how to read those lens markings . . .

What do all the numbers and letters on a digital camera lens refer to and what do they mean?

If you're new to DSLR photography one of the first things you'll need to know when buying a new lens is the meaning of the numbers and letters printed on the lens. You will need to know what they mean to ensure you buy the lens that suits your specific needs.

First I'll start explaining what the mm number means. For example on a Canon EF-S 17-85mm f/4-5.6 lens you will see the number 17-85mm written on the side. The mm number refers to the focal length. This is the length or distance from the front of the lens to the camera's sensor. The focal length is one of the most important considerations a photographer needs to take into account when buying a new lens. The mm number (distance in millimeters from the front of the lens to the sensor) will determine the style of photograph that can be taken.

For example, the smaller the mm number the wider the angle of view within a photograph.

On the other end of the scale, big mm numbers like 200mm or 300mm provide a magnified telephoto view. They are good for animal photography where you might want to zoom in on a bird yet not be too close and scare it away.

What is the difference between a lens that has a single mm number to one that has two mm numbers?

If a lens has a single mm number, for example 100mm written on it, it is said to be a prime lens. This means the focal length of the lens is fixed and cannot be changed.

If a lens has two mm numbers on it, for example 17-85mm, this tells you it is a zoom lens. In other words it can take photographs with a 17mm focal length or zoom all the way to a 85mm length.




What does the ratio number mean?

You might also notice a number written on your lens that looks like a ratio. 
For example 1: 4 - 5.6 This number refers to the widest aperture available for that lens. Generally speaking, the lower the f number the better, because in low light situations the lens can be used to give a brighter image. This number is sometimes referred to as "lens speed".

Note also, the 1: part of the number is there purely because f stops or aperture is measured in ratio's.

What does Ø58mm mean?

If you see a number that starts with the character symbol Ø, for example Ø58mm written on your lens, it simply means that a screw on filter that is 58mm in diameter will fit this particular lens.