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Focal Length Explained Simply

Focal Length Explained Simply

Focal length explained simply.

Because when you’re getting started in photography, all those numbers and terms can be pretty intimidating. They lead to a lot of questions for beginners around the subject of focal length.

Questions like:

  • What does the mm value mean on my camera lens?
  • How to measure focal length?
  • Why is focal length important?
  • What is normal focal length?
  • Do I need to worry about focal length?
  • What is focal length anyway?

Finding an article that answers these questions in a way you can understand is not always easy.

So I wrote one for you.

I think it’s best we start at the very beginning.

What is focal length?

Put simply, focal length as denoted by the numbers on your lens (35mm, 50mm etc.) is the distance between your sensor or film and your camera lens when it’s focused to infinity.

As far as I’m concerned, to keep this article as simple as possible, that is it.

Even that when it is focused to infinity bit is deeper than I wanted to go, already.

If you have a zoom lens, say an 18-55mm, you can use it any focal length between 18 and 55mm. Prime lenses are fixed at one value.

Here’s a picture to help you visualise how focal length is measured.

camera focal length diagram

Is focal length important?

My opinion is yes, it is, but only until you start shooting.

Let me explain.

Depending on what you shoot – landscape, portrait, street, travel, anything else – understanding the best focal length for the photographs you’re going to want to get is important.

My general take on focal length is this: when I’m out shooting, focal length is one of the last things on my mind.

I’ve already chosen my lens, which is always a fixed length, and am far more concerned with what I can capture with it, and how, than I am with why.

I’m not thinking about what I could be capturing with a different focal length. I’m concentrating on what I can shoot and not worrying about what I can’t.

So whilst out with whatever lens you might have, you should be adapting your mind to suit what you see through the viewfinder, adapting your ideas, adapting your subjects, adapting your composition, and taking your photos accordingly.

Understand a bit about focal length will help you choose the best lens for getting the best results you can. That’s important.

But once you’ve chosen, I recommend forgetting all about it.

How does focal length affect my photos?

Focal length affects your photos in a few different ways.

The first, which is perhaps the single most noticeable, is how close to the camera your subjects appear. It’s the basic action of zooming in and out.

As we zoom in (or use longer focal lengths) and our subjects get closer, we lose width too.

Take a look at this next diagram, showing how longer focal lengths mean narrower fields of view while shooting.

camera focal lengths fields of view

Subjects appearing closer and fields of view becoming narrower are both rather obvious effects of different focal lengths.

However, there are further, less conspicuous ways in which focal length changes how your images look. While they may not be quite so obvious to the beginner, they can be just as important depending on what you’re shooting.

One is how near or far different subjects in your image appear to each other.

With the help of some Branston Pickle, Aesop’s Fables, and two men punching each other in the face, I’ll give you another visual.

camera focal length demonstration

The first picture, shot at 18mm, appears to show a greater distance between the objects while the one shot at 55mm is somewhat flatter, with the images appearing closer together.

Notice the size of the book in relation to the pickle.

When taking these pictures, the three objects did not move. I did have to move the camera further away from the pickle as I zoomed in to the higher focal lengths, to have the bottle the same size in each picture, but that was the only change I made.

Focal length explained with real world photography

While that mini shoot was fun to do, we’ll always learn more from real world examples, such as these shots from the park.

For these three pictures, I had to stand further away from the lamppost to make it appear the same size in each picture as my focal length increased (as I zoomed in).

Notice the distance from the lamppost to the tree, and the bin, and also how the blue cafe sign comes closer as the focal length increases.

Looking carefully, the first tree in the first picture doesn’t even make it into the other shots.

camera lens focal length demonstration

Of the three park photos, I like the one shot at 18mm most. It has a depth that the others are missing.

The pictures taken at 33mm and 55mm were taken from this position only to show you the lessening distance between the lamppost and the tree.

They are in no way good pictures.

When focal length isn’t important

I said earlier that I don’t believe focal length is something to worry about once you’re out shooting, and this is where I can explain that further.

Had I been in the same park to take actual photos with a 55mm lens, and not just getting shots to demonstrate focal length, I wouldn’t have taken that third shot. I would have found other things to take pictures of. Things more suited to the focal length available to me.

Indeed, I have been to that park with a 55mm lens. The results are here.

You might think the images in that post are rubbish, or you might think they aren’t bad. I would be surprised to hear you thought they’re rubbish because of the focal length though.

I don’t recall ever looking at a picture of an outdoor scene and thinking ‘it’s all right, apart from the focal length’.

