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Lens Focal Lengths - Zoom lens vs Prime lens

There are many factors in film the determine the end result of a shot. One of those factors is the focal length of the lens.

Please note: There may be many technical terms that come up in this article, but we will explain each term because even though you may or may not be a filmmaker already sometimes the basics of why something is called a certain way is not considered and can expand our understanding of certain technical concepts – short story: hang in there with the technicalities and you can always add a suggestion box topic if something is still unclear.

The focal length of a lens is just what we say to describe how 'zoomed in' you are in a shot. If you are 'zoomed in' a long way then we call that being at a 'long focal length'. If you are not 'zoomed in' very much then we call that being at a 'short focal length'.

These 'focal lengths' are measured in numbers. These are the numbers that we see on a lens - whether it is a 'zoom lens' or what we call a 'prime lens'. In the image below this zoom lens has a focal length set to '35mm':

So, now that there are new terms to learn, what is a 'zoom lens' and what is a 'prime lens'?

Let's start with a 'prime lens'. This is a lens that has only one focal length. This means you can't get closer to or further away from a subject without physically moving the camera towards or away from the subject. You can't twist the lens to make the camera zoom in.

So how does this work?

With prime lenses photographers and videographers will then have a number of lenses that they change depending on how close or how far away they want the image to look. So, the longer the focal length number is (ie: 85mm vs 35mm) the more 'zoomed in' that focal length is. This is covered in more detail in the coming exercise.

The other type of lens is the 'zoom lens'. This is a more common type of lens to us all. These lenses usually have focal length ranges whereby if you twist the lens it will either 'zoom in' or 'zoom out' if you twist in the opposite direction. You can see in the image above the focal length of this zoom lens is 16mm - 35mm. At the bottom of the image you can see the minimum focal length of '16' and the maximum being '35' towards the middle of the camera. With zoom lenses you don't have to change them as much because you are covering a range of focal lengths in one camera.

With zoom lenses and looking to purchase one, you will see they are specified based on the focal length range it covers. For example, the lens above is what is called 16-35mm lens and the following images represent their respective focal length ranges:

24-70mm zoom lens:

70-200mm zoom lens:

Zoom lenses make a lot of sense then, so what's the purpose of 'prime lenses' then? For a long time photography and filmmaking only had prime lenses because the technology hadn't advanced to zoom lenses yet. However, when zoom lens technology became a reality there are so many components and mechanics involved in having a zoom lens that there are other elements of prime lenses that have to be compromised in a zoom lens. These could include:

  • Speed

  • Light and;

  • Sharpness

So, it's a personal choice whether someone chooses a prime lens over a zoom lens, however, most professionals will have both in their kit - a few standard zoom lens that do most the work and then some more bespoke prime lenses for those shots they can take a little more time for.

Continue on to the next article 'Lens Focal Lengths - Wide angle vs Close ups' learn about how the different focal lengths affect the look of an image.

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