Canon EOS-1D Mark IICanon doubles the resolution of their speed demon SLR, while actually increasing its speed and cutting image noise. Amazing!<<Viewfinder :(Previous) | (Next): Exposure>> Page 6:Sensor, Noise Reduction, and OpticsReview First Posted: 01/27/2005 |
Sensor, Noise Reduction, and Optics
Sensor
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The Mark II's CMOS sensor has a total of 8.5 million pixels. Of these, 8.2
million are "effective" pixels, the rest are lost to dark current
calibration and edge effects. After processing, the resulting images measure
3,504 x 2,336 pixels at full size, with a range of smaller resolutions available
as well. What's special about this sensor is not only its greater resolution,
but the improvements Canon has made in the speed of data "readout"
due to the camera's 8-channel reading technology. What this means is that the
CMOS sensor has more pathways to quickly offload the data from each pixel for
fast processing by the DIGIC II chip.
Image noise reduction
One of the most notable improvements in the Mark II's sensor technology
though, is in its noise performance. The 1D Mark II incorporates a range of
engineering advances that act together to provide dramatic improvements in image
noise, particularly at high ISOs and long exposure times. The changes involve
not only the sensor circuitry itself, but microlens design, algorithm changes,
and even the physical arrangement of digital and analog circuitry within the
camera body.
Canon's advanced CMOS sensor technology uniquely allows significant image-processing functions to be integrated right onto the sensor chip, before the image data even reaches the digitizing electronics. In the 1D Mark II's sensor, Canon has implemented no fewer than three different anti-noise processing techniques directly on-chip. While remaining very close-mouthed about the specifics of these anti-noise techniques, Canon does note that they affect both fixed-pattern and random noise generated within the sensor elements.
A common but little-recognized source of image noise in digital cameras is
electrical noise generated by the digital signal-processing circuitry. The current
spikes generated by high-speed digital circuitry can couple back into the sensitive
analog circuitry, if circuit designers aren't careful. In the EOS-1D Mark II,
Canon's engineers went to the extreme of completely separating analog and digital
circuitry onto physically separate circuit boards, each with its own ground
plane. It's hard to say just how much this reduced image noise levels, but it's
notable that Canon invested the required engineering effort and manufacturing
expense to achieve the required separation.
It's important to note here that noise reduction is only one path to producing noise-free images. What ultimately matters is not how much noise is present in an absolute sense, but how the noise level compares with that of the signal you're interested in. (In this case, the "signal" is the image data itself.) This is why engineers speak in terms of "signal to noise ratio" (SNR for short), when characterizing noise levels in an electronic system.
The concept of signal to noise ratio makes it clear that there are two ways to improve the "cleanliness" of images. You can reduce the noise levels in absolute terms, or you can increase the magnitude of the image "signal" itself. In the CMOS sensor used in the EOS 1D Mark II, Canon has worked both sides of the equation, with two enhancements to the sensor design working specifically to increase the amount of signal the sensor has to work with.
Improved Sensor Cell Layout
In any semiconductor image sensor, some portion of the silicon real estate
has to be devoted to circuit wiring and other functions not directly related
to light measurement. This reduces the amount of surface area that can be devoted
to the light-sensitive detectors themselves, in turn reducing the sensitivity
of the chip as a whole.
For the CMOS sensor used in the EOS 1D Mark II, Canon developed a more efficient pixel layout, that reduces the amount of surface area lost to ancillary functions. This increases the image "signal" directly, since each pixel is able to collect more of the light falling on it. Canon hasn't told us how much the improved pixel layout helps relative to earlier designs of theirs, but it could easily amount to 10-20%, possibly even more.
Larger Microlenses
To make the most of the light-sensitive area that they do have, most
digicam sensor chips incorporate an array of "microlenses" on the
surface of the chip. Constructed right on the surface of the chip itself, each
microlens is positioned above a single pixel. The job of the microlens is to
collect the light that's falling on the full area of the pixel, and concentrate
it on just the light-sensitive part.. (Anyone who's played with a magnifying
lens and light from the sun will understand immediately how this works.)
