous exposure, meaning you can snap an image of a white card and then base the color temperature on that image.
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.)
The Canon 7D's WB Bracketing is set on the same grid as the now familiar White Balance Correction control. Fairly sophisticated, the white balance correction tool lets you shift the color balance toward more or less green vs magenta or blue vs amber, using a +/-9 step grid format. You move a highlighted square through the grid to adjust the color balance, and bracketing adjustments spread the single square into a cluster of three.
As with other recent Canon DSLRs, the Canon 7D offers a Picture Style option through the LCD menu, which lets you select from Standard, Portrait, Landscape, Neutral, Faithful, Monochrome, or three User Defined settings. In each of the preset modes, the Contrast, Saturation, Sharpness, and Color Tone are set for specific conditions. For Monochrome, Saturation is replaced with Filter effect (options are: None, Yellow, Orange, Red or Green), and Color Tone is replaced with Toning Effect (None, Sepia, Blue, Purple or Green). The three User Defined options let you manually adjust each variable, then save it as a custom parameter. Of course, you can also set the camera's color space to sRGB or Adobe RGB.
Auto Lighting Optimizer (ALO)
First seen on the Rebel XSi, Canon's Auto Lighting Optimizer (ALO) function lets you expose for the highlights, preserving detail there, while the camera adjusts the image to open up the shadows. This happens on the fly, as the files are being written to the memory card, so there's no post-capture intervention by the user required to take advantage of this function. We weren't terribly impressed with ALO on the XSi, however newer Canon models including the 7D offer four different settings for it, and the control on the 7D seems quite effective. The ALO setting is made via Record Menu 2, where you can select options of Disable, Low, Standard (the default), or Strong.
Highlight Tone Priority (HTP)
Also included on the Canon 7D is Highlight Tone Priority (HTP for short), a feature that's been on Canon SLRs for some time now, and it's one that works quite well when dealing with subjects with important detail in strong highlights. (Think of the standard wedding dress shot, and you'll get the idea.) Digital cameras normally expose more like slide film: Once you hit a certain exposure level, detail just vanishes. This really becomes an issue when you're dealing with contrasty lighting and a subject with lots of highlights in it.
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
The Canon 7D offers two types of adjustable noise reduction. Long Exposure noise reduction can be performed for exposures one second or longer, and works by taking a second "dark frame" of equal duration with the shutter closed, and then subtracting it from the first frame. This reduces or eliminates most noise generated by the sensor during long exposures at low ISOs, but can make noise worse at higher ISOs (at ISO 1,600 and above). Available settings are Off, Auto and On, and are accessed from Custom Function II-1. The Off setting is the default. The 7D also offers the user four levels of high ISO sensitivity noise reduction. Options are Disable, Low, Standard and Strong, with Standard being the default. Though the name implies this noise reduction is only applied at high ISOs, the Canon 7D applies it to all ISOs. This explains why the 7D's chroma noise in shadows and darker tones is lower than most DSLRs at low ISOs, when using the default Standard setting. These settings are accessed in Custom Function II-2. High ISO noise reduction is not applied to RAW files.
Continuous Shooting Mode and Self-Timer
The Canon 7D's High-Speed Continuous Shooting mode is rated by Canon at 8 frames per second, for a total of 94 Large/Fine JPEG shots or 15 RAW before the buffer fills. When using a UDMA-capable CompactFlash card , the Large/Fine JPEG number increases to 126 while RAW remains the same at 15 frames. We measured 7.97 frames-per-second for 22 Large/Fine JPEGs and 7.98 frames-per-second for 16 RAW frames, when we tested with a fast UDMA card. When shooting JPEGs of a very complex scene with a lot of sharp, fine detail, images may compress less and result in lower buffer capacities, as was seen in our testing. The Canon 7D also offers a Low-Speed Continuous mode, which we clocked at 2.99 frames per second for over 60 Large/Fine JPEGs and 27 RAW files.
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
Canon has included two optional modes to reduce noise in Live View mode, first seen on the 5D Mark II. Called "Silent Modes," the first leaves the first shutter curtain open while you shoot up to the maximum 7.0 frames per second. The second mode is a single shot mode which spreads the sounds out, not re-opening the shutter until you release the shutter button.
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 .
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