I haven't seen the 5.2K Option with the EOS M though in 1.6x Crop?
The preset is called "5k anamorphic flv" and allows you to film at 1736x2928/16fps/12bit continuously. It is a full sensor readout mode meaning that the 1.6x Crop is referenced to the full frame sensor size. This mode works extremely well with the speed booster giving you only about 1.1x crop w.r.t. full frame - perfect if you like experimenting with fast prime lenses creating beautiful bokeh and shallow depth of field. In this case, you project the full frame vision onto a 1.6x croped aps-c sensor. As I said, if I want quality, this is my mode for landscape and portrait videography. Since there is no aliasing, image quality, even fine detail rendering, is superb - way better than the regular 1736x976/24fps mode.
For me, there are only two minor downsides of this mode:
1) At 16 fps, camera motion speed needs to be about 30% slower compared to 24 fps, otherwise, if you move the camera too fast, the motion blur becomes too much for a clear image while panning for example. Usually, this is not a problem if you pan from a tripod with a fluid head or a gimbal.
2) If you film at an extremely wide focal length, in the 10 to 24 mm range, the texture size of your finest detail becomes comparable with the 1x3 horizontal pixel size meaning that your camera will not be able to resolve finest detail at infinity at that resolution. The effect is perceptible only when 100% pixel peeping or if you watch your video on a very large 4k screen from a close distance, (less than 1 m). If you use your favorite 18-135 lens and stay above 24 mm, you should be perfectly fine. My favorite one is the 24-105/F4 L IS for this mode since it is heavy and provides more stability with hand held filming. Also, if you use this lens on a gimbal with a focus motor, you can achieve very smooth zooming when the focus motor is coupled to the zoom ring.
This anamorphic mode requires some practice, attention and getting used to it but once you learn how to avoid the above drawbacks and get it working the way you want, you will fall in love with it. Just spend some more time with it and you will see what I mean.
