Much Ado About Hardly
Anything. (April
2005)
Special Note: While other reviewers, and perhaps Canon themselves,
have dubbed the chip inside the 350D as being a different
chip, the factoid remains that every single pixel on its
face is precisely the same size as the one found in the 20D and
I doubt that the original part that is cast in silicon is any
different at all.
How Canon implements this chip--now that's a different story!
Perhaps a
new set of laminates atop the photosites is slightly different.
Chipswitch.
In tests with a 20D, 300D and 350D all shooting from the same tripod at the same subject with the same 50mm f/2.5 Canon Macro lens taped to the same focus point and switched to AF off (M), a phenomenon first noted on these pages (you heard it here first) is that both the 20D and, of course, the 350D cut a smaller image out of the light passed on to the focal plane.
In other words,
the 3504 pixels in the width of the 20D's image occupy a slightly
smaller dimension than the 3072 pixels found on the 300D's imager.
And the 350D is 48 x 32 pixels smaller than the image from the
20D, so of course its image cuts an even smaller slice out of
the optical image.
The literal difference between the size of the 20D frame and 350D frame is suggested above. It's art generated with a light 350D frame floating atop a black 20D frame. In other words, there's barely enough difference to conceptually grasp.
Let those
things fall where they may.
To summarize
all that, because it's a little difficult to absorb all at once:
- Pixels can
be any size.
- Smaller pixels
can pack more of them into any given dimension.
- The given
dimension of the image chip in the 300D is 22.7mm x 15.1mm with
3072 x 2048 pixels covering that space.
The
given dimensions of the image chip in the 20D is 22.5 x 15.1
mm and this is 0.2 x 0.1 mm smaller in the published specification
than the image from the 300D. But it is 432 x 288 more pixels
if image compared to the 300D. It makes an image 3504 x 2336
pixels in size.
- The given
dimensions of the image chip in the 350D is 22.2 x 14.8 mm with
3456 x 2304 pixels.
- The pixels
on the imager for the 20D and 350D are precisely, exactly, definitely,
absolutely, for sure the same size as each other.
- But that
350D image is 48 x 32 pixels smaller.
- So it interrupts
a smaller image patch of light coming from the lens than the
one delivered out of the 20D.
- And the physical
size of the image chip in the 20D is a tad smaller than that
in the 300D, so its image cuts a slightly smaller chunk of image
out of what the lens delivers to the focal plane.
Where do we
get all this?
Those tests.
The same scene was shot with the 300D, 20D and 350D with a Canon
Macro 50mm lens. No chance of image size distortion there. Pixels
from the 350D lay on top of the 20D image in a perfect 1:1 relationship.
Not a nanometer different.
What does
it all mean?
My most precise
measure of the differences put the image from the 20D as being
98.9% of the coverage captured by the 300D. I shrank the 20D
image to 3072 pixels wide and laid it atop the 300D frame. It
didn't fit, exactly but when shrunk to precisely 98.9% scale,
it now does fit.
Images from
the 350D were also scaled to 3072 and corrected into place. Of
course, these had to be shrunk a tad more than those from the
20D.
Conclusions:
1. The 20D image is 98.9%
(1.1% smaller) than the image from the 300D. The 350D image is
97.4% (2.6% smaller) than the image from the 300D.
It may not
sound like much, but when you go out looking for a 10mm wide
angle lens, you really don't want to hear that the newer, better,
more wonderful camera only delivers 97.4% of the coverage found
on last year's camera.
Take heart:
you still get 39/40ths of the coverage obtained with a 300D.
2. The 300D covers a wider
slice of the lens coverage. If you have a wide angle lens, a
Kit Lens or do any ultra critical image coverage such as slide
duplication or tight macro work, be aware that the newer cameras
are actually working with a "crop factor" or "magnification
factor" that is slightly different.
Of course, Canon is free to do anything they wish with their chips, and by and large we are grateful to them for their diligent development of bigger/smaller, better, less expensive cameras. So they can call any slight modification of the CMOS imager a "different chip." It reads better in the specs.
Above the electronic photosites of the image chip is a complex laminate of materials that treat the photons at the last sub-millimeter of their travels. Here a pair of low pass filters spread the photons vertically and horizontally to avoid aliasing artifacts. An infrared absorption filter sucks up longwave red light to avoid image contamination from the highly IR sensitive CMOS pixels and a "Phase Plate" identical to the "Retarder Plate" on the back of every circular polarizer tumbles photons so they don't hit the chip with linear polarization intact.
Could differences in the construction of the laminates on the
350D's chip be the "different chip" that the 350D is
supposed to have? In my mind simply masking out 48 x 32 pixels
doesn't really produce a "different chip," but that's
the only other difference we can see between them.
--Peter iNova (peter.inova@mac.com)
Links:
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