Distortion Grids 101: A Complete Guide for VFX Video Production

Why is some VFX work impressive and some is not? Why do some VFX look good? Why do others look “fake”? 

In my opinion, the answer is obvious: the secret of high quality visual effects is careful preparation of the material during the shooting phase and even before the start of shooting.

A VFX supervisor begins his work before (or during) production in order to facilitate the work of post-production and video editing: 

• Determine the list of cameras and lenses;
• Refining the technical parameters of the files;
• Shoots chrome, grey balls and colour checkers in each location.
• Distortion grid technical shots for each lens.

In this article, I’ll go into detail about the definition of lens distortion, its importance in VFX production, and how and why to shoot distortion grid on set. But first, let’s start with a different term.

What is lens distortion?

Here’s what Wikipedia says:

Lens distortion is a distortion of an image that is caused by the design of the lens used. It can appear as distorted lines, changes in the shape of objects, and darkening or vignetting at frame edges.

My explanation for lens distortion is this: Lens distortion is a phenomenon where straight lines appear curved. Objects appear stretched or shrunk, and the overall image appears deformed. This happens because the shape of the lens elements causes light rays passing through the lens to bend slightly.

There are two main types of distortion that occur: 

• Barrel distortion: when straight lines appear to be curved like an arc.
• Pincushion distortion – when straight lines appear to be curved inwards like a pillow.

Barrel distortion

Pincushion distortion

Fun fact: my very first Photoshop job was the correction of lens distortion in a photo (Photoshop 2.0).

Distortion affecting factors

• Focal length: Wide-angle lenses tend to have barrel distortion. This is where straight lines curve outwards. Telephoto lenses may have cushion distortion. This is where the lines curve inwards. 
• Aperture: With a wide aperture, distortion is often more noticeable.
• Distance to subject: When the subject is close to the camera, distortion tends to be more noticeable. 
• Lens design: Different lenses are designed to produce different amounts of distortion.

When shooting commercial videos, several lenses are usually used to convey the atmosphere of the shot and the director’s intention. Lenses can differ not only in focal length, but also in aspect ratio.

They have become increasingly popular with the advent of affordable anamorphic lenses such as SIRUI 35mm F1.8 1.33x M4/3 Anamorphic Lens. Filmmakers appreciate them for their cinematic wide-angle effect. This is achieved by horizontally compressing the image. Such lenses have an image aspect ratio of 1:2 (for every pixel in the vertical direction, there are two pixels in the horizontal direction). You need to know the distortion of such an image to track it properly. I have already explained what is tracking in my glossary of terms. 

In addition, each lens, especially anamorphic lenses, has its own distortion and breathing characteristics. Several lenses are often used on set to create different emotions and atmospheres in a piece of work. This may not be noticeable to the average viewer, but images shot with different lenses can bend, warp, compress and expand, which presents great challenges for visual effects artist when video editing and integrating CG elements.

How to use image warping in post-production

Take a look at the image in the header of the website. This shot was taken with a wide-angle drone lens and has spherical refraction from the centre to the edges of the frame. That’s the way all optics work, and it’s beautiful: the wider the angle of view of the lens, the more distortion there is at the edges of the image.

Now let’s imagine that we have the task of inserting several 3D billboards on the facades of buildings into this frame during video editing.

You don’t need to be a VFX expert to notice that this 3D graphic is strictly rectangular and has no curvature. However, the buildings on which the billboards are to be ‘erected’ are curved and distorted. Even though we know that in real life they are strictly rectangular.

To integrate 3D graphics into this image, we need to know the distortion of the lens used to capture the image. We then apply this distortion to the 3D graphics in the output.

There are two methods of obtaining lens distortion parameters: approximate (manual) and using a distortion grid.

In the first case, you “show” the program all the apparently straight lines with your hands, and the program calculates the distortion coefficient itself. This method is quick and easy, but not always accurate.

And it is quite obvious that it is not universal at all. Your image may not contain any obvious straight lines, or they may be blurred. So how do you get the distortion factor? You need a frame with a distortion grid for your lens.

What is a distortion grid?

So, here’s what I found on the internet for the question “What is a distortion grid?”

A distortion grid is a special test pattern used to measure and correct distortion.

I think the Distortion Grid is primarily a VFX supervisor’s tool. It helps to place images in post-production, to track the frame and to do matchmove for further VFX integration.

It’s essentially a chessboard. It has a clear centre and strictly straight lines. Using a distortion grid provides anchor points for analysing how straight lines are distorted or objects are deformed by curvature when the camera and lens are combined.

Types of distortion grids

In general, all distortion grids look almost the same, except for a few details. Below I’ll introduce the most popular ones, and I’ll also attach PDF files so you can download them.

