This video from Tony Northup explains it better:

Crop Factor: Why you multiply the aperture by the crop factor when comparing lenses
https://www.youtube.com/watch?v=f5zN6NVx-hY

I would also like to quote one viewer's comment

[gamerguy00 2 years ago]
So in short, just so people don't misunderstand this - you're actually talking about the the effective DoF, not the actual amount of light coming through the lens. Logically, an f/2.8 on a full-frame is still a f/2.8 on a m43-camera, however the effective depth of field is equivalent of a f/5.6. This has to do with the size of the sensor, as you very well explained. Great explanation, just some bits and pieces that could've been misunderstood.

I use it with Canon EOS 750D body (Rebel t6i). Awesome combination, I must say! Very good AF speeds allow me to do bird-in-flight shots. Excellent BG compression and blur when doing portraits @ 250mm.

I am into insect macro-graphy, and had bought this lens specifically to shoot butterflies. When this lens is mounted, I keep a Raynox DCR-150 handy which gives me a max mag ratio of 1.2:1 from a working distance of 16cm.

A great lens for EOS 70D upwards and EOS 700D (Rebel t5i) upwards... but unfortunately is slightly (12.5%) less sharper with EOS 7D Mk2 according to DXOmark sharpness test.

Image stabilization is good... I am able to do 250mm portraits @ 1/40s.
But I turn IS off at shutter speeds exceeding 1/500 (Bird-in-flight shots) as it seemingly affects the autofocus.