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Shooting Lens Grids

Here is a breakdown of how we go about shooting lens grids for VFX.

Introduction

You may think that every 50mm L series lens from Canon is identical, or every Signature Zoom 45 – 135mm lens from ARRI, as fancy as it is, are also built identical. In reality, anything that is physical will be unique and have its own characteristics.

In order to capture reference of the characteristics of a lens, we’ll need to capture images/videos of a checker pattern that is printed and mounted onto a board. This pattern is known as a lens grid. By shooting this pattern of evenly squared faces (that can come in different sizes), we can assess the distortion of a lens. The most common forms of distortion being:

  • Barrel
  • Pincushion
  • Mustache

Image from Drew Grey Photography

In 3d applications, all straight lines are rendered straight, therefore, we need our plate to have straight lines too, so we can sync things up. In order to do this, we need to remove the distortion from the plate. By shooting a lens grid pattern, we are able to use software to then calculate how to undistort the plate. From that point, we can track, render and comp our CG elements into the undistorted plate. Once we have added our CG elements, we are then able to re-distort the plate with the CG, so we end up with the original plate but this time, we have the awesome looking CG integrated into the shot and everything is distorted once more for final.

If you are looking for a more detailed step-by-step breakdown on all things onset related, you can check out our courses on the topic here:

In this lesson, we won’t go into detail regarding the solving of the lens grid or adding the VFX. Our focus will be on how to go about shooting a lens grid, so it can be used later on in the VFX pipeline.

Setting up the Camera and the Lens Grid

  • Mount the lens grid at the end of a track or on a wall and make sure it is centred. It should be flat with no distortion/warping, etc.
  • Use an inclinometer to check the lens grid is not tilted.
  • Place the camera on a dolly and track and check that it is level. The track should be pretty long. 15 – 22 foot or longer ideally.
  • Centre the camera with the lens grid, so when the camera moves, the centre lines up with the centre of the grid.

If you do not have a solid wall or track, then you’ll need to improvise. Ideally, you’d shoot the lens grid in a controlled shooting space what a professional camera operator but if that is not possible, do the best to you can ensure the lens grid is flat and not tilted. C-stands for example are extremely handy.

Framing the Lens Grid

For the lens grid pattern, check out this fantastic set provided by Eric Alba:

In the office, I’ve gone for 3. Base grid, with nested smaller grid and XO, of which I have printed and mounted onto a rigid board and then glued to the wall.

When shooting the grid, make sure the lens grid fills the full frame of the image. The checked pattern should go to the outside edges of the frame. You need to see a good selection of squares, not just a few. We need many squares to have a good image analysis and grid corner points to get a good solve and work out the warping.

Here are some poorly framed grids and some well framed grids:

BAD – Not enough squares

BAD – Too far away

BAD – Not to the full edge of the frame (top/bottom)

GOOD – A nice solid set of squares and with variation in size

GOOD – Closer now but the grid pattern still reaches the edge of the frame

GOOD – Even closer now but the inner grid pattern still reaches the edge of the frame

Focus

  • Framing (see above) is essential, however, do also focus.
  • The grid pattern should not be blurred.
  • Make sure to check through the camera eye piece and zoom in with the view finder.
  • Ideally, the focus would work across multiple focus positions. However, make sure to change the focus if need be.
  • To get more focus, stop down the aperture (smaller aperture opening). You will need to compensate this by adding more light or open up the shutter angle to increase exposure time (traditional) or ISO (digital)

If you have a monitor around, hook it up, so you can get a clearer idea of how things are looking in terms of focus.

Lighting

  • For spherical lenses, evenly light the grid. Do not add lots of glare onto the grid or reflection.
  • For exposure (T-stop/F-stop), aim for F11
  • For anamorphic lenses, set the aperture to T22 or better for best DOF. Light the grid with a couple of 4Ks and push
  • the camera ISO (for digital) Or slow frame rate down for longer exposure.
  • Try to avoid uneven lighting and glare. Try to bounce the lighting if you can from reflective boards

Sometimes you’ll have to work with the lighting you have, so do the best you can with what you have. But if the lighting team are about, don’t be afraid to ask for some additional lights, or even help setting up. The below example is NOT ideal as we have some glare going on. But it is not a deal breaker. We were still able to solve the distortion.