Base lens distortion grid

Base lens distortion grid with OX

The history of the VFX industry

I don’t want to go too deep into the history of VFX, but in the chronology of the article I should mention the emergence of the VFX industry. So I’ll just give a few examples of the first VFX from very old films. These are works from the time when computer graphics were just beginning to appear in films. Unfortunately, I can’t find any definitive proof, but these three films are considered to be among the first to use 3D VFX graphics:

• “Star Wars (1977): used computer graphics to create the Death Star and other spaceships. The Making of Star Wars – Pioneering Special Effects, VFX and Sound Design.

• “Alien (1979): used computer graphics and animatronics to create the Xenomorph.

• “Tron (1982): used computer graphics to create a virtual world.

Since then, the VFX industry has gained momentum, and with each project, the visual graphics have become more complex. Today, VFX and post-production are evolving at a rapid pace, thanks to healthy competition among the players. Even today, the threshold for entering the profession is much lower than it was 10 years ago.

Where to get distortion grids for shooting 

I won’t explain how to create a Distortion Grid in this article. However, if I see a lot of interest in this article, I will write a separate longread on how to make your own distortion grid. I will tell you in detail about the choice of materials, nuances and tricks. I have also prepared the files in PDF format and added them above.

You can always rent a distortion grid for your project from a local rental shop, along with all the equipment you need for the shoot. You may even be offered it along with a VFX supervisor.

Personally, I’ve printed my own.

However, I’ll give you the main characteristics if you decide to print the distortion grid yourself:

• The geometry of the cells. They should be flat and parallel. This is important for accurate distortion measurements.
• The material. It should be hard and matt to avoid glitter and wrinkles. Use a rigid base to avoid bending.
• Print quality. Each cell should be clear and the colours should be contrasting.

These three rules will help you choose a printer and get the best results when creating a distortion grid.

How to use Distortion Grid correctly

So let’s talk about how to use distortion raster correctly, how to remove lens distortion correctly, and how to capture distortion raster on location. 

Different types of lenses distort images in different ways. No joke, even lenses of the same model from the same manufacturer can have different levels of distortion.

Mounting the Distortion Grid

Use the Grip-Head or Molly to attach the Distortion Grid to the C-stand. Avoid gripping the outer edges of the grid when mounting the camera, as the outer edges may wear over time with prolonged use.

Depending on the material of your Distortion Grid, you will need to make sure that the tablet is fixed in a strictly level position. The tablet itself should be installed in a completely flat plane, with no bends. 

This is important, so check again after you have mounted the camera!

Mounting the camera

The camera must be strictly perpendicular to the distortion grid. The centre of the distortion grid should be exactly the same as the centre of the camera lens. You need to be careful when aiming here, as all the squares are the same and you need to pinpoint the centre. If you have an XO element on your board, simply align the camera viewfinder with the XO.

And here’s a little trick I’ve added to my PDF mockups: The centre squares are highlighted with a 50% colour series, which makes them contrast with the black, but also makes it easier to find the centre point in the camera lens.

Shoot in the manual mode only. Set the minimum focal length and position the camera in such a way that the rectangles are visible all over the image. You should be able to see a large number of squares, not just a few. You need a lot of squares to analyse the image properly. The most important are the edges of the image. To best capture the lens distortion, shoot the distortion grid so that only the edges of the squares are visible, and as few halves as possible.

You can see how lens distortion changes as you move away from the subject by shooting the grid as close and as far away as possible.

Don’t try to fit the whole grid into the frame. Let it extend beyond the frame.

Adjusting the lighting

Once the camera is level and the axis of the lens is exactly perpendicular to the screen of the distortion grid, you will need to adjust the light to illuminate the entire screen.

Keep in mind: You need maximum frame clarity. This means that you will be shooting with the aperture closed. So you need a lot of light.

Try to avoid uneven grid lighting and glare. Sometimes you have to work with the light you have. But don’t hesitate to ask for extra lights if you have a lighting crew nearby.

How to shoot a footage using distortion grid

Capture the distortion grid in the same format used for your primary footage, rather than the highest resolution. This is because most cameras change the angle of view (AoV) when you switch formats. Alternatively, you can shoot in Open Gate, but then be sure to properly crop the grid to the required format. Take a test shot with the camera and, in that same shot, include a tablet (or sheet) displaying the technical specifications of the lens and camera. Since you will have many such shots, organize your process so that this test shot is easy to locate and recognize during post-production.

Move the camera half a meter back, adjust the focal length, and capture a new set of shots (recording details such as the camera model, lens type, distance from the grid, and shooting format), then repeat the test shot. It is also crucial to document the full range of focusing distances, since most lenses exhibit significant changes in distortion with different focus settings. If you are using a zoom lens, be sure to cover the entire focusing range for each focal length.

