Canon PowerShot A50Canon's "Digital ELPHs" goes megapixel plus - great picture quality, superb portability!<<Optics :(Previous) | (Next): Shutter Lag & Cycle Time Tests>> Page 6:Exposure & FlashReview First Posted: 7/10/1999 |
Exposure
The PowerShot A50 exposure system covers an unusually broad range
of illumination for a digicam in its price range, although we're a little puzzled
by Canon's specifications in this area. Canon claims a "metering"
range for the camera of EV2 to EV16.5. This is both a narrower range and one
skewed toward lower values than our own calculations would indicate. Although
Canon doesn't specify an aperture range for the A50 (and indeed no aperture
value is recorded in its JPEG file headers), even the maximum aperture of F/4.0
at the telephoto end of the lens' range, combined with the ISO 100 light sensitivity
and minimum exposure time of 1/750 second translates to an EV value of roughly
18.5. At the low end of the range, an exposure time of 2 seconds at ISO 400
(in low resolution mode) and the f/2.6 maximum wide-angle aperture translates
to an exposure level of about EV5. (In our own tests, the A50 appeared to work
well down to a minimum exposure level of about EV 7, with usable results as
far down as EV5. ("Usable" here means that we could extract fairly
good-looking, albeit somewhat noisy images from the camera's pictures, using
Photoshop or some other image editing program to adjust the brightness and contrast.)
While we don't have a formal test for measuring maximum EV levels, some playing
around with a bare 60-watt light bulb seemed to indicate that the A50 works
fine up to at least EV21.5.
As noted earlier, the camera normally operates at an ISO value of 100. In low-resolution
mode, it can combine groups of 4 adjacent CCD pixels into a single, virtual,
"super pixel," with four times the light sensitivity. This results
in an ISO rating of 400 for this operating mode. Interestingly though, it appears
that the camera's manual exposure-adjustment feature works by boosting or cutting
the amplification in the camera's electronics. Thus, when you increase or decrease
the exposure with the manual exposure compensation adjustment, the shutter speed
(and presumably the lens aperture) remain the same, despite the evident increase
or decrease in the brightness of the captured image. Thus, it seems the A50
can actually behave as though it has an ISO sensitivity of 400, even in high-resolution
mode, albeit at some cost in increased image noise. Another oddity is that the
ISO 400 of the low-resolution mode doesn't seem to translate into a lower absolute
light capability, but rather to shorter exposure times at the same minimum light
level. We found that the maximum exposure time for low-resolution (ISO 400)
shots appeared to be one second, and the low-resolution mode didn't appear to
extend the ultimate minimum light level any lower than that of the high-resolution
mode. On the other hand, at higher light levels, the exposure times were definitely
shorter, making hand-held shots more practical.
We've mentioned the A50's manual exposure-compensation adjustment several times
now, and this is one area where we initially had a minor quibble with the unit's
creators, in the realm of user-interface design: We find ourselves using exposure
compensation fairly frequently with digicams, and so like to have the controls
for it quickly accessible. On the A50, this function appeared to be consigned
to the LCD menu system, requiring several button-presses to access it. We were
thus very pleased when we read the manual and discovered that you can access
the exposure compensation immediately, just by pressing the "Set"
and "-" buttons simultaneously. This is a great shortcut, and very
practical in actual use. (The A50 has several shortcuts like this: We highly
recommend you take the time to read the manual!) However you choose to activate
the exposure-compensation function, the display is very functional, showing
both a "live" view of your subject with the current compensation applied
to it, and a scale from -2 to +2 in 1/3-unit increments showing the currently-selected
compensation level. The process of making these adjustments is a little slow
though, as the camera pauses for a second or so after each incremental adjustment,
before allowing you to move the control further.
Continuous (Burst) Mode
To capture fast-changing action, the A50 provides a "continuous" shooting
mode, in which the image size is automatically set to small, and the camera
continuously captures images at roughly 1.2 frames per second until it runs
out of buffer memory - 10 frames or so for the higher-quality compression setting,
15 frames for the lower-quality one.
White Balance
Like most digicams these days, the PowerShot A50 provides several preset white
balance options, in addition to the default automatic mode. Available settings
include Auto, Daylight, Cloudy, Fluorescent, and Tungsten. White balance settings
tend a bit more toward art than science with digicams, in that you often want
the camera to NOT remove all the color cast of the original lighting, to preserve
more of the "feeling" of the scene. Thus, there's a range of acceptable
behavior for camera white-balance controls, ranging from more to less aggressive
in their removal of color casts. Overall, the white balance of the A50 tends
more toward the less-aggressive end of the scale, leaving more of the scene
coloration in the final image than we'd prefer in most cases. We liked its handling
of scenes with relatively little color cast, but would have preferred a more
neutrally-colored final result in our indoor portrait shot. To its credit though,
even under strongly-colored lighting, the camera produces image files with plenty
of data in all three color channels, making it fairly easy to clean up the images
in an image-editing program after the fact.
Flash
The PowerShot A50's built-in flash is fairly typical in its capabilities, providing
auto, always-on, always-off, and red-eye reduction modes. It has a range of
8 feet (2.5 meters) with the lens at the wide-angle end of its range and 11.5
feet (3.5 meters) at the telephoto end. It also throttles-down very well for
shooting in macro mode, with Canon rating it for use all the way in to 6.7 inches
(17cm). This agreed with our own experience. We did find in our own experimenting
with it that it's worthwhile to play with the white-balance settings when using
flash in conjunction with strong artificial ambient lighting -- Often, using
a manual white-balance setting along with the flash will produce more pleasing
results than the auto option.
Like most digicam internal flash units, the strobe on the PowerShot A50 fires
twice for each shot: A pre-flash to determine exposure and white-balance settings,
and then a second time for the actual exposure itself. Thus, you'll need a special
slave-sync (such as the unit from SR Electronics) that triggers on the SECOND
flash, to synchronize external strobe units with it.
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