Like many things, microlenses are simple in concept, but tricky to implement
well. The lens material itself is less than perfectly transparent, and the fabrication
process makes it difficult to create lenses that cover a large percentage of
the pixel's real estate. This was thus another area to which Canon's chip engineers
devoted considerable effort, with very salutary results. The microlenses on
the 1D Mark II's sensor are larger than those on previous chips, and the gaps
between them are much smaller. Here again, Canon has declined to say just how
much improvement they've achieved in this area, but they claim that the new
microlens design and fabrication techniques "greatly increase the efficiency
of light convergence, while greatly reducing birefringence." (I believe
that this last refers to the optical phenomenon that produces the "purple
fringe" seen around dark objects against bright backgrounds in the images
from many digicams.)
Image Noise bottom Line
The net result of all the aforementioned improvements in both noise reduction
and light gathering ability is immediately apparent when you examine high-ISO
images produced by the 1D Mark II. Noise levels are indeed remarkably low, and
what noise is present has a very fine-grained pattern to it, making it even
less obtrusive than it would be otherwise. While perceived (as opposed
to measured) noise levels are a very subjective matter, my own reaction to the
1D Mark II's images is that shots from it at ISO 1600 look as good as those
from many competing d-SLRs shooting at ISO 400. - Even if some readers disagree
agree with my subjective evaluation, it's safe to say that pretty much any photographer
will be impressed by the quality of the 1D Mark II's high-ISO images, when comparing
them to those from other d-SLRs currently on the market.
Lenses
The Mark II features a Canon EF lens mount, which accommodates the full range
of Canon EF lenses. I normally cite the aperture and focal length of a digital
camera's lens in this part of the review, but because the Mark II accepts a
wide range of lenses, these characteristics will vary depending on the lens
in use. My evaluation model was accompanied by Canon's 16-35mm L-series USM
lens with a maximum f/2.8 aperture setting. Since Canon seems to be promoting
this lens for use with the 1D Mark II somewhat, I shot my Outdoor and Indoor
Portrait and Far Field tests with it, as well as a fair number of our "Gallery"
shots for the Mark II. For images subjected to critical analysis though, we
stuck with the 100mm f/2.8 EF Macro lens, a lens known to be very sharp and
distortion-free.
Antialiasing Filter
Essentially all of today's digital cameras use "antialiasing" filters
in front of the CCD array to reduce color aliasing in images containing high
spatial frequencies (high-contrast, closely spaced lines). These filters work
by slightly blurring the image -- technically, by knocking off the high spatial
frequencies, while hopefully leaving the lower frequencies undisturbed. The
problem of course, arises in trying to balance the need for antialiasing with
the desire to maintain good image sharpness. Too strong a filter produces soft-looking
images, while too weak a filter results in color "twinkles" and "jaggies"
appearing in the fine details. Some high-end cameras leave the choice up to
the user, with a removable antialiasing filter that allows the user to choose
whether or not to use it, based on specific shooting conditions.
In the original 1D, Canon claimed to have taken a different approach, deliberately choosing an antialiasing filter with a higher cutoff frequency, and relying on fancy image processing to eliminate or reduce any aliasing that might creep in as a result. It's not clear whether Canon stuck with this approach on the Mark II, or if they've gone back to a more aggressive antialias filter. The softness of the Mark II's images at its default settings suggest a possibly stronger antialias filter, but it's equally likely that this is just a matter of different default settings than were used on the original 1D.
