Why EOS 5D mark IV and not 6D mk ii
In terms of Picture quality 5D Mark IV offers higher resolution of 30.4 megapixel as compared to 26.2 megapixel of 6D mark II. Higher resolution does translate to better image quality. From that point of view, even 6D II 's resolution is higher than Mark III which has long been an industry standard till the launch of Mark IV. If you were happy with Mark II performance, both 6d II and Mark IV have higher resolutions. If Higher resolution is your prime concern you may consider EOS 5DS or 5DS R at a higher price but with 50.6 megapixel of resolution.
In terms of build quality - In this area it's very simple. In general, more the money you spend more rugged is the camera body. In this case both bodies are water and dust resistant. 5D Mk4 is just about 35 gram heavier and slightly bigger in dimensions.
In terms of Usability 5d IV has a faster frame rate, more advanced Autofocus system, Full 100% viewfinder view while 6D mark II has a vari angle touch screen LCD. For street, wedding and wildlife photography, faster frame rate and better autofocus are certainly a plus point if you leave aside the full 100% view of the finder. In any low light or difficult situation, you rely upon the good autofocus system a lot. so, if reliability of focus is important, go for 5d Mark IV. Faster focusing justifies spending more if you are the kind of photographer who depends a lot on fast focusing like in case of street and weddings. If wildlife is your area of interest, you might be considering EOS 1D X Mark II as many other factors are also involved in case of Wildlife photography.
If you are mainly concerned with photography in the studio or in controlled environment where faster focusing is not going to a major advantage, 6D Mark II also should be just fine. Remember, we are talking about fast focusing in terms of fractions of seconds. A tiny bit faster or slower does not really matter in most of the photography situations. Mark IV has more focusing points which makes it easier to focus at off centre subjects without having to shift the camera.
Does better video quality matter ? If you are a Photographer who does not shoot videos, the 4K video resolution of 5D mark IV should not matter to you.
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Munish Khanna is a Delhi, India based, commercial Photographer who has been shooting Images since the time of Canon AE1, Canon AL1 and Nikkormat EL. Currently his Photography equipment includes Canon EOS 5D mark III and in the medium format category he shoots with Phase one P30+ on Mamiya 645 AFD II. Munish is actively involved in teaching Photography in Delhi and runs an online Photography course as well for Photography enthusiasts