Canon EOS 20DBy: Shawn Barnett and Dave EtchellsSlightly smaller and lighter upgrade brings greater speed and ease of use along with higher res and lower image noise. <<Optics :(Previous) | (Next): Shutter Lag & Cycle Time Tests>> Page 7:Exposure & FlashReview First Posted: 08/19/2004, Update: 11/19/2004 |
Exposure
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The Aperture Priority and Shutter Priority modes work much the same as on any other camera, allowing you to adjust one exposure variable while the camera selects the other for the best exposure. A Custom menu setting enables a "safety shift" option, which automatically adjusts the primary variable (aperture or shutter speed) in Av or Tv modes, if the setting you've selected won't permit a good exposure under the current lighting conditions. This could come into play if you were shooting in shutter-priority mode to achieve a motion-blur effect, but the light suddenly got brighter, pushing the required aperture value beyond what the lens could provide. In this situation, the camera would automatically boost the shutter speed the minimum amount needed to achieve a good exposure. Program mode keeps both variables under automatic control, while Manual mode gives you control over everything.
The Automatic Depth-of-Field mode (A-DEP) uses all nine autofocus zones to
determine the depth of field in the active subject area. Once it has determined
the range of focusing distances present across the nine zones, it automatically
computes the combination of aperture and shutter speed needed to render the
nearest and furthest points in sharp focus. This is a remarkably useful feature,
even for professional photographers. In many situations, you want to keep several
subjects in focus, while at the same time trying for the highest shutter speed
(largest aperture) that will permit that. In practice, faced with such situations,
I've usually resorted to just picking the smallest aperture feasible and hoping
for the best. With the 20D's A-DEP mode, the camera takes the guesswork out
of this process and gives you the fastest shutter speed it can manage while
still keeping things in focus. (In playing with this, I was often surprised
by how large an aperture in fact would work. I frequently would have chosen
a much smaller aperture to stay on the safe side.)
Introduced on the 10D and continued here is an ISO speed extension, which increases
the 20D's maximum ISO speed to 3,200. (Default ISO is 100, other normal options
are 200, 400, 800, and 1,600.) For adjusting the exposure, the 20D's Exposure
Compensation setting increases or decreases overall exposure from +/-2 EV in
either one-half or one-third EV increments. The default step size is 1/2 EV,
but you can set an increment of 1/3 EV via the camera's Custom menu. (Frankly,
I've always found that one-third EV compensation is just about ideal for digicams.
One-half EV steps are just too broad to set critical highlight exposures accurately.)
Automatic exposure bracketing on the EOS 20D lets you set the total exposure
variation (across three shots) at anywhere from +/- 1/2 or 1/3 EV all the way
up to +/- 2 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 set positive compensation of 0.7EV, and then have the camera give you
a variation of +/- 2/3 EV around that point. Whatever EV step size is set through
Custom menu also sets the bracketing step size.
I really like the amount of information the 20D gives you about its exposure,
not only in terms of the settings it's using, but in the form of feedback on
how pictures you've captured turned out. You can select an "Info"
display mode when viewing captured images on the rear-panel LCD screen, which
produces the display shown at right. Notable here is that you not only can see
all the exposure parameters, but you get excellent feedback on the tonal range
of the image itself. One form of feedback is the histogram display at upper
right, which shows how the tonal values are distributed within the image. Histogram
displays are useful for directly seeing how the overall exposure turned out
in an image, but I've found them to be of limited usefulness for making critical
judgments about highlight exposure.
Digital cameras need to be exposed more or less like slide film, in that you
need to zealously protect your highlight detail. Once you've hit the limit of
what the sensor can handle, the image "clips" and all detail is lost
in the highlight areas. The problem is that it's quite common for critical highlights
to occupy only a very small percentage of the overall image area. Because they
correspond to such a small percentage of the total image pixels, the peak at
100 percent brightness can be very hard to distinguish in the histogram display.
To handle such situations, the 10D blinks any pixels that are 100 percent white
on its screen, alternating them between black and white. This makes localized
overexposure problems leap out at you, making it very easy to control the critical
highlight exposure precisely. (The sample image shown in the display above is
a pathological example, chosen to show how the feature works. In practice, you'd
probably never overexpose an image that badly.)
Besides
the abovementioned exposure information and feedback, the 20D's playback options
include a thumbnail index display, normal full-frame viewing of captured images,
and a zoomed view, as shown at right. There's also a "jump" mode,
activated via the Jump button on the rear panel of the camera. Jump mode lets
you very quickly move through images stored on the memory card, jumping 10 shots
at a time. The EOS 20D's image playback can be zoomed in very small steps anywhere
from 2-10x. Once you've zoomed in at any level, you can scroll the zoomed window
all around the image area, using the large rear-panel control dial and one of
the rear-panel buttons to control direction and movement.
