Gaining the angle
An illustration on how equalling the relative, subjective framing from one format to another by way of altering distance to "see" the way that a lens looks according to a different format.
From the previous chart: Attempting to illustrate the basic concept of depth of field across formats. While mathematically the APS-C settings aren't completely accurate (due to variences in "APS-C" sensor sizes) it is a basic outline based on general crop factors, rounded to 1, 1.5 and 2x for Full 35mm frame, APS-C and Micro 4/3 formats respectively.
While we often hear that Full Frame offers shallower depth of field, it's not entirely true. That is only true if you are to use a focal length to frame a scene or subject with the same angle of view it would provide on a Full Frame, 35mm format sensor. From a fixed location, the smaller the format, with the same focal length and aperture setting, you will achieve a shallower depth of field. This can be handy if needing to shoot from a fixed location (ie: birding, sports, safari, trips to the zoo, etc) when wanting to achieve a shallower depth of field for subject isolation.
You can read more if you'd like on my blog here: Aperture, Focal Length, Sensor Size, Exposure and Depth of Field.