Understanding Focal Length and how it can Improve Your Work

You’ve got a shiny set of prime lenses or some of these massive zooms. Bravo! Now, let’s set these to work to your advantage. Let’s take a look at how to use focal lengths properly and then dive into the impact they can create for your audience. Understanding the concept of focal length in more detail than just knowing the numbers is essential when it comes to mastering the craft of filmmaking.

Introduction to focal length

The focal length of a given lens dictates what you’ll get from it in terms of the visual impact of the resulting imagery. Therefore, the choice should be well-founded. This post will not focus too much on giving a technical explanation of what is going on inside a lens, but about the impact that certain focal lengths will have on your audience and what filmmakers should keep in mind so that they can choose the “correct” focal length for their shots.

 

understanding focal length

A set of prime lenses. Samyang Cine DS 14mm, 24mm, 35mm, 50mm, 85mm, 135mm.

What Focal Length does to the Image

It’s really important to understand that different focal lengths will have very different effects on the overall aesthetics of captured images. As a simple rule of thumb, keep in mind that three effects are commonly considered most important:

  • Depth of field
  • Compression of space
  • Three-dimensional feel of the resulting two-dimensional image

When opting for a longer lens, an 85mm or 135mm for example, the depth of field will decrease—the focal plane that objects will appear in focus narrows. Of course, there’s another factor which affects the depth of field: a wide open iris will decrease the depth of field, whilst a nearly closed iris will increase it.

A longer lens will also compress the space. That is to say that objects in the background will appear much bigger and closer than they actually are.

This compression of space caused by a longer lens will also affect the three-dimensional feel. The resulting image will look somewhat flat. As you watch a two-dimensional screen, it is important to understand the impact that your choice of lens will have in terms of creating a three-dimensional look and feel to your footage.

focal length variance

Different focal lengths and their impact on the resulting image.

The examples above show the exact same framing (well, almost) on the foreground object—in this case, a lovely plushie—yet all six images differ greatly in terms of background compression, depth of field, and even distortion. For this series of stills, I moved the camera in with every lens used; the position of the plushie and the distance from the background remained the same throughout.

Using the 135mm lens, the sensor plane of the camera was 155cm (61 inches) away from the object. When I put on the 14mm lens, the sensor plane was a mere 23cm (9 inches) from it.

The dartboard in the background looks fairly massive when shot with a 135mm lens, making the resulting image look very two-dimensional—as if the plushie is located immediately in front of the board. Meanwhile, in the 14mm wide angle lens image, you can’t even see the dartboard—it is completely covered by the plushie (which looks slightly distorted itself). The background in general, however, looks further away and adds a feeling of more space to the scene.

understanding focal length

The same series of images but this time without moving in of the camera

To further clarify the impact of focal length, the camera doesn’t move as the lenses are changed in the above examples. The effect that lens choice has on depth of field is clearly visible—while the background blurs away on the 135mm, the texture of the wall and the dartboard are almost in focus on the 35mm image.

A Point to Note

All of the images above were created using Samyang Cine DS glass. While that is not a bad thing, it certainly isn’t the benchmark either. You can quite clearly see the vignetting on the 14mm. Different lens manufacturers have different approaches and ideas on how to make decent lenses. They are shipped at different price points, produce different quality images, and often bring their own unique look to resulting footage.

I guess that what I am trying to say is that there’s a fourth point to add to this article: look. In the end, though, every choice made in filmmaking comes down to budget, taste, or a combination of the two.

Focal Length Conclusion

So, there you have it! There is a very good reason for the existence of the many focal lengths out there; use them wisely. Throwing on a 35mm lens and moving the camera closer to your subject is not the same as using your 85mm lens, which is unfortunate if you’ve left your 85mm at home!

There should always be a logical reason behind your lens choice for each particular shot. By being aware of the effects that your lens will have on the resulting imagery, you can set yourself up for success with the knowledge of which focal length will compliment your creative work at any given moment.

