When you look at the object end of your lens you will see a collection of words and numbers written around it. Looking at my 70-300mm, you will see something like:
OLYMPUS  ZUIKO DIGITAL    70-300mm                  1:4-5.6                 58mm
[              Make                   ]       [Focal Length]      [Maximum Aperture]    [Filter Thread]


Focal length
This is the number you see measured in millimetres. This represents how wide an angle of view you will have from the lens.

If you have a short focal length, say 11mm, then you have a wide field of view. If you have a long focal length, say 300mm, then you have a very narrow field of view. For a landscape shot you typically want as wide a field of view as possible, and so chose as short a focal length as you can. For a wildlife shot you generally want a close up of the subject and therefore need to use a long focal length.

This is a good diagram to illustrate the differences in focal length: images.cambridgeincolour.com/t…


Zoom versus Prime
A zoom lens lets the photographer vary the focal length of the lens between a defined range. For example my long lens is a 70-30mm (140-600mm equivalent). A zoom lens gives you greater flexibility, and as such is the ideal choice for sports or wildlife photography where you need rapid focal length changes.

Prime lenses in contrast are a fixed focal length. Because the optics in a prime lens are less complex than that of a zoom, you will generally find that a similar priced prime will be better than a zoom. Prime lenses are also generally ‘faster’ (have a larger maximum aperture) than their zooming cousins. A prime will also be lighter weight than a zoom.

I actually have more primes than zooms in my collection.


Maximum Aperture Size
The maximum aperture size of the lens will be written on front too. Depending on how expensive the lens is depends what this will be. An average zoom lens will probably have a maximum aperture of f3.5-f5.6, as you increase the focal length of the lens the maximum aperture gets progressively smaller. You can buy zoom lenses which have the same maximum aperture across the whole of the focal length range. As you might expect, these are considerably more expensive than their counterparts.

The maximum aperture is often written in the form of a ratio, such as 1:3.5-5.6 rather than using f numbers.


Filter Thread
The other number you will find on the front of the lens is the filter size. It will also be measured in millimetres and will look something like ‘Ø49mm’. You use this number when you buy a filter for you lens so that you buy the correct one to fit.


Choosing a Lens
So how do you chose your new lens? You have a couple of things to consider.

Firstly the cost. How much can you afford to spend? Like with most things, you get what you pay for. A cheap optic will not yield as good an image as an expensive optic. A cheaper optic can cause colour aberrations, blurring, reduced contrast, vignetting or distortion. However it is worth noting that sometimes lenses are chosen for these characteristics. The ‘toy’ cameras like the Holga produce technically poor shots, but still have cult following for that very reason. Like all photography, what is ‘correct’ is up to you the photographer and what you want to achieve from your shots.

As a rough guide on quality, the more lens elements the better the lens. But as those are the expensive part, the more elements you have the dearer the price will be.

Prime or zoom. What are you hoping to use the lens for? Do you need to zoom in and out for that?

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