The 5D Mark IV is a far better camera than the 5D III, in my opinion. Skipping the 5D III was a relatively easy decision for me, as my 7D and 7D Mark II cameras had met my actual needs, and the 7D II had added new features that I reckoned would be in the next 5D-series body, after the 5D III. (I actually kept using original 5D cameras, skipping both the 5D II and the 5D III.) I added a 5Ds R in early 2016, about half a year after the introduction, and then added a 5D IV in March 2018.
In my case, I had added a pair of pre-owned 1D Mark II N cameras, for birds/wildlife in good light, at the end of 2012 and very early 2013, which also made skipping the 5D Marks II and IIi an easy decision. One very desirable feature that the 1D II N offered, that the original 7D could not, was the ability to AF with an f/8 combination of lens and Extender, such as my EF 400/5.6L and Extender 1.4x III. The improved capabilities of the 7D Mark II largely relegated my 1D II N cameras to reserve status, especially after I added my second 7D II body. (I used a pair of 7D cameras, and then a pair of 7D II cameras, to photograph crime scenes and crime victims, from 2010, until my retirement in January 2018. This is not the same thing as being a "professional photographer," but I strived to be a very, very good photographer as part of my public service career.)
I do, however, love the large, sturdy form factor of top-tier pro-body cameras, and when a little-used, near-pristine 1Ds Mark III arrived at a local camera store, it seemed to call to me, from the lower shelf of the display of pre-owned Canon equipment. I handled it several weeks ago, then tried it with a demonstrator lens lens about two weeks ago. I returned last week with a memory card, one of my 600EX-RT Speedlites, and my EF 35mm f/2 IS lens*. After a few test shots at typical distances, I placed a dollar bill on the countertop, extended the Speedlite's bounce card, and shot field-expedient, very-close-range images of the dollar bill. At that moment, I knew I was going to be buying that 1Ds Mark III.
I had, since late 2010 or early 2011, considered the 1Ds Mark III to be a grail-quest camera, as it was Canon's flagship at that time. Starting with the arrival of the 1D X, I had followed the internet discussion, regarding whether the 1D X cameras had replaced the 1Ds Mark III, or whether the 5Ds-series cameras had actually succeeded the 1Ds-series. Well, I have no 1D X cameras, but will now have the opportunity to compare the 1Ds Mark III, 5Ds R, and 5D Mark IV.
*This is a very special lens. Perhaps all EF 35/2 IS lenses are as good as mine, or perhaps mine is a particularly nice sample, but this one behaves very well on every camera on which I have tried it, up to 50MP, and is actually one of my two most-favored lenses for my 5Ds R.
