Beginners often ask me “why should I buy those lenses with the low F-numbers? Why not buy the standard lens with the 5.6 on the front? They say the lens with the 2.8 on the front is better but I see it takes the same pictures: it’s just more expensive, right?”
Not exactly. The low F-number means the lens has a larger maximum aperture. This means two things: it lets in more light, meaning faster shutter speed; and the ability, should you choose this, to create blurrier backgrounds.
Here’s my hand at F/11. Recently in Scarborough, while the other instructor, Christine, was explaining the effects of the Aperture setting. My hand is as close as the lens will allow while still achieving focus. I am in Aperture mode (“Av”) and have set the camera to a setting of F/11:
Now the same at f/5.6: a much blurrier background, see:
And now, since I have an expensive lens, I can go even farther, to the extreme end of this lens, namely f/1.4. Meaning very wide open. Meaning very blurry background – and when you look carefully, even foreground:
So that is why people buy these lenses with those low F-number – i.e. “fast”lenses. If you want to blur the background dramatically. Or if you want to have the resulting faster shutter speed (at the same ISO, f/1.4 gives a shutter speed four times faster than f/2.8, and 16 times faster than f/5.6).