AUTOFOCUS
There are two modes that I commonly use with the Canon cameras: One Shot and AI Servo. The midrange and low-end Canon DSLR's also have another mode: AI Focus, but I have never had any luck with that mode for birds-in-flight, plus the Mark III doesn't have that mode so I will not reference it any further.

One Shot mode is useful for perched birds that are stationary. I find this mode useful for owls, birds-of-prey or other birds that aren't constantly moving. I also find this useful while photographing birds in trees where branches and leaves can play havoc with AI servo mode. All of the Canon DSLR's that I have ever owned do a very good job in One Shot mode.

AI Servo mode on the other hand has been open for debate no matter which camera is used. AI stands for Artificial Intelligence. The camera attempts to ( through software algorithms) predict the motion of an object as it moves and attempts to maintain accurate focus.

There have been numerous complaints about the Mark III not doing a very good job of this with high-contrast subjects with fast lenses (f/2.8 or f/4). My personal experience has not shown any problems with the Mark III when it comes to AI Servo mode. It works as stated. With that said, I mostly shoot with a teleconverter on f/4 lenses, so my f-stop is usually 5.6 or above.

The Mark III has many more autofocus options than the 40D, although most of which I keep at the default settings. (It is my opinion that the 40D's autofocus settings are set similarly to the Mark III's default settings.) There is only one setting that I changed on the Mark III with regards to autofocus: C.Fn III-2. CFn III-2 adjusts the system sensitivity to sudden subject changes. For example: more likely than not, while tracking a bird in flight, the bird (or you) will wander off of a focus point or grouping of focal points. When this happens, the camera will hunt to regain focus after "x" period of time. With the Mark III, this is adjustable whereas the 40D is not. I prefer to keep this setting as slow as possible when photographing birds-in-flight using the Mark III.

The 40D does not have any option to control this setting and I believe that it is set to match the default setting of the Mark III, which is set to midrange: not too fast, not too slow. This is not a deal-breaker in my opinion, but the Mark III has the advantage here.

The other advantage often stated by photographers who use the professional series from Canon has to do with selecting multiple focal points. You can either select the center circle of focal points (expansion points) or select all of them at once (sometimes referred to as the ring of fire). This may come in handy for photographing a bird against a blank sky, but it can also become a detriment when attempting to photograph a bird against a busy background. Quite often when multiple focal points are selected, the camera will end up focusing on some background object instead of the bird.

At first I found myself using the expansion point option and/or all focus points on the Mark III to track a bird against a blank sky. That worked just fine until the bird would swoop down low and move between me and a busy background. Poof! The bird is now out of focus. For that reason, I now shoot exclusively using only the central focus point no matter which camera I am using.

So how does the 40D stack up to the Mark III using AI Servo with only one focal point enabled? It works just dandy in my opinion. The only disadvantage is that because it is a 1.6x crop, the focal points are smaller than that of a 1.3x camera like the Mark III. This means you have to be more accurate with keeping the bird centered on whatever focal point you choose. In most cases, you should be photographing a bird-in-flight that at least fills half the frame, so this shouldn't be too difficult.

The 40D does a very good job keeping the focus locked on the bird even with busy backgrounds.

The 40D's autofocus is much more accurate and fast compared to its predecessor—the 30D. The Digic III processor is probably the reason why. The Digic III is also used in the Mark III. Actually the Mark III has two Digic III's that are used in tandem.

The bottom line is that the Mark III is the winner in this category, but the 40D is a very close second when it comes to photographing birds-in-flight.

By the way, I believe there is no autofocus system on the market that is 100% fool proof. All DSLR's that I am aware of use visual methods for determining focus. It would require some form of sonar system to actually lock on to a moving target and keep it in focus.

IMAGE QUALITY
Comparing image quality between the Mark III and 40D really boils down to ISO noise levels. The Mark III has inherently lower noise levels due to the size of the pixels, and this feature is easy to see on images taken from ISO 1600 on up. With that said, the 40D is no slouch at ISO 1600 and even ISO 3200. Both cameras now have 14-bit color depth compared to their predecessors which were 12-bit. I find that both the Mark III and 40D produce attractive, color-rich images.