Another feature deserving comment is the 20D's separation of the autoexposure
and autofocus lock functions. In consumer-level digicams, half-pressing the
Shutter button locks exposure and focus simultaneously. You can use this to
deal with an off-center subject by pointing the camera at the subject, locking
exposure and focus, and then reframing the picture before finally snapping the
shutter. The only problem is that you sometimes need to perform a more radical
recomposition of the subject in order to determine the proper exposure. For
instance, you may want to zoom in on the subject, grab an exposure setting,
and then zoom back out before taking the picture. Situations like that require
locking the exposure independently of the focusing, and the 20D provides for
just such eventualities by way of a separate AE lock button on the back of the
camera, right under your right thumb. (The "*" button.) A very handy
feature indeed, for those times you need it.
The EOS 20D offers a full range of White Balance settings, including six presets, an Auto setting, Custom setting, and Kelvin temperature setting. The six presets include Daylight, Shade, Cloudy, Tungsten, Fluorescent, and Flash. The Custom setting bases color balance on a previous exposure, meaning you can snap an image of a gray card and base the color temperature on that image. The Kelvin temperature setting lets you get even more specific, and offers a range of temperatures from 2,800 to 10,000 degrees Kelvin.
The EOS 20D also offers a Parameters option through the LCD menu, which lets you select Adobe RGB color space, or set up as many as three Parameters setups. Each setup lets you adjust Contrast, Sharpness, Saturation, and Color Tone, but the custom setups are all based on the sRGB color space.
One
of the 20D's more unique features is its two-dimensional White Balance Shift/Bracket
control. Conventional white balance "tweak" adjustments are generally
limited to adding blue or red, or perhaps just shifting a color temperature
setting that's calibrated in units of degrees Kelvin. The problem is that controls
of this sort treat color as if were a one-dimensional entity, when it's really
three-dimensional in nature. I've often been frustrated when trying to adjust
a camera's color balance, for instance wanting to shift it toward green, when
the camera offers options of only blue or red.
On the 20D, Canon offers a two dimensional adjustment for tweaking white balance, as shown above right. The current color balance is represented by a white cursor floating in a rectangular window representing color space. Moving the cursor up or down results in a shift toward green or magenta respectively, while moving it left or right produces a shift toward red or blue. Each adjustment step in the yellow/blue direction corresponds to 5 mireds worth of color conversion filter, and green/magenta steps are of a similar magnitude, although the green/magenta axis doesn't translate to the color-temperature shift units of mireds.
At first glance, you'd think that a two-dimensional color adjustment tool still wouldn't cover a three-dimensional color space, Canon's approach actually does just that. That's not to say that it is entirely straightforward though. To understand the control, it's important to remember that color (hue and saturation, as opposed to brightness) in an RGB image is determined by the relative amounts of red, green, and blue present, not necessarily by the absolute values of each color channel. The 20D's color shift display lets you control the green channel with either positive (green) or negative (magenta) adjustments, and the red and blue channels with positive-only tweaks.
The positive-only adjustments for the blue and green channels are where it's easy to get confused, but keeping in mind the relative nature of color balance, it's easy to see that you can effectively cut the red in an image by boosting the relative levels of green and blue together. This would correspond to a cursor position somewhere in the upper left quadrant of the Color Shift display's color space. Likewise, you can compensate for a blue cast in an image by boosting green and red together, placing the cursor in the upper right quadrant. Canon's color adjustment tool thus lets you dial in any white balance shift you'd like to make, even though it's only a two-axis control.
"But wait, there's more!" (To steal a line from TV infomercials.) The Bracketing aspect of the White Balance/Bracketing control comes into play when you turn the Quick Control Dial right. This expands the single cursor dot into a horizontal row of three dots, with slightly variable spacing. These represent the successive color values that will be used for a set of three shots that bracket the white balance. You can thus set whatever basic color balance you want, and then bracket with more or less red, or more or less blue, depending on where you are in the color space. Not enough? Turning the quick dial back left switches the set of three dots from a horizontal to a vertical array, letting you bracket with more or less green/magenta, rather than red or blue.
About the only possible remaining option would be the ability to rotate the set of three dots to arbitrary angles, but I guess the Canon engineers had to stop somewhere. Regardless, the EOS 20D's white balance adjustment control goes far beyond anything we've previously seen on a digital camera, regardless of price point.
