The Basics of Film Camera Lenses Explained

Filmmaking 19/07/2019 8 min read

Today, we’re going to be looking at something that every film creator uses, but many have no idea how they work! We’ll be covering the basics of lenses for filmmakers. 

Most of us know lenses simply as that expensive thing in front of your (also very expensive) camera. But what do they do, why are there so many different options, and more importantly, which lens is right for you? There are a lot of questions, but we’ll cover some of the basics here today. Take this as simply an introduction, not a comprehensive resource on lenses, so it’s perfect if you want to start learning about them! 

What is a Lens and Why Do You Need One? 

If you were to take the lens off the camera, do you know what happens? You get a blank, white nothingness. Light is hitting your camera’s sensor, but there’s really nothing else going on. 

The job of the lens is to focus the light, so that when it hits the sensor of your camera, there’s a discernible image. This is exactly what the lens of your eye does — focuses light, which hits the retina in the back of your eyeball, so that you can make out the shape of the expensive camera you just bought. 

So, why won’t just any old lens work for the scene you’re shooting? Why are there so many options, and what’s the difference between them all? We’re going to cover three main things that are important when it comes to a lens’s functions:  

  1. Focal length 
  2. Aperture 
  3. Zoom vs. Prime lenses 

1. Focal Length

Focal length, by definition, refers to the distance between the center of the lens and its focus.  But for our purposes, it refers to how wide or how zoomed-in it makes your image look. A wider lens, like 16mm, will show a very wide amount of area for you to look at. 

16mm Focal Length

Alternatively, a 200mm lens positioned in the same place will see way less of the scenery and make your subject appear much bigger in the frame by comparison. 

200mm Focal Length

There are a crazy number of different focal lengths that you have at your disposal, and they each give slightly different viewing characteristics. In addition to being wider or more zoomed-in, there are also a few other characteristics about focal length that you should be aware of. 

The Wider Your Lens, The More Distorted Your Image

If your subject is too close to your wide-angle lens, you can actually start to see it make a real mess of their face, making it look rounder and more alien than it would in the case of a 200mm lens. The difference becomes very apparent and shows just how the focal length actually changes the way your subject’s face looks structurally. 

The reason it changes the way the face looks is because of how each lens displays the distance between objects at different lengths. We notice it right off the bat with faces because we’re so used to looking at them in everyday life! 

Distance Appears Differently 

When we take an object in the distance filmed on a 24mm lens, and then change the lens out for a 200mm lens, it now looks like the object is much closer. This is because wider lenses make things look farther apart, whereas longer lenses compress the distance between objects, making them look closer together. 

So what does that mean for you? How does that influence what lens you choose? 

Well, it means that if you want to want to really exaggerate how far away something is, use a wide-angle lens. But the opposite is true too — if you want to cheat and make it look like something is closer than it really is, then a longer lens is the way to go. This is really useful for showing stunts where you want to keep your actors safe. That way, you can make it look like your subject is really close to a dangerous situation when they’re really a safe distance away! 

But unless you’re shooting something incredibly stylistic, you probably won’t be shooting all the time at these two extremes. All throughout the middle of the scale, in between these ranges, is a variety of focal lengths that can serve your purposes for different occasions. Keep in mind the characteristics that you saw at each of the extremes, and as you leave those extremes and go closer to the middle zone, you’ll see those effects diminish, all the way up until you reach about 50mm.  

Why are there no effects at 50mm? It’s less about the lens and more about your own human eye. It’s been said that shooting through a 50mm lens gives about the same experience to viewing with a human eye! 

2. Aperture

When you hear people talking about lenses, you probably hear them talking a bunch of numbers. If they’re talking 24mm, 35mm, 50mm, 200mm, you probably understand those a bit better now in terms of focal length.

But there are other numbers that people refer to often when they talk about lenses. Those are usually known as either a T-stop number or an F-stop number, but both of those have to do with the lens aperture. 

T-stop and F-stop are different terms and they mean slightly different things, but for our purposes, we’re going to use the term F-stop. The difference isn’t necessary to know to understand how aperture works! 

Basically, the F-stop refers to the diameter of the opening of the lens. This number can change because lenses have aperture blades that can artificially make the hole larger or smaller. If the lens itself is marketed with a particular F-stop number, that means that’s the widest open the lens is capable of being. 

When you’re actually shooting with it, you might see an F-stop number on your camera display, which will refer to how wide open the lens is at that particular time. This can be a little confusing because it seems counter-intuitive: the lower the number, the wider the opening. The higher the F-stop number, the smaller the opening. 

