The S1 and the S1R have the same body.
The S1/S1R has one of the biggest and heaviest bodies among the full-frame mirrorless crowd -- the only one that's close is the Leica SL -- and is even larger and weightier than many competing DSLRs, such as the Nikon D850. The size is partly due to the sensor-shift stabilization mechanism, which can get pretty big with a full-frame sensor. It's not the mount, because the L-mount diameter (51.6mm) is smaller than both the Nikon Z (55mm) and Canon RF (54mm) mounts.
One of the reasons it's so deep is the hefty, comfortable grip.
One of the lenses that will be available at launch, and which is the kit offering, is a 24-105mm f4.
It retains a top status LCD like the G9, though the information is laid out a bit differently.
The camera has two card slots, an XQD and SD. The XQD slot will be firmware upgradable to CFExpress at some point in the future.
In addition to the obvious differences on the outside -- a switch to a round eyepiece and a bayonet mount eyecup -- on the inside it's a lovely, bright OLED with a relatively high 5-megapixel resolution.
The back control design and layout is an updated version of the Lumix G9's, but the movie record button's been moved to what seems like an awkward location next to the viewfinder.
The camera improves on the G9's joystick with the ability to navigate diagonally as well as horizontally and vertically.
The viewfinder is also one reason the camera's taller than some.
The camera has a USB-C connection that supports charging and a full-size HDMI connector, as well as mic and headphone jacks.
It has the same drive mode dial as the G9, though the 6K and 4K burst photo modes have been relocated.
While it has essentially the same locking mode dial as the G9, Panasonic drops the Creative Control mode, which is less important on a pro model.
Panasonic finally adds a control to quickly lock and unlock the other controls on the body, rather than forcing you to go into the menus.
Like the G9, the switch on the front of the S1/S1R provides quick access to saved custom settings.
Panasonic has always gone with the swing-out-and-twist variety of display, but for the S1/S1R it switches to the type of three-way multiangle tilting display that Pentax launched with its K-1.
While these three-way displays can be set to a lot more angles than traditional articulated ones, they always feel awkward and confusing to position. This one's no different.
However, given the flexibility of positioning the display, it's usually worth the cumbersome movement.
The front buttons are differentiable by feel.
The optional grip only holds one extra battery.