Low Light Capability & Image Noise Performance
When operating the camera in full-manual exposure mode, the EOS 20D offers
a Bulb exposure setting for very long exposures. Normally, exposure times are
limited to a maximum of 30 seconds in Aperture- or Shutter-Priority modes, but
in Manual mode, you can expose for as long as 999 seconds by selecting Bulb
mode and holding down the Shutter button for as long as you want the shutter
to remain open. Obviously, 999-second exposures aren't a practical reality,
as sensor noise will totally swamp the signal long before that point is reached,
but the 20D does seem quite able to take very long exposures with very
little image noise resulting.
A full discussion of image sensor noise is beyond the scope of this review,
but the simple story is that the most obvious and objectionable noise you'll
see in long digicam exposures is so-called "fixed pattern" noise,
caused by variations in "dark current" between sensor pixels. "Dark
current" is just what it sounds like. Current (a signal) appears even when
the sensor isn't being exposed to light. When you look at a long time exposure
shot with a digital camera, you'll often see very bright pixels, where minor
manufacturing defects have resulted in unusually high "dark current"
levels. Often called "hot pixels," these flecks of color are very
distracting visually.
The normal way to deal with hot pixels is to take an exposure with the camera's
shutter closed, immediately after shooting the subject. If this "dark frame"
is exposed for the same time as the subject was, you can largely eliminate the
hot pixel problem by subtracting the dark frame information from the actual
exposure. In practice, this works fairly well, but has the disadvantage that
you have to wait for the dark frame exposure to be taken, requiring an appreciable
amount of time in the case of long time exposures. (If you shot a one-minute
exposure for the photo itself, you'll have to wait another minute for the dark
frame exposure to be made.)
While most other high-end digicams on the market use a dark frame subtraction
method to deal with image noise, previous d-SLRs using Canon's CMOS sensor technology
apparently did something quite different, as there was very little delay between
the end of the primary exposure and the writing of the image file to the memory
card. There was clearly no "dark frame" exposure involved. I suspect
that this advanced noise reduction processing was another consequence of the
"active pixel" CMOS technology Canon developed internally. Having
active circuitry associated with each pixel in the sensor array allows lots
of fancy processing that would be impossible otherwise, and it looks like Canon's
noise reduction system takes advantage of this.
In the EOS-20D though, while apparently still using the sophisticated on-chip
noise reduction processing we saw in the 10D, Canon has also added an option
for conventional dark-frame subtraction as well. Accessed via Custom Function
02, the "Long exposure noise reduction" seems to operate just the
same as dark-frame subtraction on other cameras we've seen it on. The difference
with the 20D though, is that there's precious little image noise to be subtracted
out, at least at exposure times of 30 seconds or less, where I did essentially
all my shooting. I can imagine the dark-frame subtraction option being useful
for astronomers doing 5-minute exposures with the 20D, but it will add little
to most users' image quality.
Flash
The
EOS 20D's built-in flash has a guide number rating of 43 feet (13 meters) at
ISO 100, translating to a range of about 15 feet at ISO 100 with an f/2.8 lens.
(Reasonably powerful, but not dramatically so.) The new flash pops up higher
than the flash on the 10D, offering greater clearance over lenses and also somewhat
reducing the likelihood of red-eye at closer ranges. Unlike the Digital Rebel,
which has a similar pop-up mechanism, the new mechanism doesn't rattle noticeably
when the camera is moved, and it pops up more quietly as well. The 20D gives
you a great deal of control over flash exposure, allowing you to adjust flash
and ambient exposure independently of each other, in one-half or one-third EV
increments. This makes it very easy to balance flash and ambient lighting for
more natural-looking pictures. The 20D also uses E-TTL II control for both the
built-in and compatible external flashes (according to Canon this includes the
current 550EX flash, as well as the new 580EX), a new standard that promises
better, more balanced exposures. Custom Function 14 turns this mode off and
returns to an average metering system. E-TTL II is only available with the built-in
flash or when the camera is paired with either the 550EX or the new 580EX flash.
Another nice touch is the Flash Exposure Lock button, which fires the flash
under manual control before the actual exposure, to determine the proper exposure
setting. This struck me as very handy, akin to the more conventional autoexposure
lock function for handling difficult ambient lighting conditions. A Flash Exposure
Compensation feature controls the flash exposure +/- 2 stops in 1/2 or 1/3-stop
increments.
