Understand Focal Length in 5 Minutes

In Photo Basics by Jim Harmer

Focal length is something that we talk about constantly as we discuss different lenses and styles of photography in our weekly free podcasts.

It can be a bit confusing as a beginner to understand focal length because there are a few twists and complexities, but I'll do my best to explain it in 5 minutes or less.  I'll start with the most basic information and then build up to the more advanced stuff.  Be sure to read the entire article because there are some cool examples down lower that will help you get the right focal length for when you're taking pictures of people.

The focal length scale shows that this lens can go as wide as 10mm and as long as 24mm (zoomed in). Right now, it is set at 18mm.

The focal length scale shows that this lens can go as wide as 10mm and as long as 24mm (zoomed in). Right now, it is set at 18mm.  Almost all lenses have a scale like this on them.

What is focal length?

In short, the focal length of the lens is the measure of how “zoomed in” your lens is.  Much like looking through binoculars, you may be at 40 mm and be able to see an entire mountain, or zoom in to 400mm and only see one tree on the mountain.

The focal length measurement tells the photographer what the angle of view will be.  The angle of view means how wide of an area is visible in the picture.  It also conveys the magnification of far-away objects in the photo.

If you shoot at 20mm and the person you're photographing is 30 meters away, the person will be small (magnification) and you will see a lot of area around the person (field of view).   However, if you zoom in to 300mm, the person will be large in the picture (magnification) and there won't be much scenery on the sides of the model showing in the frame (field of view).

Focal length is measured in millimeters, but the measurement is not of the actual physical length of the lens, but rather the magnification properties of the lens.

The actual measurement of focal length is the distance, in millimeters, between the convergence point and the imaging sensor.  The convergence point is the point at which the light rays combine in the lens.  No worry, though, you'll never need to know that again.  There's no pop quiz at the end.

Knowing the Focal Length of Your Lens

All lenses show the focal length right on the lens.  First of all, you'll see the range of the focal length that the lens can achieve in the name of the lens.  If your camera came with an 18-55mm f/3.5-5.6 lens, then you know that the widest your lens can go is 18mm and the most zoomed in your lens can go is 55mm.

As you turn the barrel of your lens to zoom in, you can look at the scale on the end of the lens that connects to the camera to see what specific focal length you are shooting at.

After you've taken a picture, almost all cameras will save the focal length information in the metadata of a picture.  So if you want to go back and see what focal length you used to get a certain look, then you can go into the properties of the photo and see that.

Keep in mind that not all lenses can zoom (change the focal length).  Some lenses are prime, which means they cannot zoom in.  A common lens that cannot zoom (one which you likely own) is a 50mm f/1.8 lens.  It's a fantastic lens, but it cannot zoom in or out.  It's a fixed focal length lens.

full frame vs. crop sensor comparison photo

The inner rectangle is the photo taken with a Nikon crop sensor DSLR, the full outer picture is taken at 18mm as well, but with a full frame camera.  This shows that, AT THE SAME FOCAL LENGTH, a full frame camera is much wider.

Focal Length and Crop Factor

If you shoot a crop sensor camera (Nikon D3300, D5500, D7200, or Canon Rebel, 70D, 7D, Fuji XT1, or Sony A6000 just to name a few), then your camera's crop factor will make your camera more zoomed in when compared to full frame cameras at the same focal length.

So suppose I take a picture of a building on a crop sensor camera at 18mm.  If I put the same lens on my full frame camera and shoot at 18mm, the full frame picture will be much wider.  However, I could easily get a 13mm lens and put it on my crop sensor camera to match the exact same field of view as the full frame camera.

The point is that full frame cameras are not capable of shooting any wider or more telephoto than a crop sensor camera.  The difference just means that if the same focal length is selected on the lens, the full frame camera will produce a wider shot.

Please understand that full frame cameras are neither superior nor inferior to crop sensor cameras.  They both have benefits and drawbacks.  I used to shoot a full frame Nikon camera, but ended up switching to a crop sensor Fuji XT1 camera which I'm really enjoying right now.  Don't let anyone tell you that a full frame camera is better.  It's simply different.

I also want to make sure it's clear that you can achieve the same wide angle of view on a crop sensor camera as on a full frame camera just by using a wider lens, so there is no advantage to a full frame camera for landscape photography.  And on the long end, it can be handy to have a crop sensor camera because it makes a 400mm lens turn into a 640mm lens without paying thousands and thousands of dollars on a lens that long.

I wrote an entire article about how the crop factor affects field of view, which you can read if this isn't quite sinking in yet.

In this example, I first stood VERY close to the model to shoot at 10mm, then stepped back and zoomed in for each consecutive shot. This keeps the size of her face the same in all the shots, but as you can see, the photo looks COMPLETELY different in each shot.

In this example with my lovely wife, I first stood VERY close to her and shoot at 10mm, then stepped back and zoomed in for each consecutive shot. This keeps the size of her face the same in all the shots, but as you can see, the photo looks COMPLETELY different in each shot.  NOBODY is going to look good in a photo if you shoot them with a wide angle lens from up close.  Scoot back and zoom in when shooting portraits!

Using a Proper Focal Length for Portrait Photography

Wider lenses show more distortion (unnatural bending of objects in a picture–especially around the edges of the frame).  Also, the wider field of view of a wide lens (10 – 18mm) will make objects which are close to the camera seem much larger, and objects further from the camera much smaller.

