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You might think that it would be pretty easy to compare images from a Canon
EOS 5D (full-frame 36mm x 24mm sensor) with a Canon EOS 20D (APS-C 22mm x 15mm
sensor), but it's actually more difficult than you might think. You need to
compare apples to apples, not apples to oranges and that's not quite so easy as
it sounds. There are perhaps four different ways that you could use to compare
1 - The first way is that you could use a different lens on each camera, each
giving the same field of view. For example a 35/2.0 lens on the EOS 20D and an
50/1.8 lens on the EOS 5D would give you almost the same image in each
case. However you'd then be testing the image quality of the two lenses as much
as the image quality of the two sensors.
2 - The second way is using a zoom lens and equalize the field of view. So if
you set the zoom to 70mm for the Canon 5D you would set it to 112mm for the Canon
20D. However you'd also be testing the image quality at two different focal
3 - The third way would be to use the same lens on each camera, but crop the
EOS 5D image so it showed the same view as the EOS 20D. However if you do that
you're testing a cropped full frame image, which isn't really fair.
4 - The fourth way is that you could use the same lens on both cameras, but
shooting from different distances. Say a 35mm lens shot at 10ft from a subject
with the Canon 5D and at 16ft from the subject with the Canon 20D. This is pretty
fair test method since image quality isn't a strong function of focus distance in
this range. The practical drawback it's not always possible to do this in the
real world. You can't usually back up from 0.5 miles to 0.8 miles when shooting a
landscape for example! However when just doing testing, this seems like the most
reasonable way to look at differences caused by the sensor.
All lenses vignette, especially fast lenses shot wide open. It's
usually worse with fast, wideangle lenses and better with slower, telephoto
lenses. At apertures of f8 and smaller vignetting should be slight with any
decent lens. Vignetting comes from a number of sources. First is the theoretical
"cosine to the 4th power" falloff which all wide angle lenses exhibit, but more
important is usually physical vignetting of the aperture due to the construction
of the lens. A typical 50mm f1.8 lens can show up to 3 stops of vignetting at the
corner of a full 35mm frame. Stopping down to about f8 can reduce this to about
1/2 stop. Since vignetting is a strong function of distance from the center of
the frame, the "cropped" APS-C sensor shows less vignetting, especially when used
with lenses designed for full frame illumination.
Center Resolution - Test Method 4
Based on test method (4) as described above we can look at
the center of images shot with a 50/1.8 lens at f8 from different distances.
While the 50/1.8 is a cheap lens, its performance is good, especially when
stopped down to f8, so this should be representative of good lenses.
Clearly, I think, you can see that the full frame sensor of the
EOS 5D shows more detail than the APS-C sensor of the EOS 20D.
Edge Resolution - Method 4
If we now look at the edge of the frames, we see the
The first obvious thing is the vignetting again. The corner of
the full frame shot from the Canon EOS 5D with the lens at f1.8 is clearly a lot
darker than the center and a lot darker than the corners of the APS-C shot from
the EOS 20D. In terms of resolution though, the full frame images are again
Looking at a pretty average (or below) lens, here are a couple of
shots from the corner of the frames using an EF 22-55/3.5-5.6 lens at 22mm and
f3.5. This is a low cost, plastic mount, lens that was originally designed for
use with APS film bodies, though which does have full frame coverage.
As you can see, again the full frame corners are darker due to
vignetting. In this case I think the lower image quality at the corners of the
full frame shot cancels out any size advantage.
Center Resolution - Test Method 3
Here's a series of shots using test method 3, i.e. with the same
lens at the same aperture and at the same distance from the subject:
As you can see, in this case the 20D image shows more detail.
This situation (same lens form same distance) is the situation that nature
photographers often find themselves in. They're using their longest lens
(typically a 500mm or 600mm, maybe with a multiplier on it) and they're as close
to their subject as they can get. They might use a longer lens if they had one,
but they don't! Under these circumstances, where you'd have to crop one or both
images to the same field of view, results from an APS-C sensor with a higher
pixel density (such as the EOS 20D) can give better results than those from a
full frame camera with a lower pixel density (such as the EOS 5D)
Real World Shots - Test Method 2
Here's a real world image comparison. shot using test method (2), i.e. a zoom
lens at different focal lengths on the Canon 5D and Canon 20D. This would
probably be the practical situation if you had both a 20D and a 5D. You'd
probably use the same zoom, but at different settings. I tried to equalize image
quality by shooting at f11 and shooting at the wide end of the 70-300/4-5.6 IS
USM, where image quality is high and doesn't change a lot. In addition to
comparing image quality in terms of resolution, you can compare EOS 5D vs. EOS
20D noise levels since these images were shot at ISO 3200. First, here's the
The area shown outlined in red is shown as a 100% crop in the two
And finally here's the same two images, but with the EOS 20D shot
upsized by about 125% (using bicubic interpolation) so that the subject is the
same size in both shots:
The superiority of the Canon EOS 5D image is clear. Now they were
shot at different zoom settings, but since they were also both shot at f11 and
the zoom is pretty good in this focal length range, I think what you see in the
images reflects the difference between a full frame 12.7MP sensor and an 8.2MP
sensor rather than any difference in lens quality.
It's a win, but not a total "slam dunk" for the full frame sensor, at least as
far as the EOS 5D vs. EOS 20D goes. With good lenses the results from the Canon
EOS 5D are better, though with noticeably higher vignetting when lenses are used
wide open. Of course you can mostly compensate for vignetting in software, but
still you may lose something. With low cost lenses shot wide open, the full frame
advantage disappears, and in situations where you have your longest lens in use
on an APS-C camera, switching to a lower pixel density full frame camera and
cropping will lead to lower resolution images.
However there's no denying that under the right conditions, the images from a
full frame camera like the Canon EOS 5D can beat those from an APS-C camera like
the Canon EOS 20D, as the real world examples above show. Given the budget, I'd
own both a 5D and a 20D and I'd use each in the situation for which it was best
suited. Since I do a lot of nature work, the higher pixel density of the EOS 20D
is useful for me. If I didn't shoot extreme telephotos, I'd certainly pick the
Canon EOS 5D over the EOS 20D, especially if I had a good set of lenses to go
with it. Good here doesn't mean all "L" series either. It's clear that the better
"consumer" grade lenses can also take advantage of the higher performance of the
full frame Canon EOS 5D.
Canon has made a significant step in DSLR technology by producing a full frame
camera which is selling for under $3000. They seem to be on a path which may end
up - in maybe 3 to 5 years time - with a full frame "prosumer" DSLR sellng
perhaps in the $1500 region. However currently, and for the forseeable future,
the EOS 5D is second only to the $7100 Canon EOS 1Ds Mk II in terms of image
quality and is, I think, the best bang for the buck out there if you're looking
for the highest image quality from a DSLR.
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