There was a thread with the same title on the old forum so I thought I would restart it here:
Something I have learned this week on this forum that I was staggered by is that f numbers are not standardised by the amount of light they let through. I would have expected f2.8 to be different diameters on different lenses but defined so that you could take your 18-105mm f3-5-5.6 and your 35mm f1.8 and when set at the same aperture and ISO, you would see the same shutter speed. Turns out I was wrong. Can see why aperture is not standardised by light, even if somebody explains it I will still not think that it makes sense for the reason stated above, but there ya go - never too old to learn!
Some points raised here are like 'DX doesn't have more reach - it is a cropped sensor' are just pointless to sweat over. Those who say DX magnifies the image or has more reach do understand the effective
difference between DX/FX so what the heck, but that aperture point was a 'What? Wow!' moment for me. 40 years of taking snaps and I didn't know and would never have even suspected that.