When I decided to buy my first lens, I taught myself what all the different numbers on the lens meant. This is important to understand, so you're buying the lens that you want.
What does the mm mean?
What is the difference between one number and two numbers mm?
What does the ratio mean?
my post about exposure.
What does the f mean?
When you buy a camera lens online, the aperture is shown by f and a number, instead of a ratio. Just like the ratio example above, the f describes the maximum aperture.
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