This would probably be a good point to discuss the softness of the Canon EOS-1D
Mark II's images: Out of the box, the Mark II's default image processing parameters
produce rather soft images, when compared to those from other d-SLRs, including
its predecessor the original 1D. Much has been made of this in some internet
forums, but the fact is that it has everything to do with the settings to use
for the Mark II's defaults, and almost nothing to do with the actual amount
of detail the camera actually captures. Setting a sharpening level of 2 or 3
in the Parameters sub-menu of the Capture menu sharpens the images a fair bit,
while introducing only negligible artifacts in the process. Images captured
with the default sharpening setting take strong/tight unsharp masking in Adobe
Photoshop(tm) or other image-processing software very well, revealing excellent
detail. Bottom line, the Canon EOS-1D Mark II delivers excellent resolution
and detail, you just need to allow for Canon's deliberately conservative approach
to in-camera sharpening.
Autofocus
The Mark II employs a very fast autofocus system, with a myriad of focusing
options. At the heart of Canon's advanced AF is an unusually large AF frame.
(The region over which the AF system can take readings to determine focus.)
As in the EOS-1v, no fewer than 45 autofocus points cover a large portion of
the overall field of view. As shown in the diagram at right, seven of these
are "cross" sensors, sensitive to both horizontal and vertical detail,
while the remaining 38 respond to texture in the horizontal axis only. The Mark
II can select the specific AF point automatically (based on the closest subject
with reliable autofocus characteristics), or the user can select a specific
AF point manually. The AI Servo AF system can also track moving subjects anywhere
in the AF frame, if they're initially locked-in via the central AF area.
In the Manual/45-point
mode, you can select any one of the 45 AF points manually, and the camera will
use that point exclusively in its focus determinations. Selection is made by
using both the front and back Command dials.
No question, 45 focus
points are probably a lot more than you need for manual selection, and choosing
just one of them could be more time-consuming than you'd like, particularly
in a fast-paced shooting situation. Custom Function menu item 13-1 and 13-2
simplify the AF choice, reducing the number of available AF points to 11, and
arranging them in a cross pattern. Selection is again made using both the front
and back Command dials.
When speed is really
the issue, Custom Function menu option 13-3 further reduces the available AF
points to nine, all located around the periphery of the AF frame area. The advantage
of this is that only the Quick Control dial is needed to select an AF point,
rotating the selection point around the edge of the AF area as it is rotated.
This allows for a much faster selection, and is particularly useful for situations
where the photographer is dealing with an off-center subject.
Sometimes, you
may want to use more than one focusing point (for example, if your subject is
fast-moving, low-contrast, or if you're working in low-light conditions). For
these situations, Canon has provided the AF Point Activation Area option, available
as Custom Function menu items 17-1 and 17-2. In these modes, you can still manually
select a single focus point, but the camera then looks at up to six surrounding
points to determine focus. Only the selected point lights up in the viewfinder,
but as many as six other points are used to determine focus.
In addition to being able to select the AF area, you can also set the camera
to One-Shot AF or AI Servo AF. One-Shot AF is intended for stationary subjects,
and locks in focus on one area. In AI Servo AF mode, the camera tracks moving
subjects, keeping the subject in sharp focus no matter where it appears in the
frame. (As fast as 186 mph or 300 km/h when using the 300mm f/2.8L IS lens,
according to Canon.) I admit that I'm not quite sure what this spec means, as
the focusing behavior of the lens would obviously depend on the range from camera
to subject, something not specified in the brochure where this performance figure
was mentioned.)
Depending on the lens, you can also employ standard Manual focus, controlled
by turning the focus ring encircling the lens barrel. Canon's USM autofocus
lenses support a camera mode that autofocuses when the Shutter button is first
pressed, but that allows you to focus manually while continuing to depress the
Shutter button, to permit manual tweaking of the focus point after the AF system
has put you into the ballpark.
A Depth of Field Preview button on the camera's front panel allows you to check
the depth of field and focus for the current aperture setting. The preview is
displayed in the viewfinder, and is helpful for determining how much of the
scene will be in focus.
Overall, the Mark II's AF technology is very impressive, as it employs the same technology as the 1D, 1Ds, and 1v, but with the added benefit of two powerful signal-processing chips to run the AF-related computations, vs the single chip used in the earlier 1D. It's both flexible and very fast, well-suited to the demands of professional sports shooters and photojournalists.
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