I have slowly been growing tired of the Canon EOS 5D's body and control limitations. Not too long ago, I photographed my son's school field day,and had a nuisance of a time with the 5D's back button autofocus controls. To me, the menu system dependence Canon d-slr cameras have is a terrible disadvantage over Nikon cameras. On Canon cameras, the controls on the back of the camera and the top right front of the camera actually CHANGE FUNCTIONS, depending on which exposure mode the camera is set to. I liken this to switching the control of the gas and brake in a car, depending on which gear the transmission happens to be in: an absolutely stupid idea.
Yes, it is true. The inconsistent performance of my two Canons has cost me many photographs over the last few years--at times when I really,really wanted to make a photo, I have brought my 20D or 5D up to my eye, and had the camera let me down, usually in terms of its inability to autofocus fast enough to get the shot. But also, at times, there has been a need to make an exposure change,and the fucked up Canon control system has cost me the shot. To my way of thinking, only ONE control should adjust the shutter speed, an only ONE control should adjust the aperture. Always. No matter which exposure mode the camera happens to be set to. But that is NOT the way Canon cameras work. Nope...
A few years ago, after being disappointed with Nikon's sensor problems, I purchased a 20D, just to see what Canon was all about. I bought a 20D and a Sigma 18-125mm zoom, a Canon 50mm f/1.8 EF-II, and a used Canon 100mm f/2.8 EF USM Macro. I liked the 20D's picture quality across a wide range of ISO settings, and I liked its forgiving 8.2 Megapixel sensor. With the 100mm EF Macro, the images were sharp,and file size was an excellent compromise. My main complaints were with the 20D's focusing system,which often failed me,and which did not allow me to select off-center AF points very well. The 50mm f/1.8 EF-II lens was totally unacceptable on several fronts,and I gave the lens away to a nephew.
Then,about 18 months later, I decided to buy the EOS 5D and 24-105mm f/4 L-IS kit. I also added the 50mm 1.4 EF, the 85mm f/1.8 EF,the 135mm f/2 L, the 135mm f/2.8 Soft Focus, and the 70-200mm f/2.8 L-IS lens. I also added the 580 EX-II flash unit. Unlike the 20D, the 5 was a full frame d-slr, and I greatly relished having my favorite focal lengths "back again".
Many who took up serious photography only since the advent of the d-slr have no idea of how or why a 50mm lens differs from an 85mm lens, or "what a fast 135mm lens does". The crop-body d-slr forces the user to stand very far away when using lenses like the 85 and 135mm. The crop-sensor d-slr delivers a lot of depth of field at smaller apertures like f/8. The crop-body d-slr is very easy to work with when one needs a LOT of depth of field.
My problems though with the Canon 5 system are that the body itself has a very centrally-oriented AF system,and one which has poor controls over setting the autofocus area, and over controlling the AF and the AE lock functions. I'm just not happy with the menu-driven nature of the 5D or 20D line of Canon d-slr's. I do not like having four buttons, each of which has TWO different functions! I am not happy with a camera that rotates aperture,shutter,aperture,shutter between two different controls. I never again want to bring the camera up to my eye, and have the thumb wheel move the aperture when I EXPECT IT TO MOVE the shutter speed. Or vice-versa. With a Canon, that is a fact of life. Design flaw, if you ask me. I want predictability and single-purpose controls. I want my next new "good" camera to be a Nikon.