Several of the more impressive features of the Canon flash
system depend on the dedicated 550EX or 580EX speedlight. Among these are true
FP (focal plane) flash sync, flash exposure bracketing with external flash units,
flash modeling, and E-TTL II exposure control. FP sync requires a flash unit
to provide uniform light output for a relatively long period of time, long enough
for the focal plane shutter curtain to fully traverse the "film" plane
(sensor plane in the case of the 10D). On the 20D, this requires a flash duration
of 1/250-second. Uniform, long-duration flash pulses like this permit use of
shutter speeds as high as the 1/8,000-second maximum that the 20D is capable
of. This can be invaluable when you want to exclude ambient light from the exposure.
(FP sync mode is referred to as "high speed" mode on the Canon 550
and 580 flash units.)
Here's the rundown on Canon Speedlights and their compatibility with the 20D:
| Speedlight Model | On-Camera Capability | E-TTL Wireless Compatibility |
| 580EX | All | Master or Slave |
| 550EX | All | Master or Slave |
| 480EG | External auto plus manual operation | None |
| 540EZ | Manual operation only | None |
| 430EZ | Manual operation only | None |
| 420EX | All | Slave Only |
| 420EZ | Manual operation only | None |
| 380EX | All | None |
| 220EX | All | None |
| 200E | Not Compatible | None |
| 160E | Not Compatible | None |
| MR-14EX Macro Ring | All | Master Only |
| MT-24EX | All | Master Only |
| ST-E2 transmitter | E-TTL, attach to camera | Master Only |
| Non-dedicated shoe-mount units | Manual operation only | n/a |
| Studio strobe packs | Manual operation only, connect via threaded PC sync socket on camera body | n/a |
You'll note the references to "E-TTL remote"
capabilities in the table above. Canon's Speedlight system permits TTL flash
metering with multiple remote units, and even allows you to set differential
power ratios between the slaved units, over a six-stop flash exposure range.
The "Flash Modeling" feature of the 550/580EX speedlights is quite
useful. With a F550/580EX connected to the 20D, pressing the camera's Depth
of Field Preview button causes the speedlight to fire at 70 flashes per second
for about one second. This creates the illusion of a constant light source for
your eyes, letting you preview the lighting on your subject when the flash fires.
VERY handy, and likely to save lots of shoot/check/reshoot time!
As alluded to above, the "X-sync" speed
of the 20D is 1/250-second. (This is the maximum shutter speed that can be used
on the 20D when working with a non-dedicated, FP-capable speedlight. It has
been increased from the 10D's 1/200 sec.) When used with higher-powered studio
strobe systems, Canon recommends a maximum shutter speed of 1/125-second or
slower, to accommodate the variable time/intensity profile of such units. Finally,
via a Custom menu setting, you can program the 20D to use a shutter speed of
1/250-second in Aperture-Priority exposure mode regardless of ambient light
levels. (I guess this is useful, if you know you're going to be hopping in and
out of flash mode, but other than a convenient preset for the shutter speed,
it's little different from simply using Manual mode to set both shutter speed
and aperture simultaneously.)
Another benefit of the dedicated Canon speedlights is that they carry powerful
autofocus assist illuminators that can extend the range of the built-in AF assist
light of the 20D significantly. As an example, the AF assist beam on the 550EX
is rated as good to about 50 feet, versus the roughly 13 feet of the
lamp on the 20D itself. (Note that the ST-E2 wireless sync transmitter can also
be used for AF assist during non-flash photography, a handy trick.)
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20D with E-TTL II |
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Digital Rebel without E-TTL II |
Continuous Shooting Mode and Self-Timer
Among digital SLRs currently on the market, the 20D is above average in terms
of shooting speed, very competitive with units it'll be stacked up against in
the marketplace. The Continuous Shooting mode is rated by Canon at five frames
per second, a number that matched almost exactly the 4.8 frames/second that
I measured in my own tests. This actually exceeds the ability of most d-SLRs,
which typically come in at about 3.0 frames per second, but it's nonetheless
slower than the blazing 8.5 frames per second of Canon's own EOS-1D Mark II.
Professional sports shooters will doubtless want more (they being a primary
target of the 1D Mark II), but for most situations, I expect that the five frames
per second of the 20D will be plenty fast enough. The 20D also has an unusually
"deep" buffer, as it's able to capture up to 31 large/fine JPEG images
or 6 RAW or RAW+JPEG ones before having to pause for the memory card to catch
up. The 20D also seems well-able to take advantage of fast memory cards, as
its buffer-clear time is only 19 seconds with a Lexar 80x CF card.
The camera's Drive setting also accesses a Self-Timer mode, which opens the
shutter 10 seconds after the Shutter button is pressed, giving you time to dash
around in front of the camera.
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