Photography authority DPReview posted their very exhaustive review about Canon’s EOS 5Ds and EOS 5Ds R.
From the conclusion:
Without a doubt, most photographers don’t need 50MP but some do, and this places the 5DS and especially the 5DS R firmly in medium-format territory for studio and fashion photographers. Canon has an excellent family of glass and accessories that pros trust: for example, Canon’s radio-triggered TTL flash system (600EX-RT / ST-E3-RT) is simply unparalleled in its versatility among Speedlites. Landscape photographers are always looking for more resolution for larger prints, and Canon’s excellent new glass in the 11-24 F4L, 16-35 F4L IS, and 70-200 F2.8L II potentially make this camera an easy sell for nature shooters. But the 5DS/R don’t exist in a vacuum, and that makes them a bit of a conundrum. Read the review…
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