Shoot Tips

  • Shoot like you would the shots
  • RED – ISO 800
  • ARRI – IS0 800
  • Film – depends
  • CANON 5DS – ISO 800
  • Shutter speed: 24fps – whatever you will film at (You just need a still – so you can drop to 12fps)

Slate Info

On the slate, include the following:

  • Project Name
  • Date
  • Camera body + serial number
  • Lens + serial number
  • Lens focal length and/or range of lens
  • Actual focal length – for zoom increments
  • Actual focus distance of camera for focus increments
  • Roll
  • T-Stop
  • FPS
  • Shutter
  • ISO

For Spherical Zooms:

  • Add the lens’ zoom barrel markings on the slate. If you have no space on the front, use the back.

For Anamorphic Lenses:

  • Add all the lens focus distances on the slate

Lens Naming Convention

Create a fixed naming convention that pulls in the info from the data sheet. At CAVE, our convention is as follows:

<cameraBody>_<lens>_<focalLength>_<focusDistance>_<roll>_<clip>_<take>.<fileFormat>

For example:

canon5ds_fuji24_10to24mm_5ft_r001_c001_t001.mov

arriAlexa_arriMasterPrime50_50mm_6ft2_r001_c002_t001.mov

All the info for the naming convention should be visible on the slate.

Lens Data Sheet

Create a spreadsheet that grabs the following information per clip:

  • Camera Body
  • Camera Serial Number
  • Lens Name/Type
  • Lens Serial Number
  • Focal Length
  • Focus Distance (in Feet)
  • Roll number (film stock/memory card – 001, 002, etc.)
  • Clip Number (video number – 001, 002, etc.)
  • Take (001, 002, etc.)

Shooting Spherical Primes – Barrel and Pincushion Distortion

  1. Position the camera as far back from the grid as possible whilst ensuring the grid still fills the frame.
  2. Shoot/Roll the camera.
  3. Slate the lens and shoot the grid once – make sure it is in focus.
  4. Stop the camera.

Shooting Spherical Zooms – Barrel and Pincushion Distortion

Here are 2 methods for shooting spherical zooms, but before you begin, do the following:

  • Go to the widest zoom and dolly the camera back from the grid – make sure the whole grid fills the frame.
  • Focus on the grid.
  • Shoot/Roll the camera.
  • Get the slate in frame.

Then…shoot using Method A:

  1. Pull the zoom from wide to long. Pause for a second on each marking on the zoom barrel
  2. Stop the camera

…or…shoot using Method B:

  1. Set barrel marking
  2. Shoot/roll
  3. Slate
  4. Stop the camera
  5. Repeat for each barrel marking

 

For example (using a 70 – 200mm lens):

  1. Set to 70mm
  2. Shoot/Roll
  3. Slate
  4. Stop the camera
  5. Set to 100mm
  6. Shoot/Roll
  7. Slate
  8. Stop the camera
  9. Set to 135mm
  10. Shoot/Roll
  11. Slate
  12. Stop the camera
  13. Set to 200mm
  14. Shoot/Roll
  15. Slate
  16. Stop the camera

For a bonus, do a rack/zoom for each lens to see the lens breathing:

Shooting Anamorphic Primes – More Levels of Barrel Distortion, Moustache Distortion

  1. For anamorphic lenses, set the aperture to T22 or better for best DOF. Light the grid with a couple of 4Ks and push the camera ISO (for digital) Or slow frame rate down for longer exposure.
  2. Position the camera as far back from the grid as possible whilst ensuring the grid still fills the frame.
  3. Add the first focus distance to the slate and get the slate into shot (use disto).
  4. Focus on the slate so it is readable.
  5. Shoot/Roll the camera.
  6. Move the slate away and pull focus to the position that is marked on the slate.
  7. Stop the camera.
  8. Do this repeatedly for a number of focus distances (6 would be good but could be more). Make sure to also include infinity and some near focus settings. Add 4 – 5 (or more) spreads within the range of the lens.

Shooting Anamorphic Zooms

Do the same for Anamorphic Primes and:

  1. Repeat the steps for each zoom barrel marking

So for each zoom barrel marking, you will slate and shoot a set of focus distances (6 for example).

All the results will be graphed to give you a curve that represents the distortion.

Onset Training

For more information on all things onset related, you can check out our courses here:

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Resources

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