Ugh, the terminology the guy was using in the video was horrible. If I was new to photography, I would come away from that video completely misguided. When I look at the aperture of a lens, I think primarily how much light it lets in, and know that the effect of a larger aperture gives me a narrower depth of field.
I think that's unfair. New photographers are not familiar with depth of field or circle of confusion as they are with aperture and background blur. He was just using more familiar terms but he did not use them incorrectly. If one listens carefully, he shouldn't be misguided.
When he tells you that when you compare cameras of different sensor size, he keeps telling you that you also "need to multiply the crop factor and the aperture..."
"... because it allows you to compare lenses in terms of background blur."
That's correct, isn't it?
even calling out manufacturers for not being as honest when selling their crop factor lenses, that the aperture isn't the true value for the lens.
Actually, he said manufacturers multiply ONLY the focal length by the crop factor (to give equivalent FoV) and NOT the aperture (to give equivalent DoF). And he gives an actual illustration from Amazon. He urges them to convert BOTH if convert at all.
The amount of light coming through at f/1.8 is the same for a micro 4/3s as it is for a full frame. He never clarifies that.
The amount of light may be the same (and he doesn't comment on that) but the light gathered by the sensor is different, due to different sensor sizes (and that, he does specifically state).
He makes it sound like aperture and depth of field are the same thing, when they're clearly not.
Actually, he keeps saying multiply aperture by crop factor to get same amount of blur.
I was oblivious of the effect of crop factor on depth of field for a long time until Neuro corrected me. I think this is still a pretty common mistake among beginners. Northrup was trying to correct that without going into technical details of circle of confusion. I am sure there are better ways to explain it, but I think this was a decent effort.