Written by Snehal Patel
As a filmmaker, there is a lot to learn. It takes a lifetime of experience and education to continue creating compelling content. One of the most complicated topics to discuss about filmmaking is the photography. Cinematographers, who are in charge of delivering the photography, are always some mix of artist and technician. There are many topics to keep track of and lots of variables to control, which have to be done with good taste and deliver the artistic intent of the story.
One of the variables that Cinematographers control is lens choice. A subset of lens choice is determining which focal lengths are needed for a project. Prime lenses always come in sets of 3 to 15 focal lengths, or more. So, let’s “focus” on prime lenses and focal length choice by answering some basic questions.
What are focal lengths?
Focal lengths, marked in millimeters (mm) are best categorized as magnification choices. Wider focal lengths like 12mm, 15mm, etc. have less magnification, and longer (or more telephoto) focal lengths like 135mm, 150mm, 200mm, etc., have more magnification. Each focal length in between has its own flavor.
Watch this YouTube video by ZEISS Cinematography which shows you how focal length magnification is designed to work.
In this example, the camera position is fixed, and each subsequently longer focal length of Supreme Prime lens is bringing distant objects closer by magnifying them. Notice how the women standing by the car never distorts. Her features remain the same, no matter how close you magnify, because the camera is not being moved. The things that do change are the depth of field and the compression between background and foreground.
Now, in this next video example, the camera position is changing between each subsequent focal length change.
Starting from a wide 18mm up close to the actress, we back off the camera position to move it further away from her as the focal lengths increase. This keeps her head the same size from top to bottom but definitely distorts her face. Since we are using a full frame sensor, the normal lens is going to be the 50mm at 7-12 feet distance. As you can see, she looks pretty realistic when the 50mm lens is used. That’s probably how she looks in real life. All the other focal lengths manipulate her features. She’s rounder-looking and more distorted when there is a wide-angle lens up close to her. She has flatter facial feathers in the longer focal lengths. And her face changes shape a bit with each focal length choice. Notice what happens to the foreground and background objects as well. Using the 18mm or 21mm lens, would make the garage look quite large and spacious, with plenty of room between vehicles. Using a long telephoto lens compresses the foreground and background, making the space feel crowded and smaller.
Which focal length should I use?
This is where the artistic choices come into play. Cinematographers that study and use a lot of lenses will start to form opinions as to how focal lengths should be used to further the story. Different situations call for different focal lengths because placement of the camera, framing, and magnification all work hand in hand.
Let’s take the example of a common scene – the interrogation of a suspect by a uniformed police officer. The room is small, with a table and one chair on either side and a bright spotlight from above. How can we show that the suspect is really a criminal who is in over her head? Let’s start by choosing a higher angle looking slightly down so that our perspective is from above her. Then let’s move in the camera up close to the actress and use a wide-angle lens. This will distort her face and make it look bulbous. This choice of magnification, along with a little makeup magic, we can make it look like she is really sweating under the pressure of the moment. Plus, her background will look imposing as you will see a lot of the room with the wide-angle lens, which can be bathed in darkness. Kind of like the darkness imposing on her.
Now let’s switch camera angles to be on the officer. If we want him to look large and in charge, place the camera a bit below the eyeline, back up the camera position and use a longer focal length of lens for a medium close up. Maybe a 50mm or 85mm depending on the sensor size of the camera. This way, the officer looks very normal and realistic in terms of their facial features. There will be no distortion in the image, so their uniform will look perfect and straight. The lower angle of the camera will make him look taller and imposing, in contrast to the angle on the criminal. The depth of field will be pleasing and hide the background. Plus, the compression of the frame will eliminate the imposing darkness we created above.
This is just one example of how someone can make decisions about focal length, camera placement and framing. You can now think about what you would do for each scene in your film.
Do focal lengths change when using different sensor sizes?
No, they remain the same. A 50mm magnification, is a 50mm magnification, is a 50mm magnification. Magnifications don’t change, but what changes is how much of the projected image is being captured. This is because all lenses project a round image onto the film plane, and different sensors see different rectangle crops of this projection.
See this image (pictured above) of a theoretical image circle from a CP.3 lens. Notice the soft white circle which represents the kind of circular image that comes out the back of the lens. Notice that different sensor, of various sizes, will see different rectangular cutouts of the circle. So, the difference between a Full Frame sensor and Super35 sensor is that the Super35 sensor is smaller and will see a smaller crop of the image circle. This does not change the magnification of the lens itself in any way. It just means that you will make different magnification choices to suit your story, based on the camera system used. Most cinematographers, for example, will end up using a longer focal length of lens on a full frame sensor to get the same “angle of view” as they would get on a smaller sensor.
See this last video example of how we recreate the same “angle of view” used in S35 with a full frame sensor and Supreme Prime lenses.
Notice that when matching angle of view, you will get less depth of field because you have to use a longer focal length of lens on the larger sensor to get the same framing as before. So, it’s not the sensor size that is determining the shallow depth of field, but the magnification choice itself.
Hope this helps!
Snehal Patel is a film and television professional with over two decades of experience creating content and adapting new technology. He started the first Canon Bootcamp in Los Angeles during the Canon 5D DSLR craze and has over twenty years of experience in cinema. Snehal has lived and worked in Chicago, Mumbai and Los Angeles as a freelance Producer & Director. He was a camera technical salesperson at ARRI, and currently is the Head of Cinema Sales at ZEISS. He represents the Americas for ZEISS and is proud to call Hollywood his home.