Everything you need to know about the focal length

One of the most important characteristic of a lens is its focal length. You’ll hear a lot about it, but you only need to care about very few things in fact.

You’ll see it on the lens itself

The focal length is the attribute that usually makes up the name of the lens among with the F-number and some other marketing details, so it’s probably one of the first things you will know about a lens by just looking at it.

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Field Of View

We can describe focal length in many ways, but probably what you need to know from a photographer’s interest is how it affects your photo.

In short, the focal length influences how wide or narrow the FOV (Field Of View) is. The FOV is basically the angle of view.

I say ‘influences’ and not ‘determines’, because you would also need to consider the sensor size to exactly determine the FOV.

Precisely, shorter focal length will produce wider FOVs and vice-versa. Also, since we brought sensor sizes into discussion, larger sensor sizes will produce wider FOVs given the same lens and vice-versa.

FOV-2

That’s good news, because it means you can always find a lens that will produce exactly the same angle of view on your sensor as other lenses on different sensors. In theory at least.

Crop factor

The ratio between the 36 mm sensor size and your sensor size is called the crop factor.

Why do you need to know this?

Because if you have a crop sensor camera (and the chances are that as a beginner you do) and plan to buy a lens, you’ll have to account for the fact that the lens you’re buying will look one way on a Full-Frame or 35 mm format camera and differently on yours. Essentially narrower.

If you need a lens that would produce exactly that angle of view you’re seeing on the FF camera, you’ll have to divide the focal length of that lens by the crop factor and buy that one instead. The general idea is that smaller sensors need shorter lenses to produce the same look.

For example, your friend John has a Nikon D700 camera which is a FF camera and a 50 mm lens. You have a Nikon D3100, which has a 1.5x crop sensor.

You’ll have to buy a 50 / 1.5 = ~33 mm lens (closest you’ll get is 35 mm) to obtain the same FOV as John on his FF camera.

Another example. You have the same Nikon D3100 and decided to buy the rather cheaper 50 mm lens after all. Mary has a Canon 6D, which is a FF camera. Mary likes how your portraits look like and decides she wants the same lens, Canon version.

You will then tell Mary that if she wants the same FOV in her pictures, she will have to account for your crop factor and get a 50 * 1.5 = 75 mm lens instead. Because her sensor is 1.5 times bigger than yours and captures more angle.

Way to go! You saved Mary from buying a wider lens than she wanted!

Why is it called cropping?

Imagine that you have a physical photograph of about 36 x 24 cm in size. You decide it’s too big, so you get a scissors and cut it down to become 24 x 18 cm. Now you cropped it.

That’s exactly what your sensor does simply by just being smaller in the first place. Hopefully no one will physically crop your FF sensor with a scissors. Ever.

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So basically, the cropping factor indicates how much your small sensor is cropping the otherwise larger image that would have resulted from a 36 mm sensor given the same lens.

When multiplying a focal length with the crop factor, you’re essentially computing what is called the 35 mm equivalent focal length. You’re also answering to the question “What lens would I need on a FF camera to produce the same image as this lens on my crop camera?”.

By the way, it’s called 35 mm format, but it’s actually 36 by 24 mm.

Other sensors may be even smaller, case in which their crop factor will be higher. For example, a 9 mm sensor will have a 36 / 9 = 4x crop factor and to compute the 35 mm equivalent focal length of a lens, you would multiply the actual focal length of the lens by 4 to understand what lens would produce the same image on a FF sensor.

Misconception: The larger the lens’s aperture, the wider the FOV

Definitely wrong. It’s easy to fall in the trap of thinking that a lens is like a window, hence the bigger the window, the wider the view. That is correct for windows, but lenses produce images in a different way.

Aperture size has nothing to do with FOV and the easiest way to prove it is simply by reducing or closing the aperture, process called stopping down the lens. Doing so will not affect the FOV by any means.

Lenses can have a range of focal lengths (for example 70-200 mm), case in which they would be called zoom lenses or just a single focal length, case in which they would be called prime lenses.

Misconception: Zoom implies magnification

Note that a lens is called a zoom lens not because it magnifies the subjects, but because it allows you to zoom from one focal length to another by the means of a zoom ring.

