The Canon EOS 20D is, in my opinion, a very nice upgrade from the Canon 10D.
My biggest complaint (albeit not that big) with the Canon 10D was the sometimes inaccurate autofocus. The 20D seems to have cleared up this issue. Autofocus is both fast and accurate. Although the Canon 1D Mark II is still superior for focus speed, I'm impressed with this upgraded feature on the 20D - it is excellent for a non 1-Series camera body.
Not lost in the 20D upgrade is excellent image quality. Results from the 20D are impressive. All of the "Auto"s are excellent including Auto White Balance, Auto Exposure and as I mentioned already, Auto Focus.
The 20D gains the 1D Mark II's excellent low noise high ISO quality. Direct high ISO noise comparisons between these two DSLRs are very similar - both excellent - with the Mark II having a slight edge. And low ISO images are very smooth. Color is similar between the two DSLRs - both excellent. The 1D II seems to have a higher dynamic range.
The 20D also gains an 8 megapixel sensor.
Although they are both 8 megapixel DSLRs, the 20D and the 1D II do not share the same field of view crop factor - 1.3x for the Mark II and 1.6x for the 20D (the Mark II has a larger sensor). If you are shooting wildlife or sports with long lenses and wide apertures, you will welcome the 20D's field of view crop factor. While retaining the same 8 megapixel digital image size, the 20D will give you a noticeably tighter crop than the 1D Mark II. The 1D Mark II requires a longer focal length (and likely slower, more expensive and/or heavier) Lens to get the same image framing as the 20D. If you are shooting wide landscapes, you will likely prefer the Mark II's 1.3x factor.
Another interesting difference between these field of view crop factors is how they use a lens. Lenses generally perform best near their center. The 1.6x field of view crop factor body uses the sweet spot in the lens (the center of a lens generally shows the best sharpness and fewest aberrations in the final image). However, since it is concentrating the same number of megapixels into a smaller area of the lens, it requires the lens to be higher quality to not show softness and aberrations. My 1D Mark II tended to give a slightly sharper image than my 20D when using the same lens wide open. The difference was slight, but it was there. The two bodies produced very similar images once the lens was stopped down. Again, both excellent. As always, good lenses make a difference.
The first thing I noticed when removing the 20D from the box was how small and light this DSLR is. Slightly smaller and slightly lighter than the 10D and without a Canon BG-E2 Battery Grip, much smaller and much lighter than the 1D II. This is both an advantage and a disadvantage.
The disadvantage is that my pinky finger does not comfortably fit onto the grip. I don't feel like I'm in full control of the 20D when I have a large/heavy lens attached. With a Canon 550EX Flash attached, I felt like I should sit the 20D on its side to prevent it from falling over. Having exclusively used the 1D II for several months, I have grown used to the full size landscape and portrait grips. They offer excellent control over the body and lens Adding the Canon BG-E2 Battery Grip should resolve this problem for the 20D.
Smaller size and lighter weight makes the Canon EOS 20D less of a burden to take with you. Hanging the 20D on your shoulder all day is not at all burdensome (unless you attach a huge lens of course). Adding a small lens to the 20D creates a nice lightweight package. The Canon EF 70-200 f/4 L Lens and 20D are an example of a great matched combo.
Turn on the power switch and the 20D is on. Instantly - no waiting - very nice. Startup is much faster the its predecessor, the Canon EOS 10D. If you use the rear thumb wheel (doesn't everyone?), make sure you turn the 20D power switch to the second position. I don't like this feature but can easily live with it. I would prefer to enable/disable the thumbwheel from a menu option.
The 20D utilizes the Canon BP511A Li-ON battery introduced on the Canon PowerShot Pro 1 which contains about 27% more energy than the Canon BP511 Battery. Canon estimates battery life at about 700 shots (depending on use of course). The battery indicator on my 20D dropped from full charge at about 500 shots and was still going strong at 600 when I recharged it. My second charging lasted 530 shots.
I really like the new battery charger - no more cord! The Canon BP511A Batteries slide into the small charger which plugs directly into an outlet. Very nice. Charge time is about 100 minutes.
As for look and feel, the 20D is similar to or slightly lower than the 10D. That is good - but not up to the higher quality of the 1D II. The look and feel of the Mark II's buttons and grip are definitely higher quality. I find the 20D's rubber remote release/video-out door very difficult to get closed properly. Everything must line up exact for proper closure.
I like the easy menu navigation and camera setting changes the 20D allows. Even after months with the 1D II, I still feel camera setting changes are awkward on this body.
With the 20D, Canon introduces a joystick controller near the right thumbtip. Examples of where it can be used include focus point selection and image review during zoom.
I would describe the shutter sound as medium-loud. It is definitely not as loud as a 1D II, but has a more plastic-sounding click. It is a bit louder than the 10D.
Listen to the 20D's shutter sound and frame rate compared to the Canon EOS 350D Rebel XT ...
Although not bad, the 20D's viewfinder is not as bright/smooth as the 1D II's or even the 10D's for that matter. The 20D shows about 95% of the actual image captured in the viewfinder (same as the 10D) vs. 100% on the 1D Series. I keep picking up things I don't want in my 20D images (things that were not visible in the viewfinder).
The 20D's 5 fps (Frames Per Second) capability is very good - fast enough for most general purposes. The frame buffer is a bit less adequate in my opinion. The 1D II's frame rate is 8 fps and has a larger image buffer.
The 9 focus points employed by the 20D are a nice upgrade from the 10D's 7 focus points, but not close to the 1D Mark II's 45 points. I missed some of the focus points when utilizing AI Servo to track action at a recent soccer match. I sacrificed my framing slightly to get the focus correct. And as I said before, focus accuracy is very good. My hit rate is noticeably higher with the 20D than with the 10D.
The 20D is Canon's second DSLR body to support E-TTL II. Although not perfect, I am not displeased with E-TTL II when used in Averaging Mode (a Custom Function). White dresses and black tuxes still need FEC (Flash Exposure Compensation).
The 20D sports a built-in flash. This can be useful for fill-flash and emergency/snap-shot use (with smaller lenses/lenses without hoods attached). I recommend an accessory flash such as the new Canon 580EX Flash for any serious flash work.
I can't think of any feature the Canon EOS 10D had that the 20D doesn't, but this is still not a Canon EOS 1D Mark II. For those considering the more expensive 1D Mark II, I'll attempt a list of some of the missing features ...
Much more comfortable eyepiece
Spot AE metering with multispot capability
AF Point linked AE metering
Better operating temperature range
More rugged build
More durable shutter
Autofocuses down to f/8 (Vs. f/5.6)
Faster shutter release
Shorter viewfinder blackout time
Dual memory slots - CF + SD Cards
Dual higher resolution LCD Displays showing more information
Dual timer modes
Dual frame rate modes
Dedicated FEL button
ISO is displayed in the viewfinder
More custom/customization options
Ability to save options
1D Mark II Batteries build memory and are more expensive
What did I miss?
On the software side, the 20D includes point-version upgrades to the software included with the 1D II, EVU (EOS Viewer Utility) and DPP (Digital Photo Professional). I only noticed minor changes so far (I haven't gone looking), but EVU still produces smoother results.
The 20D is going to make a lot of non-professionals who were considering the purchase of a Canon EOS 1D Mark II DSLR reconsider. It is easily capable of creating excellent images.