I think you are missing a characteristic of lenses called the f-ratio, which is sometimes described as fast or slow. This is not specific to cameras; it is a general term used in optics. The f-ratio is the ratio of the aperture to the focal length. Even telescope lenses and mirrors are described similarly. For example, a telescope with an f/4 mirror can be described as a fast telescope.
Not only that, a lot of the terms have been misappropriated in camera lingo. The camera folks express aperture as an f-ratio, which is extremely confusing.
Thank you, you are absolutely right! I should have said "f-ratio" everywhere I used "aperture".
I'll leave my previous comment in its incorrect state in the hope that people will see yours to correct and clarify it. :-)
> The camera folks express aperture as an f-ratio, which is extremely confusing.
Isn't it the other way around? I think the mistake we self-proclaimed "knowledgeable" camera folks make is using the term "aperture" where we really should say f-ratio.
For example, I have two Olympus Micro Four Thirds lenses that are both f/1.8: a 45mm and a 75mm. Looking at these lenses it's pretty obvious that the f/1.8 is an f-ratio: the front element of the 75mm is much wider than the 45mm, as you'd expect.
So thanks to your comment, I will endeavor to use "f-ratio" where I've been misusing "aperture".
Not only that, a lot of the terms have been misappropriated in camera lingo. The camera folks express aperture as an f-ratio, which is extremely confusing.