To upgrade or not - the 5D mark III
If you shoot events:Then the bigger increase in AF points and improvements in the cross points are noticeable. Low light shooting improvements in both higher ISO and in getting accurate lock are noticeable over my Mark II. The improved tracking makes it more possible to use this for sports events, and for those who shoot indoor arenas will find this a dramatic improvement. Locking onto horses, dog shows, ballet, etc... Wedding and infant photographers will really appreciate the new silent shutter mode. It's a dramatic improvement in sound and great for very discrete shooting.
|Tracking action is no problem with the new 61pt AF.
If you shoot wildlife:The increased 61 point AF is a great improvement. The AI Servo now has the newest generation of AI Servo III, which is found in the 1Dx. It's better than the 1Dmk4 which uses AI Servo II. The new AF system definitely makes it a more reliable Birds in Flight camera, and you rely less on your skills with a 5Dmk2 to do the same (I've had great success shooting with the 5Dmk2 with wildlife and other photographers I know, but you do still need to rely on your skills of tracking with the mark 2). The 6 preset profiles are very welcome as the AF system is highly advanced so it's nice to have specific conditions to choose from. The AF system alone is the biggest reason to upgrade. Silent shutter is also very quite and beneficial here and also the increase to 6 fps in regular shutter mode from the 3.9 fps is enough to capture fast action. A trick I also discovered by using the movie mode, you can actually set your system up to work as a video remote/still capture device. With my wireless video remote, I'm able to wake the camera to see what it can see. If there's something that is interesting to photograph, I can hit the trigger on my video remote. This is extremely handy if I want to set up my camera and go to a hide or sit indoors. I only just tested this recently and looking forward to setting this up in some discrete wildlife shooting conditions.
|Catching birds in flight and the improved metering are a cinch.
If you shoot still life or studio:This is the best thing about the 5Dmk2. Improvements for these kinds of photographers are slight and for most might not even be worth the upgrade. However if you shoot interiors, improvements like 7 bracketed shots with a huge ±9 EV may interest you. The increased dynamic range also doesn't affect studio photographers, but if you do shoot high contrast subjects, you'll appreciate the improvements in this area. In my tests, I see about an 8-15% increase in dynamic range. More colours and more tones means more leverage in post processing.
If you're a landscape shooter:7 bracketed shooting, multiple exposures, increased dynamic ranges are the primary improvements for landscape shooters. If you're not concerned over dynamic range then this upgrade is not for you. Minor points would be the level indicator which measures two axis (across your plane of view and forward and backwards). HDR mode is a nice and welcome addition if you like that style. It works relatively well and gives you an idea of what range you have to work with while shooting bracket RAW shots. Also some minor improvements in the live view simulations for those who use tilt-shifts will like the new improvements. No real improvements in resolution or clarity. In carefully analyzing the files, there were no real major improvements over the 5Dmk2, however I did get my hands on a Nikon D800 and I compared the files which I also found little advantage with the higher resolutions due to diffraction limits that softened the images on that camera (f/8 and up). Good for the 5 series that they didn't bump up the resolution as it kept the clarity of my images. I haven't printed a large canvas yet, but I look forward to doing that soon.
|Dynamic range has been dramatically improved. Especially in the shadow details.
If you're a macro photographer:The silent shutter will be very welcome here if you shoot shy insects especially, but if you have a macro lens, then you also have macro mode Ai Servo (traditional servo shuts off at 1m). Found in the 7D, 1Dmk4 and the 1Dx, this completes all the axis of motion while shooting macro. It's nice to be able to lock on the subjects and even in slight windy conditions your macro subject will be locked and remain in focus. Even if you're a person who likes to focus with your feet can appreciate the new macro mode. Single button magnifier was annoying at first but when you set it to do 100% on focus point, this was really nice to have it check and confirm focus. A benefit across all types of shooting I find.
|Handheld macros is definitely improved by the macro AI Servo mode.
Casual or recreational shooters:The Auto mode has been improved dramatically. It also now has focus and recompose and half shutter presses for locking exposure. It's like using P mode but even simpler. The ability for it to track what kind of scene is pretty amazing. I've seen how well it detects action and automatically starts to track things. This is great for family shooters and shooting active children. I usually never use the green box mode, but with active kids it's very handy to just switch to that and catch the action. For those casual shooters, the Auto+ mode is a welcome upgrade. They removed the creative auto mode which I think confused more users than anything. The simplified approach is very nice especially when I hand over my camera to a novice user.
Videographers:At first it doesn't appear to have a lot of improvements, but the biggest thing is the new button toggle switch, and removing the start/stop from the set button. The ability to optionally use the shutter button to start/stop now is very welcome (which I mentioned can be used as a video remote camera in the wildlife section). Now you can use E3 remotes to trigger video operations (vs the horrible front of camera only IR remotes). The silent touch dial is also a major benefit. Although they did a great job to reduce click noise off the dials, the touch dial is very welcome for very discrete quiet shooting which allows you to change exposure, sound levels, aperture, shutter etc... Improvements also to video and audio controls is also another thing to note.
Many of the improvements in the 5D Mark 3 is found in usability, so images don't necessarily demonstrate all of these features, but it's no less important. In a time where the details of a camera's image over another is merely microdetails now, the important thing to focus on is certainly on the rest of the system and I welcome the fact that Canon spent more time on those things rather than just improve the images which for the most part were already outstanding on the 5D Mark 2. If one of these conditions suits you and the improvements are what you're looking for then you'll not be disappointed by the upgrade.