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Shooting Modes & Lens Distortion: Does Cropping Open Gate Work?

Shooting Modes & Lens Distortion: Does Cropping Open Gate Work?

In this series of tests, we will answer the most popular questions about camera tracking that arise among CG artists.

The goal of the experiment #1 is to determine whether switching formats results in a proportional crop or if other manipulations are applied to the captured image.

The key idea is that distortion grids should always be captured using the full available sensor (Open Gate mode). Afterward, cropping can be performed during post-processing if necessary. This eliminates the need to shoot grids separately for each mode.

METHODOLOGY

01

For testing, we selected the most popular cameras with the largest sensor sizes and captured four sets of distortion grids in all formats. You can download the samples by clicking the button below:

01

For testing, we selected the most popular cameras with the largest sensor sizes and captured four sets of distortion grids in all formats. You can download the samples by clicking the button below:

ARRI Alexa Mini LF (ARRIRAW, ProRes)

Sony Burano wintout stabilization (RAW)

RED Monstro 8K (RAW)

02

Next, we took the grids captured in Open Gate mode and cropped them in Nuke using our script CropDistortionGrid

02

Next, we took the grids captured in Open Gate mode and cropped them in Nuke using our script CropDistortionGrid

Reminder: It is crucial to crop based on the sensor size ratio, not the pixel aspect ratio.
Most cameras have varying pixel densities per millimeter of the sensor.

Some cameras also perform downscaling – reducing the resolution of the digital image while maintaining proportions. This is done to ensure the required recording speed, for example, when shooting in slow motion (high frame rate). downscaling ≠ crop!

How crop math works

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You need to take the original sensor size (original_sensor) and the size of the sensor you want to crop the image to (new_sensor).

03

You need to take the original sensor size (original_sensor) and the size of the sensor you want to crop the image to (new_sensor).

The formula for calculating the crop factor based on width is:

The formula for calculating the crop factor based on height is:

04

Then, the new image dimensions (in pixels) after cropping are calculated as follows:

04

Then, the new image dimensions (in pixels) after cropping are calculated as follows:

For width:

For heigth:

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As a result, we obtain a cropped grid based on its original Open Gate (OG) image and the sensor size to which we are adapting the original OG grid.

EXAMPLES

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Examples of grids captured on the Alexa Mini LF in ProRes:

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Examples of grids captured on the Alexa Mini LF in RAW:

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Examples of grids captured on the RED Monstro 8k:

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Examples of grids captured on the Sony Burano:

COMPARISONS

05

Alexa Mini LF

05

Alexa Mini LF

06

RED Monstro 8k

06

RED Monstro 8k

07

Sony Burano

07

Sony Burano

CONCLUSIONS

There was no visible difference between the cropped OG grids and those captured in the target format, which means that the cameras apply the crop without additional transformations. Therefore, we can shoot grids in Open Gate mode, then crop to the desired format and calculate the distortion afterward.

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