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So I want to make the jump to Full frame, I am considering getting either a used 5D or a used 5D3, Can someone that knows more about these cameras give me any advice on which one would work out better for me. Sure the 5D is about 700 less than the 5D3. What other pros or cons exist within the two cameras.
Thanks in advance.
If cost is an issue and you need full-frame (and many who think they do don't, and end up giving up useful features that they could get on a cropped sensor body), the original 5D is a fine camera that can produce excellent image quality. If cost is not a limiting factor, the 5D2 and 5D3 include improvements and additional features that are useful and perhaps compelling - with the biggest changes coming between 5D and 5D2 - and getting one of those can make more sense.
5D: Excellent image quality. If you know what you are doing, you can produce quite large prints from 5D originals. (How large depends on a lot of variables including your own quality expectations. I feel that it can produce a very fine 16" x 24" print, and larger is possible.) Compared to the later 5D models, it lacks some features - more on these below - and it has a tendency to be afflicted by dust on the "sensor." You'll have to become adept at sensor cleaning - not too hard to do, but far more necessary on this camera than on newer models.
5D2 - Significantly better than the 5D in many ways, especially if you need/want the specific feature improvements in provides. The 21MP sensor is perhaps not as critical as some might have you believe - unless you shoot very carefully and frequently push the upper boundaries of print size, in which case it makes a real difference. The 5D2 can, in my view, produce excellent 20" x 30" prints or even 24" x 36" prints, and larger is possible depending upon your expectations. The camera has improved high ISO performance, perhaps by a margin of one or two stops over the 5D. The addition of a dust reduction feature is very useful. (I used to clean my 5D more than once per month. With the 5D2 it is more like once every 6-12 months.) For shooting landscape and certain other similar subjects, the "live view" feature is tremendously useful. The camera adds very high quality video capability. There are others, but I've written enough already!
5D3 - Reflects solid improvements to the 5D2, though the magnitude of the improvements over the 5D2 is much smaller than the magnitude of improvements of the 5D2 over the 5D. It might be said that the 5D3 mostly does a very fine job of addressing things that 5D2 users might have wanted improved or fixed, but it is not (in the estimation of most) a radical or major upgrade. Image quality from the 5D3 is reportedly only slightly improved over that of the 5D2, and many would rate the difference here as being insignificant. Functional improvements seem more important here, including such things as the improved AF system and the faster burst mode capability - both of which extend the usefulness of the camera in practical ways for certain kinds of shooters.
Someone earlier suggested big difference in the interface among the three cameras. I cannot speak to this regarding the 5D3, as I have decided not to upgrade for my own work. However I shot the 5D for about three years and the 5D3 for over 3 1/2 years and still own both cameras. Aside from the features that the 5D does not have (video, live view) the interface is not really all that different, and I'm quite comfortable moving back and forth between the two cameras.
If cost isn't an issue and you want the best, newest version of the 5D series, the 5D3 is a fine camera. If you want the equivalent image quality at a significantly lower cost and don't find the improvements over the 5D3 to be compelling enough to pay the extra cost, the 5D2 will do a great job for many types of photography. If the cost of the 5D2 or 5D3 is just too much for you, look around for a good-quality 5D, learn to clean the sensor, and you'll be good to go.
On the off chance that you are thinking you need full frame because "it is best" or "it has best image quality..." Well, yes. Or, maybe. But it isn't that simple. Full frame sensor provide some specific advantages for certain types of photography. For example, if you intend to work carefully from the tripod with excellent lenses and excellent technique and then regularly produce very large prints of the sizes I mentioned above, full-frame cameras have some very real advantages. On the other hand, if you won't shoot this way and you aren't about to start printing at huge sizes, you may give up some things with the full frame camera that might be more useful than the larger sensor. For example, you'll likely be able to use shorter focal length lenses with the cropped-sensor cameras. You may get features including faster burst mode, improved AF systems, and so forth at a lower price point. You might even save enough money to get additional lenses or electronic flash equipment - stuff that may end up having more value to you than the larger sensor.
And especially if you are more in the hobbyist category than the "serious photographer" category (I know that is vague, but you probably know what I mean) you may be better served by something else. Especially if your goals are more casual - say photographing the vacation or the kids sports or friends and so forth, and you mostly share photos online and in email - it really is not necessary to get an expensive full frame DSLR.
Good luck with your decision.