You need to understand that point to select a proper focal length for portrait photography.  In the (horrible quality) animated gif above, I took four pictures of my wife from different focal lengths.  After each shot, I STEPPED BACK and zoomed in.  By doing this, her face remains the same size in the picture, but as you can see, the photo looks COMPLETELY different!

When on the wide end, you see much more of the backyard around her.  You see the side of the house, all of the fence, and you can't even see the trampoline because she's covering it.  However, as you scoot back, you see much less of the yard (narrower field of view).  Also, notice that the face looks badly distorted when shooting with a wide angle lens up close, but as you scoot back and zoom in, everything looks normal and proportional.

One last thing to notice, and this one is a bit more advanced, is that the background appears blurrier and blurrier as the focal length increases, despite the camera settings staying the same for all of the shots.  To learn more about that, read my article on the many things that impact depth of field.

Choosing the Correct Focal Length for Different Situations

There is no “right” or “wrong” focal length for any given situation.  It's all a matter of personal preference and what you want to capture.  However, there are certainly norms of the focal lengths typically used in different situations.

All of the focal lengths below are the common focal lengths for crop sensor cameras, since the vast majority of the people reading this article will be shooting crop sensor cameras.  These are rough numbers, intended to give you an idea of what focal length you'd want).

  • Landscape photography (10mm to 18mm)
  • Full body portrait of a person (24 to 45mm)
  • Headshot (55 to 140mm)
  • Night photography (10 to 18mm)
  • Close-up photography or macro photography (70 to 150mm)
  • Wildlife photography (200mm to 850mm)
  • General lens for shooting family and kids (35 to 90mm)
  • Outdoor sports photography (200 to 400mm)

Now that you've read through this handy cheat sheet of focal lengths for different situations, you should really consider checking out my free lens finder.  It asks you 5 questions about what camera you use and what you want to take pictures of and your budget, and it gives you my personal recommendation of the perfect lens for you.  Find your perfect DSLR lens here.

Notice the HUGE difference between 10 and 15mm, but the tiny difference between 135 and 140mm. As your focal length gets longer, the difference in field of view is less of a change.

Notice the HUGE difference between 10 and 15mm, but the tiny difference between 135 and 140mm. In fact, the 135 and 140 marks are so close together that you can't even tell them apart on this web-sized version.  As your focal length gets longer, the difference in field of view is less of a change.  That's the front of my house, by the way.

How Differing Focal Lengths Affect Field of View

There is one last thing you need to understand about focal length in order to have the basics under your belt.  Here it is: As focal length increases, the change of the field of view decreases.  As focal length reduces, the change in the field of view increases.  Or in other words, there is an inverse exponential relationship between focal length and the change in field of view.  Clear as mud?

All this means is that on the wide end of your lens, zooming in 5mm will dramatically impact the field of view (how much scene you can fit into the frame).  However, when you're zoomed in tight, zooming in 5mm will only make a minute difference in how much scene is in the picture.

If you understand this principle, it can save you a lot of money on lenses!  When I suggest to newer photographers that they buy a wide-angle landscape photography lens, they sometimes say something like, “Why would I spend $500 on a 10mm lens when my camera already came with an 18mm lens?”  Aside from the optical quality differences, the difference between 10 and 18mm is HUGE and will make a significant difference in how much of a landscape fits into the frame.

However, if you are interested in shooting sports photography and you already have a 250mm lens on a crop sensor camera, it would be mostly worthless to spend $500 on a 300mm lens (assuming the optical quality is the same) because it will barely make a difference in the magnification and field of view.

Understanding “Focal Length Jargon”

As you listen to the Improve Photography podcasts each week or watch our Youtube videos, you'll often hear us discussing different focal lengths for different purposes.  In this article, all the examples of focal lengths I've used have been with crop sensor cameras, because that's what most of you will be using on your cameras.

However, most photographers stick to the standard of the 35mm imaging sensor (full frame).  So if you hear a photographer recommend shooting at 200mm, they probably mean 200mm on a full frame camera.  35mm has long been the standard.  The good news is that you can use some very simple 3rd grade math to know exactly the focal length equivalent on your camera.

Nikon, Fuji, and Sony crop sensor cameras have a 1.5x crop factor.  Canon crop sensor cameras have a 1.6x crop factor.  So if someone recommends a 200mm focal length, you can rightfully ask whether they mean full frame or crop sensor.  However, the focal lengths photographers talk about are usually in full frame.  So you can do some simple math to know what focal length you should use to achieve the same field of view as 200mm on a full frame camera.

200mm on a full frame camera is the same thing as about 135mm on a crop sensor Nikon camera, because the Nikon has a crop factor of 1.5x.  On a Canon crop sensor camera, however, it would be 125mm.

If you're still learning the fundamentals of photography, I really highly recommend you take a minute and read through my photo basics series.  It's a collection of 8 posts I wrote for newer photographers.  It'll walk you through getting a good exposure, how to set your camera up to get crisp sharpness, composition, and more.  Read the photo basics series here.

About the Author

Jim Harmer

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Jim Harmer is the founder of Improve Photography, and host of the popular Improve Photography Podcast. More than a million photographers follow him on social media, and he has been listed at #35 in rankings of the most popular photographers in the world. He blogs about how to start an internet business on IncomeSchool.com..