An 8-16 mm lens would still be called a zoom lens even if it’s quite wide and makes things really small. By contrast, a lens of a fixed focal length of 400 mm would not be called a zoom lens even though it magnifies subjects considerably. It would rather be called a “tele” lens. Probably because of the resemblance with a telescope due to high magnification.

But since they exist, why would you buy a prime lens in the first place? It seems rather a limitation than an improvement.

Well, because engineering a lens that can change it’s focal distance is significantly more expensive. It’s likely that a prime will be optically superior to an equivalent zoom in the same focal range for the same money.

Zoom factor

The zoom factor of a lens is simply the ratio between its longest and its shortest focal length. For example a 50-200 mm lens would have a 4x zoom factor.

You would usually not hear photographers talk about the “zoom factor” of a lens simply because it offers no useful information besides telling you how much your longest focal length divided to your shortest is, which isn’t very helpful in practice unless that really matters somehow.

You’ll see the zoom factor instead on every entry consumer video and photo product because just like mega-pixels, it’s a marketing gimmick made to create artificial racing between similar products with unimportant differences. Just ignore it.

Background compression

Since a longer lens will produce a narrower FOV, the background will appear to be magnified, but at the same time closer to the subject than in an image taken with a wider lens given the same subject framing.

This is called background compression and it’s the illusion of the background being closer to your subject.

In comparison with a wider image where background objects seem far from your subject, in an image shot with a longer lens, the space between your subject and the background looks compressed due to appearing closer to your subject, hence the “compression”.

Types of lenses based on focal length

Lenses can be categorized based on a lot of things, but judging strictly by focal length, here is a list of how lenses are divided:

  • Wide and ultra wide lenses: These are lenses with very short focal lengths, therefore have very high angle of view. On FF, anything below 35 mm would be considered wide. On a DX / APS-C, anything below 24 mm would be wide. Super-wide lenses can go as short as 12 mm or FF or 8 mm on DX / APS-C and produce a view angle of almost 112 degrees.
    • A special sub-category makes the fish-eye lenses. These also have very short focal lengths, but present extreme barrel distortion, which gives the feeling of looking through the eyes of a fish, hence the name. They can reach very high angles of view that would not be possible with a rectilinear lens (a normal one). For instance, Samyang 12 mm F/2.8 Fisheye for FF or Samyang 8 mm F/2.8 for 1.5x crop both have 180 degree of view.
  • Standard lenses: These lenses have focal lengths in the range 24 to 70 mm for FF and 18 to 50 for DX / APS-C.
  • Tele and super-tele lenses: Anything between 70 and 200 mm on FF or 50 to 135 mm on DX / APS-C would be tele. Beyond 300 mm on FF and 200 mm on DX / APS-C would be considered super-tele.
  • All-around lenses: They are zoom lenses, but cover a rather wide focal range, so you can use them in a variety of situations, hence the name. For example, 18-300 would be an all-around lens quite useful in a vacation where you would not want to change the lens once every minute. These lenses tend to trade optical quality for versatility, so mind that.

Of course, these ranges are not rock solid and the list is here just to give you a hint of how the lenses are divided based on their focal length.

The Lens Angle app

Until you get your hands on actual lenses, download the Lens Angle application that I’ve created especially to allow you to see how lenses look at various focal lengths in various scenes.

It’s not a mobile app, so you would have to download it and use it on a PC running Windows.

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The graphic content rendered by the 3D engine is copyrighted to id Software.

Before moving the camera, click the 3D viewport first, so that it gets the keyboard focus.

Then use A / D / S / W to move the camera left-right-backward-forward, Q / E to move up and down. Simply move the mouse while holding the left click in the 3D viewport to rotate the camera.

Additionally, middle click changes the rendering interpolation method from near to trilinear filtering and the right click toggles wire-frame overlay rendering on and off.

Change the sky or the map according to your preferences.

The Vantage Point app

Here’s another app for you to play that will allow you to experience true-to-life scenes at the focal lengths of your choice while being able to rotate freely. Various sensor sizes can be tried out. Download it and run it on a Windows machine before actually buying a lens.

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But I want to view my own scenes

Until you decide to create your own panoramas to use with the app, download some equirectangular pictures from the Internet or create some and drop them in the ‘Textures’ folder. Restart the application and you’ll be able to view them.

All right, have fun with the apps, enjoy your lenses and leave any thoughts or questions you may have in the comments section below.


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