It doesn’t happen.

If it looks good, it’s a good photo.

If it looks bad, I highly doubt thinking about the focal length would have been the best way for the photographer to make it a better picture.

Not when you can always find a different angle or composition or, if it’s really not working, to just forget it and find a different subject.

When is focal length important?

When using the wrong focal length makes your pictures bad.

When I’m out shooting on the street, I can adapt my images to suit the focal length. However, if you’re shooting something that needs a particular look, such as portraits choosing the right focal length is vital.

One reason the focal length doesn’t matter when shooting street is that nobody knows, or cares, how near or far things really are in your images, so long as they look good.

When shooting portraits however, this distortion matters.

Even though everyone is different, we do have an idea of the boundaries of normality when it comes to the human form. And no model wants to have their nose or chin enlarged by the camera.

punch and judy

As we saw above, shorter, wider angle lenses will add depth. Longer, narrower lenses will flatten the image. So what lens do we use for portraits?

Longer ones.

As well as allowing for a shallower depth of field to blur the backgrounds, which is a whole other article, longer lenses can help make people look better.

If a wider lens will make their features more prominent, longer lenses can, and often are, used to make them look more petite.

Talking about flattening or exaggerating features leads us nicely to the next question.

Which focal length is normal?

Another way to word this question would be ‘which focal length is the same as the human eye?’

And the simple answer would be 50mm.

However, the more useful answer would be 50mm on a full frame or FX camera.

And that leads us down the next rabbit hole. Because if you’re going to understand exactly how the focal length of your lenses will make your images look, you’re going to need to learn a bit about crop factor.

What is crop factor?

Crop factor relates directly to the type of sensor you have in your camera, and will alter the effective focal length of the lens you’re shooting with.

In short, the 50mm lens you buy might not give you a 50mm focal length.

The first step to understanding this is to ask, what kind of camera do you have? And more importantly, what size sensor does it have?

The main options for cameras with interchangeable lenses are FX and DX in DSLRs, APS-C in Sony (and other) mirrorless, and m43 in Olympus (and other) mirrorless.

While not actual size, this diagram shows you their relative sizes.

camera sensor size guide

If you have a DSLR and aren’t sure, it will probably be DX. These are the more common type, and if you never asked when you were buying it, you probably got a DX.

Most people with an FX camera will know that they have an FX camera.

So to repeat that earlier statement, on a full frame or FX camera, a 50mm lens will give you photos that look pretty much as the human eye sees the world.

How does crop factor affect focal length?

Camera sensors other than full frame, or FX, alter the effective focal length relative to their size.

In simple terms, FX sensors are roughly x1.5 the size of DX and APS-C, and double the size of the m43.

These sizes can be used to calculate the effective focal lengths of lenses when used with these sensors.

  • A 50mm lens on an FX camera will give a 50mm field of view
  • A 50mm lens on a DX or APS-C camera will give a 75mm field of view (50mm x 1.5)
  • A 50mm lens on an m43 camera will give a 100mm field of view (50mm x 2)

The following diagram gives a visual representation why.

camera sensors size diagram

This all means if you want to know you’re shooting the focal length as stated on your lens, you’re going to need a full frame or FX camera.

Crop factor can be an annoyance when it negates truly wide angle lenses on non-full frame cameras, but it can usually be worked around for more common focal lengths.

For example, a 35mm lens on a DX or APS-C sensor will give something close enough to 50mm when x1.5, and a 50mm lens will give something close to 85mm, which is handily a great length for portraits.

So do I need to know about focal length?

For me, focal length is something you should probably know about as a photographer. And know about it well enough to explain it to someone else.

However, I also believe it’s something you should forget about as often as possible. Especially when out shooting.

Use it to pick the right lens for what you’re going to shoot, then concentrate fully on the creative side of things. Not the maths.

Personally, I take pictures to suit the focal length I have at my disposal, without even having the words ‘focal length’ enter my head.

I adapt to what I have and if this is a prime lens then we’re essentially trying to make the best of the restrictions we have given ourselves.

Photography should be fun. Forget the numbers once you’re out there. Go enjoy it, and don’t come back without some great photos – whatever focal length it is you’re using.

I’d love to know what you think about focal length and about this article.

Did it live up to the promise of focal length explained simply? Did you learn anything from it? Do you worry about focal length when you’re out shooting? Do you have anything to add that I missed?

Share in the comments below or hit me up on Twitter!

Punch and Judy photo used under Creative Commons from Marc Roberts

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