 
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Steve K. Bruno Reply
Steve K. Bruno March 19, 2016

Köstliches Video dazu!
https://www.youtube.com/watch?v=FJAb5gmlQs0

Olaf von Voss Reply
Olaf von Voss March 19, 2016

kind of old school but fun to watch, thanks!

Patrick Murray Reply
Patrick Murray March 19, 2016

Is that a blue bias on the 50mm?

Olaf von Voss Reply
Olaf von Voss March 20, 2016

not really, the problem was that the weather changed real quick from overcast to sun and back again. I processed each image the same. T-Stop was 3.1 since the 14mm is only T3.1, white balance was 6300K.
You’ll get some CA and color shifts with these lenses when shooting wide open, though.

A good source for comparing lenses is this site: http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx
(only photo lenses, though)

David Rabeder Reply
David Rabeder March 19, 2016

Bilduntetschrift: 14, 24 —25?—- 50, when opting for a longer lens […] the depth of field will in-crease (?)

Reply
Steve Crow March 19, 2016

I think what they are trying to say is MORE shallow but it is awkward and confusing language which I would suggest changing becausexas written its backwards

Olaf von Voss Reply
Olaf von Voss March 19, 2016

thanks for pointing out. That was a typo, indeed.

Reply
Steve Crow March 19, 2016

No worries!

Reply
Johann Hütter March 19, 2016

it’s also interesting, how faces change with focal length:
https://media.giphy.com/media/l2JJu55Y2LSvkbBqo/giphy.gif

 William Alexander Reply
William Alexander March 20, 2016

Thanks for posting this link. Very cool!

Olaf von Voss Reply
Olaf von Voss March 20, 2016

sweet, thank you. Here’s another one, even more extreme with shorter focal lengths.
http://www.lesjones.com/www/images/posts/stepheneastwood-tile1.jpg

Joachim Richter Reply
Joachim Richter March 20, 2016

Focal length determines the relationship between foreground and background.

Ramon Solorio Reply
Ramon Solorio March 20, 2016

How about a case for that set? Any recommendations?

Olaf von Voss Reply
Olaf von Voss March 20, 2016

I’ve added some product links to the article. When you order at cvp, you’ll get a pretty decent pelicase style hard case which holds up to 6 lenses – the kit shown above plus the 135mm for example.
EDIT: actually, it’s not the case that I meant there.. But they should have some decent hard cases for that purpose. And I definitely would recommend to use a proper hard case for your investment..

Ramon Solorio Reply
Ramon Solorio March 20, 2016

Hmm, I have the lenses, searching for a case now.

 Zografakis Dimitris Reply
Zografakis Dimitris March 21, 2016

great post !

Reply
William Sommerwerck March 23, 2016

“A longer lens will also compress the space. That is to say that objects in the background will appear much bigger and closer than they actually are.”

Wrong, wrong, wrong, wrong, wrong, wrong, wrong. A lens’s focal length has nothing whatever to do with the apparent compression or expansion of space. This is controlled solely by the distance of the camera from the subject.

A 20mm lens N feet from the subject produces exactly the same perspective as a 200mm lens does. The relative sizes of the objects in the image (which is what “perspective” is) do not in any way change.

Olaf von Voss Reply
Olaf von Voss March 23, 2016

William, thanks for pointing out!
You’re totally right from a technical point of view but in reality your camera will be closer to the subject when using a shorter lens and further away when using a longer one most likely.

Sorry for being inaccurate there.

 Chris Orsi Reply
Chris Orsi March 27, 2016

As someone who is brand new to indie filmmaking, what is the general consensus when choosing a lens for medium or closeup framing? Apart from room space that is. Would you opt for an 85mm for an extreme closeup, or a 50mm and just move closer to the subject? What tends to be more flattering to the face? Just wondering the “go to” focal lengths are for modern dramas and such.

Reply
William Sommerwerck March 27, 2016

The closer you are to a face — regardless of focal length — the more unflattering the result (particularly with respect to the nose).

For a 35mm full-frame camera, 85mm is generally considered the shortest focal length suitable for close portraits.