Is the high-framerate crop mode in the Red Scarlet camera practical? I'm pretty excited by the new Red Scarlet, but one point seems very limiting about its (high-)framerate capacity:

It's actually cropping the image on the frame when shooting at framerates higher than 30fps  (see its tech specs

This of course will reduce the resolution, but that's acceptable. Even the Vision Research Phantoms do so - yes, starting with extremly higher resolution/framerate ratios of course!

The problem is that cropping means that if you have to match another scene angle of view you'll have to move your camera and/or change lens.

There's a nice graphic on the Red Forum that shows the cropping and I'm reading here:


  The "usefull" motion modes on the Scarlet are 4K and 3K. 5K at 12fps
  max is for timelapse and stills. 4K uses less of the sensor, making
  your lenses, "longer." The ratio goes from 1.3 to 1.6. A 30mm lens
  goes from, effectively a 39mm to a 48mm.
  
  But a lot of us want to use 3K to get raw capture without huge files.
  3K captures a 4.97MP frame using 36% of the 5K sensor area. The file
  size will be 56% the size of 4K.
  
  However, the increasing crop factor makes a 35mm lens into a 70mm. So
  we are looking at options.
  
  We need to know if S-16 glass is an option here. Some say it is. Some
  say some of it is, but not all.


I'd like to underline that 4K is the highest available resolution for video.
It might be also important to point out that the there is also a limitation in the extremely useful HDR-X as well explained on this phil bloom post. It's okay, I think I can live without it if I get a good camera for a decent price.

What I'd like somebody to shed a light about is to: clarify what lenses would reasonably work for high framerate modes, and what should be done to ensure normal speed vs. high speed shots for the same scene will edit nicely together..

Also would be nice to have a real hands-on in the answer not only theoretical, about quality of the cropped videos and practical issues with the cropping factor, but it's not mandatory!
This question is pretty obsolete by now. The new SCARLET-W cameras support 5K WS (2.4:1) video at 60 fps, 4K WS at 150 fps, and 2K WS at 300 fps.  Matching field of view is a simple matter of mathematics, not magic.  If you want to match a 50mm lens on a 6K RED camera using a 3K crop, a 25mm lens will do well.  And the price of the SCARLET-W is now below what the price of the original SCARLET was in 2011, so you get a lot more camera, as well as more sensor, for less money.

It is relatively well-known that when shooting slow-motion footage, it is often useful to use an even lower compression level than for normal motion footage, which is unfortunate.  But the DRAGON sensor, in a WEAPON body, makes SCARLET-W 10:1 as good as SCARLET DRAGON 8:1, which in turn has been estimated to be as good as SCARLET MX 6:1.  You will want to use special noise reduction techniques if you shoot at the limit of the SCARLET-W, but if you shoot 4K WS at 60 fps, the 6:1 REDCODE is really quite good.  And 5K WS at 60 fps REDCODE 9:1 is also not bad, either.