It can happen that the interval between the shooting of a video and its post-production takes months, and sometimes even longer. So don’t rely on your memory. It’s better to shoot everything with the camera and keep the shots in a folder with all the footage. 

Personally, I try to record the approximate size of some objects during the shoot, or compare them to something I know. This helps me with the matchmaking and creating a plate in Nuke in the future.

Checklist “How to shoot Distortion Grid”

During my work on my latest project I decided to create a checklist for myself on how to correctly shoot Distortion Grid. I’ll be refining this checklist from project to project, so I’ve decided to add a box here with the date of the last update, so you can see how up to date this information is.

1. Write down the camera settings, format and sensor size.
2. Adjust the framing.
3. Reduce or close the aperture and increase the ISO to get the right exposure and sharpness.
4. Focus on the centre square so that the screen is sharp. Use manual focus only. The grid should not be out of focus.
5. Set the lens to the minimum focal length and position the camera perpendicular to the grid.
6. Check the image for complete filling of the squares, especially at the edges. Make sure that there are complete squares at the edges.
7. Take a test shot using the tablet or lens settings sheet.
8. Change the focal length, move the camera, take a new shot and record again.
9. Repeat until you have reached the maximum focal length of the lens.

Notes on using the distortion grid

• Notes on using the distortion grid
• The camera does not record the lens data and you will not see the information in the metadata when you use metabones.
• Do not use grids with different resolutions! Instead, print a double-sided grid and turn it over as needed. Spend an extra minute with the zoom lens on set and save hours in post-production.
• Never tape over the crosshairs.
• The distortion grid should be one solid piece.
• There should be many squares instead of a few large ones.
• Avoid wrinkles in the pattern – a strictly flat surface.

Summary

As you can see, lens distortion is something that causes VFX artists a lot of trouble in post-production. However, these difficulties can be avoided by using the Distortion Grid for technical shots during the shooting process. Yes, this is only the first stage of creating visual effects, tracking and integrating CG objects into your video footage. But the painstaking work of a VFX supervisor saves time in post-production. 

So the next time you see a film with incredible visual effects, remember the titanic work that was done long before the first image appeared on screen.

Frequently asked questions

What is a Distortion Grid?

A Distortion Grid is a tool commonly used in visual effects (VFX) production, consisting of a board with a grid pattern. It is employed to measure lens distortion and streamline post-production processes. By capturing this grid against real-world scenes, distortions caused by optics can be calculated and corrected, crucial when integrating computer graphics into video footage. Accurate distortion parameters facilitate the creation of scenes with natural perspective and visual consistency.

What is a lens distortion grid?

A lens distortion grid is a visual tool used in cinematography and visual effects to capture and analyze lens distortions during the filming process. It typically consists of a grid pattern displayed on a flat surface, and when filmed, this grid helps VFX professionals understand how the lens distorts straight lines and objects, allowing for precise correction and enhancement during post-production.

What is distortion in imaging?

Distortion in imaging refers to the deviation or alteration of the original shape, size, or position of objects in a photograph or video. It occurs when the optical characteristics of a lens cause a misrepresentation of the scene, often resulting in issues such as barrel distortion or pincushion distortion. Distortion can impact the accuracy and visual quality of images, making it essential to use corrective measures like distortion grids in cinematography to ensure precise image representation during post-production.

What is the working principle of a Distortion Grid?

The Distortion Grid operates by providing a known geometric pattern during the filming process, serving as a reference for analyzing lens distortion. It allows for the precise measurement of how straight lines in a scene become distorted, helping VFX professionals accurately calculate and correct the distortion introduced by camera lenses. This information is essential for achieving seamless integration of computer-generated elements into live-action footage, ensuring visual coherence and realistic perspectives in post-production.

How do you shoot a distortion grid?

To utilize the Distortion Grid, attach it to a flat surface during filming, ensuring it is well-lit and fully visible in the camera frame. Set up the camera perpendicular to the grid, adjust the focus to the central squares, and capture multiple frames with varying focal lengths. This process provides valuable distortion data that aids in post-production tasks like matchmoving, VFX integration, and lens correction, enhancing the overall visual quality of the final product.

Where to buy Distortion Grid?

While I haven’t discovered the ideal store to purchase a Distortion Grid, I opted to create my own. Additionally, I’ve included PDF files in my article, allowing you to download and print your Distortion Grid at a local printing facility.

Where is Distortion Grid used?

Distortion Grid is commonly employed in the field of visual effects (VFX) and post-production. It serves as a crucial tool for capturing technical footage, aiding in lens distortion analysis, matchmoving, and ensuring accurate integration of computer-generated objects into video footage.

Where can I find more information about Distortion Grid?

Explore the most intriguing and insider details directly on my website. Additionally, delve into my Glossary of video terms for a comprehensive understanding of related concepts and terminology.