When I made the switch to Sony, one of the features that appealed to me at the time was the ability to shoot true 60p video in 1080p. Several years later and video frame rates are a hot topic with modern cameras right up there with 4k.
Currently cameras like the 5K MKIII still can't shoot 60p at 1080p resolution while others such as the GH4, a7s, and others can shoot HD upwards of 120 frames per second.
I've recently had the opportunity to use a Sony a7s along side an FS700 to do some high speed tests comparing 60, 120, and 240 frames per second; the maximum the FS700 can record at 1080.
The FS700 was a rental that I had for the weekend. This is the first time I'd used one for a professional gig having only played with them before in-store. The FS700 is an amazing camera and produces amazing imagery but it's built in an awkward package with awkward controls and menus. It would take me a long time to get comfortable with the form factor if I actually owned one. The basics are simple enough and the powerful features help you to see past those shortcomings. It can record video at frame rates up to 960 frames per second but the resolution and quality suffer greatly. 240fps is the highest usable limit.
The a7s is a camera I've recently purchased as it's a camera that's great both for video and for low light event photography. It can record video up to 120 frames per second though at a reduced resolution of 720p. Combined with the best ISO performance of any camera available today, picture profiles which include SLog2, and 4k video via an external recorder make the a7s one of the most interesting cameras on the market today. I prefer this camera because it's so much like my a7r...identical save for a few tiny features and the extra video settings in the menus. The learning curve was nowhere near that of the FS700 which I'm still trying to figure out. It should also be noted that when in 120p mode, the sensor crops to a Super 35 size creating a crop factor. This can be combatted with a Speed Booster if you've got one. This cropping introduces some aliasing and moire to the image but the overall quality is quite impressive even upscaled to 1080p.
I have experience overcranking 60p footage and have always felt that it wasn't quite enough. It can be very useful for subtle effects but has little to no effect on very rapid movement. I wanted to see how exactly the differences between 60, 120 (at 720p), and 240 frame rates compared to each other recording the same movement.
The below video shows some video of a toy quadcopter taking off at 4 different frame rates. I went with this demonstration because it has a combination of slow smooth motion and high speed rapid rotation of the rotors.
The most apparent difference that became apparent to me in these tests is that there is such a thing as too much. I made this video but watching the 240fps clip bores me to no end! In this case the 60p clip looks smooth but not neccessarily 'slow' like the 120p and 240p clips do. I shot as many different things as I could and found that for some applications, 240 isn't enough...like a popcorn kernel popping...it's just too fast. For normal human motion like people dancing, I tended to think the 120p frame rate was better than 240p. Faster motion like my dogs shaking, lapping water, or barking, 240p was the best. The FS700 can carefully control it's frame rate and shoot below 24 frames per second down to 1 frame per second. This is useful for making in-camera timelapse videos.
One of the differences of the FS700 is that it's Super Slow Motion mode has a convenient 7 second buffer that can be set to trigger at the end of the press of the record button. That is to say that when you press record, the previous 7 seconds are saved to memory. This prevents overly long clips and waste of memory space...though it also prevents longer recordings should they be desired. The ability to play back the clips slowed down is a huge help especially if you want to show client the work as you're recording it. None of the other Sony cameras I've used have this feature.
Another thing which became very apparent to me was the need for light. I recorded people dancing at a party Halloween night and in front of a white seamless with 2000 Watts of Tungsten light, I still had to run at 2000 ISO with the kit lens wide open at F/3.5 to get a proper exposure. To shoot high speed indoors with my choice of F-stops, would require a LOT of light. The a7s combats this need with it's amazing ISO performance and I'm looking forward to shooting slow motion video in dimly lit conditions where no other camera can.
It's apparent to me after using both that the a7s was the right camera to buy (I can't always still rent the FS700). 120 frames per second is plenty for most applications even if it is only 720p and the low light performance is more useful than I ever thought it could be.