When comparing Canon 5DS / R vs Pentax K-1, the Slant community recommends Pentax K-1 for most people. In the question“What are the best full frame DSLR cameras?” Pentax K-1 is ranked 3rd while Canon 5DS / R is ranked 8th. The most important reason people chose Pentax K-1 is:
K1 has an alternate shooting mode that can significantly reduce noise and increase sharpness. It's called Pixel Shift Resolution. Once enabled it will take 4 shots moving the sensor by 1 pixel each time in order to capture all color information for each pixel instead of distributing it over 4. A visualization can be found [here](https://youtu.be/gIGqwxEdU4I). Pixel Shift Resolution shooting mode takes longer as it's taking 4 shots instead of one and requires a couple of seconds after the shots have been taken to process the images into one so it won't work for sports or action, but it's great for landscape photography.
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Pro Flicker detection
The camera can detect flickering light sources and sync up with them to shoot at peak brightness in order to avoid underexposing the image by accident.
Pro Good autofocus system
61-point autofocus system with 41 cross-type points that performs well in low light. In addition, 5 central autofocus points turn into dual-cross-type points with fast (f/2.8 or faster) lenses increasing the precision even further.
Pro In-camera timelapse
Pro Excellent build quality
The 5DS/R has a magnesium alloy shell that makes it durable.
Pro Programmable Auto ISO
Pro Natural skintones
Pro High resolution
50.6 MP compared to 22.3 MP for the 5D Mark III.
Pro Can take super low noise, high sharpness images with Pixel Shift Resolution enabled
K1 has an alternate shooting mode that can significantly reduce noise and increase sharpness. It's called Pixel Shift Resolution. Once enabled it will take 4 shots moving the sensor by 1 pixel each time in order to capture all color information for each pixel instead of distributing it over 4. A visualization can be found here.
Pixel Shift Resolution shooting mode takes longer as it's taking 4 shots instead of one and requires a couple of seconds after the shots have been taken to process the images into one so it won't work for sports or action, but it's great for landscape photography.
Pro In-body image stabilization allows stabilizing fast prime lenses
You can potentially do night photography without a tripod by using a fast prime lens wide open with in-body image stabilization.
Pro Great interface
Pro Good dynamic range
Pro LCD can be pulled out of the cradle
K-1 has a cross-tilt LCD.
Pro The K-1 + one of the Pentax Limited full-frame, all mechanical lenses is the dominant design in photography
It's not the camera alone; it's the camera and the lens together. Pentax's best lenses are the FA31/FA43/FA77 Limited series. There is no other camera + lens combination that can compete with the K-1 + one of the FA Limited lenses on a price/performance basis. It is not even a close competition.
Pro Built-in GPS
Great searching through picture libraries by location.
Pro Manual sensor shift allows taking tilt-shift photos with any lens
Pro Has LEDs that illuminate key parts of the camera
There's an LED that shines light on rear controls, there's another one that illuminates the lens mount, and an LED that highlights the card bay and remote release port. These can be really helpful in the dark.
Pro Automatically fixes horizontal level
Con Limited video capabilities
No HDMI output, no headphone socket.
Con Dynamic range could be better
According to tests performed by DxOmark 5DS / R has 12.41 stops of dynamic range in print and 11.08 stops on screen for images taken at 100 ISO while cameras in this price range commonly push 14 in print and 13 on screen.
Con Low max ISO
The cameras go up to 6400 (or 12800 in extended) ISO.
Con AF subject tracking could be better
Con Low-light performance could be better
Con Images at base ISO nosier than competition
Con Autofocus could be better
Autofocus won't be able to consistently keep up with fast-moving subjects so it's not the best camera for action or sports. Additionally, it only has 33 AF points which by today's standards is on the lower side of things and they're not spread around as much as in other cameras.
Con Video capabilities could be better
It's reasonable to expect more than 1080p30 in a 2016 flagship camera.
Con Limited lineup of full-frame lenses
While Pentax has among the best selection of lenses created for APS-C sensors, this is their first full-frame camera and there aren't that many of them.