Why does that matter? Well, just like with focal length, changes to your aperture parameter will actually drastically change the look of your footage. When you raise and lower an F-stop number, there are a few different things that happen. 

Your Exposure Will Change 

If you close it down to a larger F-stop number (making the hole smaller) you’ll probably be able to understand that this means less light can get through the smaller hole. Therefore, your image is going to be darker, or underexposed. Conversely, if you open up your lens wider (to a lower F-stop number) your image gets brighter. 

Your Depth of Field Will Change 

When you change your F-stop to a larger number and make the hole smaller, more and more of your image is in focus. More specifically, it gives you a wider depth of field. This wider depth of field will allow you to have multiple objects that are further apart relatively in focus at the same time. This is great if you want to capture an image where you can distinguish multiple different elements of the scene at different depths. 

The opposite will happen if you open up your aperture. The smaller that number gets, the narrower your focal range until you get a very fine, razor-thin area that’s able to be in focus at any given time. Having an out-of-focus background and a very in-focus subject is a popular film technique. The more out-of-focus the background, the more your subject can pop out by comparison. 

Learn more about how aperture affects depth of field here.

Shopping for F-Stop 

Typically, a rule of thumb is that the lower the possible F-stop on the lens, the more expensive the lens is. That’s because as engineers create a lens with a wider aperture able to produce a shallower depth of field, it gets exponentially harder to improve upon that as they go! That’s why a Canon “Nifty Fifty” 50mm, f1.8 is priced at $100, while their f1.4 model is $350. The Canon 50mm f1.2 model is $1300-2000! Yikes. There are, of course, other factors that come into play, but that gives you a bit of a general idea. 

Of course, getting to that incredibly slick f1.2 shallow depth of field doesn’t automatically make your images better. Shallower depth of field can actually come with some tough challenges, such as: 

  • Keeping focus on your subject if they’re moving. 
  • Making sure your scene doesn’t blow out (exposure-wise) if you open your lens as wide as it goes. 

In the end, it’s more about knowing how aperture works so that you’ll be able to utilize it properly when you’re actually filming for your next project! 

3. Prime Lenses vs Zoom Lenses

A prime lens just refers to a lens that’s fixed at only one focal length that you can’t zoom in and out with. A few common focal length examples are 24mm, 35mm, 50mm, 85mm, and 135mm. 

On the other side, a zoom lens simply refers to a lens that is able to change between a range of different focal lengths within the same lens body. You’re able to choose any specific focal length in between the maximum and minimum range of the lens’ capabilities. These lenses are usually labeled with a range instead of a single number, such as 50-200mm. 

Which Lens is Better? 

If it’s really useful to have a lot of focal lengths at your disposal, and to have your lens open up super wide, why wouldn’t you just always have a zoom lens? Perhaps one that goes from 30mm all the way up to 300mm, that can have a really low F-stop at each of those focal lengths? 

Well, it’s borderline impossible to have such a comprehensive lens. The few lenses that actually are good enough to pull that off have a huge price tag attached to them. It’s a feat of engineering that’s insanely hard to pull off, so your chances of finding something that can do everything is practically zero! 

A lens like the Canon 70-200mm at f2.8 will cost you around $2000-3000, depending on which model and where you get it from. Even then, you’re not getting the ability to see any sort of medium to wide-angle. If you’re not made of money, prime lenses are probably something you should look at. 

What’s the biggest benefit of a prime lens? 

  • High-quality lens 
  • Low F-stop 
  • Lower price 

On top of that, when it comes to filming a movie, zooming with the lens is just not often done. More often, the zoom comes from physically pushing closer with the camera itself, or is simply done during the editing process. 

The Verdict

The truth is that you’re not going to be doing a lot of zooming in your filming career. A cheaper, higher quality option is to actually use prime lenses and whenever you need to frame your subject differently, either move the camera or change the lens to a different focal length. 

For some of you, zoom lenses might be essential to what you do. For example, if you’re a wedding filmmaker and you need to be able to adjust your framing during the ceremony in an instant. You don’t have the luxury of taking the time to be switching out your lens. Nothing wrong with that! 

We can’t say that one kind of lens is better than the other, but it’s necessary to know that they’re different, they’re both available, and they each have their different pros and cons. The goal is to get you understanding even more about how lenses work and what some of the benefits of each are so that you’re more prepared as you move along your path of filmmaking! 

Hopefully, this overview has helped you understand the basics of lenses. You should have a better idea by now of the ins and outs of lenses for filmmakers, what you should look at when buying one, and what the most essential functions of a lens are. 

If you can understand focal length and aperture, and learn how to use these to improve the quality of your filmmaking, you’ll be well on your way to looking like a pro in no time!