Categories

# How I understand the Focal Length of a Lens (Part Two): Focal Lengths

The simplest explanation of what the focal length of a lens really is. Part Two shows how focal lengths affect storytelling.

In Part One we tried to get a simple understanding of what the focal length of a lens means. In this part, we’ll take this knowledge further, so you can make use of it practically.

## Depth of field

Let’s look at the last picture again:

I’m not going into the detail of depth of field (DOF). There are enough articles on the subject.

If you want to see your foreground and background at the same time, you need depth in your lens. One way to achieve this is to stop down your lens (close your aperture). This isn’t always practical (because it needs tons of light), so the other ‘solution’ is to use a wide angle lens. A wide angle lens really does not change the depth of field. To know how this works, read this excellent article: Do wide angle lenses really have greater depth of field than telephoto?

Suffice to say, a wide angle lens gives the impression of greater depth of field, while a telephoto lens does not. We’re just suckers for it.

If you use my box analogy, the more blue you have, the greater the DOF, and the less blue you have, the lesser the DOF.

## The idea of perspective

Look at this image:

What’s the difference between what the two people see? Location. They are physically located in a different place, and therefore, we say they have two different perspectives.

I had trouble remembering if, whether and when a perspective change happens with a change in focal length. Actually, the answer is quite simple: Any time you physically change the location of the camera, you are changing the perspective. Remember:

• Changing the focal length does not change perspective. It’s just a different slice of the same perspective.
• Changing the location of the camera changes perspective.

A change of perspective can also happen if you move a camera towards or away from an object. If one has never seen a lens or camera, one could argue that this is the same as changing the focal length. This argument can be met easily if one actually uses different lenses in the field.

By moving closer or further away from an object, you are also changing the object’s relationship (your viewpoint, not in reality) with other objects in the scene. This does not happen with a focal length change. It only happens with a change in location, i.e., perspective.

We saw in Part One that each focal length is a slice of the box:

The angle from the back (the infinity point) to each slice doesn’t change at all. That’s perspective.

## Angle of view

Now, look at it from your viewpoint (or the camera’s viewpoint):

Because you’re only getting a slice from the total possible view, the angle of view for each focal length is different. When you choose a focal length, you are also choosing a suitable angle of view for your scene.

Choosing a lens is part necessity and part art. As to the necessity part, read What lens to get? In it, I go into great detail about spaces and how that affects your choice of focal length.

Obviously, the size of the sensor determines to some extent what the angle of view is. To know more about this and the crop factor, read the Chapter on lenses in the Comprehensive Guide to Rigging ANY Camera.

Now we have enough information to tackle some of the complex terminology we saw in Part One:

• “higher or lower magnification” – means a different slice of the box.
• “wide or narrow angle of view” – means a different slice of the box.
• “high or low optical power” – means a different slice of the box.
• “distance from the lens to the film or sensor plane” – the technical definition of focal length.
• “focus on infinity” – the infinity point.
• “focal lengths in millimeters” – they are written as millimeters.

## The art of choosing focal lengths

All said and done, lenses are simply tools that you use to tell a story. Just because a focal length is written in millimeters doesn’t mean you have to master science and math. They could be called anything, who cares?

Let’s see how lenses affect your ‘box’ (scene).

Super-wide and Fish-eye lenses

These lenses tend to distort the image simply due to the nature of optics. A special case of such lenses is the fish-eye lens:

These lenses are used when large expanses have to be covered (large buildings, planets, etc.) or when you want the ‘fish-eye’ effect. Either way, you need a solid reason for using them, and their use in cinema and video is rare.

Wide angle

The wide angle makes things in the scene look further apart:

The easiest way to know if a shot is a wide angle is to compare the foreground and background. In the above image, the foreground character is framed normally, but the background character appears smaller than usual. This gives the feeling of greater depth and distance between the two.

Wide angle lenses tend to distort objects as well, and sometimes this is used in the case of close-ups. Obviously, a person’s face distorted is rarely flattering!

Since wide angle lenses cover more of the scene, they are used for long shots, masters, mid shots and so on. Also note the greater ‘depth of field’.

Normal lens

A normal lens is somewhere in the region of 30mm to 50mm (35mm full frame equivalent). It gives a similar view (how objects are arranged according to our eyes) to the human eye:

This is probably the most used focal length range in the cinema or video world, because the ‘risk’ is less.

Portrait or short telephoto (85mm to 100mm)

A portrait lens is somewhere in the 85mm range (full frame equivalent):

You can see that this is no longer a ‘normal’ view, as objects are beginning to get compressed and closer to each other. But it’s still not off by much. Notice how the depth of field is reduced considerably under normal lighting conditions. To get more depth, Ozu would have had to completely stop down his lens.

Telephoto lenses

A telephoto lens compresses space:

You are no longer seeing a large scene, but only a small slice of the box. Everything (everyone) appears closer to each other, and relationships take on a different meaning. Characters far or near ‘share the same fate’, if you will. Compare it to the wide angle shot shown earlier.

This frame was stopped down and bucket loads of light was used, hence the greater depth of field.

There you have it. I hope I have given you enough ideas so you can get started thinking about lenses. Obviously, my goal with this article was to help you understand what the focal length of a lens means in practical terms.

What next? You could read a technical explanation of focal length, if you feel inclined. After a certain point, you will need to stop reading and start practicing with some real lenses. It’s only then that the art can begin.

brimmsays:

Great Info…

brimmÂ Thanks!