The Mark III wins here with lower ISO noise especially at higher ISO settings.

BATTERY LIFE
The Mark III's LP-E4 battery life is remarkable. I could easily take thousands of photographs before the battery would run down. Although I had not had a chance to use the battery in temperatures below freezing, I am quite confident that the new battery used on the Mark III is superb.

The 40D uses the same battery as with its predecessors—the BP-511. Although this battery has a decent run-time in temperate weather, it loses power quickly in cold weather. With that said, I have not had a chance to try the BP-511 with the 40D in cold conditions. In warm weather conditions, the addition of the BG-E2N battery grip along with (2) 2,000 mAh batteries netted over 5,000 images before the battery level indicator on the camera was down to the last bar. With a fresh set of 2,000 mAh batteries in warm weather conditions, it seems to be on par with the Mark III.

FRAMES PER SECOND
One of the more often talked about attributes of the Mark III is its ability to take 10 frames per second. Although this is an amazing attribute, how practical is it to shoot at 10 FPS? It certainly can clutter up your memory card and/or hard drive in a hurry! Although I have used 10 FPS on occasions, most of the time the camera is set at 6 FPS which ironically is what the 40D can achieve. I just found 10 FPS was more than I needed while photographing birds-in-flight. On the other hand, if I wanted to capture a batter hitting a 95 MPH fastball, 10 FPS is warranted.

The 40D at 6.5 frames per second is plenty fast to capture this American White Pelican as it comes in for a landing on the Mississippi River near La Crosse, Wisconsin.

LCD DISPLAY / VIEWFINDER / LIVE VIEW
Both the Mark III and the 40D have a 3-inch, 230,000 pixel LCD display that makes reviewing images a pleasure. I was very pleased to see Canon use this LCD on the 40D! Also the viewfinder on the 40D has been improved from the 30D, making it easier to see the birds that you are photographing.

I was also surprised to see that Canon has also incorporated Live View on the 40D. Although I haven't used this function for wildlife photography, it does come in handy for doing astrophotography. Looking through a viewfinder to see if a star is in focus is not a fun thing to do. Liveview has solved that problem.

I give both cameras a big thumbs up for these features.

DEPTH OF FIELD
A little known fact is that if you are shooting a bird from an equal distance with a 1.6x, a 1.3x and full-frame camera with the same lens, the depth of field is different. I never gave this characteristic much thought until I purchased the Mark III and found that the area that appeared to be in focus was shallower than a 1.6x crop camera. I would typically find myself using f/8 or higher for close-up bird photographs with the Mark III in order to keep the majority of the bird in focus.

I raise this point here because it is real, not imagined and needs to be considered when shooting birds at close range with full-frame and 1.3x cameras. 1.6x cameras have a greater depth of field and I rarely need to go higher than f/5.6 to achieve an acceptable depth of field. Something to keep in mind.

FINAL CONCLUSION
Someone on one of the photography forums referred to the 40D as the Mark III's little brother. I think that is a good analogy, although I might add that the "little brother" doesn't seem to get sick as often as its big brother. (I had to add that considering that I had two Mark III camera failures.)

The 40D doesn't have all the options that the Mark III has when it comes to configuring the camera for various photographer's needs. On the other hand, with less options it's easier to configure and prevent inadvertent configuration screw ups.

I'm not about to state that the Mark III is not worth the extra bucks. That is something that everyone must weigh for themselves. In my particular case however, I feel quite comfortable recommending the 40D camera to anyone who wants to do bird photography. Its ability to accurately track birds-in-flight was a deal maker for me plus I am able to do this with a 1.6x crop factor sensor at 10 megapixels—plenty of resolution power.

After doing the comparisons between these two cameras, I have returned the Mark III and purchased two 40D's with battery grips. I am quite happy with the trade!

Happy Birding,

—Alan Stankevitz