What's Wrong With The Canon 5D? I've uncovered five weaknesses during my several months of ownership and use. First,the accessory grip BG-4's ON-OFF switch has virtually no detent at the ON position,and in the course of very normal shooting and handling of the camera, the switch quite frequently makes its way far enough toward OFF that the grip's shutter release button is dead when I go to use it. This is a simple engineering failure; the EOS 5D grip's ON-OFF switch moves with very little force. On several occasions,over five months of use of the camera,I've gone to shoot with the vertical release,and it's accidentally slipped to OFF in the course of just handling/using the camera. It's annoying. [Nov 26,2007 Update:Since this writing, I've had three more instances of "dead release button when trying to shoot'.]
The 5D's pentaprism allows a LOT of stray dust into the viewfinder system. I have a 30-year old Nikon FM which was last serviced 1987 and it has about as much crap inside it as my months-old new 5D has accumulated. Seriously.I suspect the 5D's eyepiece window is the culprit. The 5D's viewfinder/pentaprism/eyepiece system is one dust-leakin' mutha. I read about this problem before buying a 5D,and knew going in that it allows a lot of dust to get inside the camera and onto the viewfinder screen. It's a poorly-sealed system.
Here's a web article illustrated with good photos that show exactly how to remove the viewfinder screen and to clean out all the crap that'll accumulate inside your 5D's viewfinder. http://www.ascent-design.com/photo/Clean5D/
The third thing that's wrong with the EOS 5D is the autofocus system. The 5D uses a diamond-shaped AF point array that's too-centrally weighted,and which does not allow the use of off-centered AF points in the sure,positive manner that I am used to with the Nikon D2x. Also, the 5D does have some difficulties in ascertaining focus under flat,lower-contrast lighting, and also under some low-light scenarios,especially when only an outside,single AF point is in use. The outermost AF sensor brackets in the 5D simply do not meet my performance expectations under low-light or low-contrast lighting scenarios. The left and right edges of the frame are "served" by one AF point at the outermost point of the diamond; again, the 5D's main AF limitation is that it is weak when using off-center AF points. By contrast,the D2x allows you to use grouped off-center AF points, and with those groups of AF points doing the computing, the D2x is amazingly strong at nailing off-center targets. The D2x AF approach gives a higher priority to the edges of the frame,and particularly when photographing people in the portrait orientation, the D2x's grouped AF points can more-reliably pull focus under soft,low light levels like under studio flash modeling lamps. The heavily center-weighted,diamond-shaped AF point array Canon uses in its consumer and prosumer cameras is simply not as sophisticated,nor as costly, as what Canon or Nikon puts in some other camera bodies.
While the 5D's AF performance is reasonably good MUCH of the time,it does not function all that well with the side AF points indoors in a lot of my shooting scenarios using either the 50mm f/1.4 EF, 85mm f/1.8 EF, 135mm f/2 L-series, nor with the 24-105mm f/4 L-series zoom. The 5D's AF system is simply not as sophisticated, nor as user-adjustable, nor as user-controllable as the autofocusing system in Nikon's D2x. That's my take on the 5D's AF system; while the 5D has a Very Good AF system, the D2x has an Excellent AF system. Compare the price,and compare the market niche,and this all makes sense.
The 5D's fourth major flaw is a two-part problem with the viewfinder system. First problem area is the skimpy viewfinder information system. There's no information displayed except plus and minus on the exposure scale and any exposure comp dialed in,and the frames remaining in the buffer. There is NO DISPLAY INFORMATION that tells the user which exposure mode is selected! Yes,that's right: there is NO indication of which exposure mode is selected when looking thru the viewfinder! No metering pattern info, no ANYTHING, except for the exposure setting readouts and frames remaining. The 5D has minimal shooting information visible in the finder. Weird. Very,very weird.
