Using ML (Magic Lantern), there is a really nice option to focus using a manual lens with the EOS M.
With "Focus Peak" enabled, the camera will show where focus is achieved. Really cool, cool as a feature that is...
This option comes with a price. At least my EOS M warms up quite a bit when running ML. Actually, the heat created within the camera made me worried about using ML in this configuration.
However, there is always something to be learned.
The "Focus Peak" option of ML works best with the aperture fully open (I wonder why ;-).
Of course, with the aperture fully opened, the focus plane is rather obvious, even on a small screen on the back of a camera.
What can we learn from this?
To achieve reasonable focus/DoF, open you lens as wide as possible, adjust the focus and finally stop down the lens to the desired DoF.
Using old lenses, of course, you can look up the DoF on the lens housing itself!
Still, the focus peak of ML is a really cool tool... I wished it would keep the camera cool too!