>>3381270>Chart is pointless, you don't convert aperturesHere's why you DO convert apertures:
First, there are two things that your max aperture determines: The shallowest DoF you can get, and the amount of light your lens can suck up.
First, depth of field. Assuming you're keeping your framing and perspective constant (I.e., taking "the same" picture), your depth of field depends on aperture and the format size. So if you multiply your aperture by crop factor, you get a 35mm-equivalent aperture that will tell you what depth of field the lens is capable of on the smaller sensor.
Second, light gathering ability. As we all know, for a given aperture, the same amount of light will be hitting every point on the sensor. That's why f/2.8 gives you the same exposure regardless of if you're on a cellphone or 8x10, or a 10mm or 1000mm. But when we're comparing cameras' abilities, max aperture isn't really what we want to know for low light ability--it's gonna be max aperture plus how noisy it is at a given ISO. And so, again, you'll find that *generally*, noisiness of a given ISO correlates with crop factor. Not always--m4/3 punches a little above its weight, and Canon full frame underperforms a bit, but as a rule of thumb, it works. So for low-light ability, you can simplify out the ISO noise issue by multiplying by the crop factor. And hey, you can move that multiply-by-crop-factor over to the aperture just as well and get the camera system's easy-to-compare low-light ability.
So in terms of the things that aperture is really useful for--both depth of field and low light ability--it makes sense to compare camera systems based on crop-factor-normalized apertures.