Part Two of the fourth major flaw of the EOS 5D needs to be in all-caps: THE 5D's LCD INFO PRETTY MUCH DISAPPEARS IN VERY BRIGHT LIGHT. It's damned near impossible to see the metering readouts of the 5D when shooting under very bright outdoor lighting. The 5D's viewfinder LED's are so weak and underpowered I am amazed that this serious flaw was not found and rectified during beta testing of the bodies. Using this camera outdoors in very bright light conditions during July and August was a pain in the ass. The metering LED's are very underpowered.
The viewfinder IMAGE, of the framing and focusing of the scene the lens sees, is quite,quite good. I can see almost all of the image area,all the way out to the corners while wearing my glasses. The viewfinder screen is bright,and has a smooth texture. I wish the screen were a bit more coarse and contrasty,and thus more geared toward manual focus ascertainment, but that's just my personal preference. The screen is optimized for autofocus operation with fast prime lenses,and under ideal to good conditions the viewfinder screen allows "reasonable" manual focus ascertainment. Under more-difficult conditions, the screen is not too good for manual focus ascertainment. I've shot the 5D with the Nikkor 85/1.4, 105 DC, 50/2 HC,105/2.5 AiS,and 300/4 AF-S and 400/3.5 and a few other manual focusing lenses. Under optimal to good conditions, the 5D's screen works reasonably well with the above lenses. Good Nikon glass performs very well on the 5D's sensor.
To recap the viewfinder system's weaknesses: 1-The 5D is really is prone to allowing dust and junk into the finder system,to the point you can easily,easily see the crud as you use the camera. 2-In bright light, it can be VERY difficult/impossible to see the 5D's exposure info through the viewfinder. The LED's can virtually disappear and become virtually invisible when shooting under very bright, i.e. under "summertime" conditions. On the plus side, the finder image is large,clear,and not 'squinty' or 'tunnel-like'.
Summary: All told, what's wrong with the EOS 5D is almost all body-related stuff. The camera is generally pretty good in almost all areas,except for a few things. The body and its subsystems are really not as sophisticated as those found in Canon's or Nikon's top-grade bodies,but then the 5D does not retail for $4,500 to $7,999 as do the top-grade bodies. The 5D allows dust into the viewfinder system MORE readily than any body I've ever used. The 5D has only minimal shooting information visible in the finder. And the finder's LCD readouts are seriously underpowered,and pretty much blank out to invisibility,or near-invisibility,when using the camera under bright ambient light shooting conditions. And the 5D is fitted with an AF system that's kind of low-rent compared with what I expect these days from a "pro" body, but this is not a "pro Canon", it is Canon's entry-priced,entry-level Full-Frame digital SLR body.
On the plus side, the sensor's performance is excellent. Truly excellent. The low, medium, and high-iso ranges are all excellent performers. The frame rate is fine,and in social situations the 3FPS rate gives a nice separation between frames. I like the 5D's ability to remove the grip,and to have a half-height camera when portability or smaller profile is advantageous. Shutter lag is acceptable for social photography. Card write times are fine. There's not a lot really "wrong" with the 5D except when you try and shoot with it under bright, outdoor light conditions which make the viewfinder information display's faded-out green characters almost impossible to see without resorting to physically shrouding the area surrounding the viewfinder eyepiece. Aside from the five major flaws I point out,the 5D has a few minor flaws that are related to Canon's goofy button-and-menu control system and Canon's goofball ergonomics and camera control 'decisions'; overall, the EOS 5D is a very,very good image-maker. The 5D makes beautiful images. Across a wide range of ISO's and a wide range of light levels. The camera's sensor is MUCH, much better for higher-ISO shooting than the Nikon D2x,for example. An excellent sensor like the 5D's in a body as excellent as the Nikon D2x would make for an incredible professional camera.
Addendum, August 10,2008: See this thread detailing problems other 5D owners have found at http://forums.dpreview.com/forums/readflat.asp?forum=1032&thread=28908041&page=1