1 The idea
An anamorphic lens adds a cylindrical element that bends light in one axis only. It squeezes the picture horizontally onto the sensor (for example 2×), and post stretches it back. The side effects the eye spots at once: out-of-focus points become vertical ovals, and bright lights smear into long horizontal streaks.
The streak colour comes from the lens coating, not the geometry. Blue is the best known, but some lenses streak amber. Use Split to compare spherical and anamorphic on the HDR sparks, then measure the plate’s bokeh to find the squeeze. With real lens bokeh on, every spark gets the bokeh traced through a Double-Gauss 50 mm at its own spot in the frame: towards the edge it turns into a darker cat’s-eye, and at f/5.6 it is round again.
bokeh height ÷ width ≈ squeeze · horizontal-angle equivalent focal ≈ focal ÷ squeeze2 Hands-on
Task: match an anamorphic plate whose bokeh measure 20 px wide × 30 px tall, with a blue streak
Bokeh height ÷ width ≈ the squeeze
3 Deep dive
An element that works in one axis
A spherical lens bends light the same in every direction. An anamorphic lens adds a cylindrical element with power in the horizontal axis only, so the picture is squeezed horizontally onto the sensor and stretched back for viewing or in post. Every anamorphic trait comes from that single fact.
The lens behaves as if it had two focal lengths: vertically it is the number on the barrel, horizontally it is wider by the squeeze. A 40 mm 2× gives a 40 mm vertical angle but roughly a 20 mm horizontal one (same sensor width).
| Trait | Spherical | Anamorphic |
|---|---|---|
| Bokeh | Round | Vertical ovals |
| Bright lights | Round glow, ghosts | Horizontal streaks |
| Distortion | Lens dependent | Often strong and characteristic |
| Breathing | Lens dependent | Often more |
Streaks: horizontal, coloured by the coating
The cylindrical element does to stray light what it does to the picture: it stretches one axis, so a point of light becomes a horizontal line. The colour comes from the coating. Blue is the one people remember, but some lenses streak yellow or amber, and modern lenses with very good coatings streak less and cleaner.
Physically, an anamorphic lens streaks every bright source. In practice you often keep streaks only on the lights you want to feature. Blue streaks on a spherical show are physically wrong, yet a director may still choose them as a style.
Matching an anamorphic plate
Read the plate first: measure the bokeh’s height ÷ width, note the streak colour and length, see which lights streak, read the distortion from straight lines. Then match every one on the CG. Round CG bokeh on an anamorphic plate stands out at once.
- 1Measure bokeh height ÷ width
- 2Streak colour and length
- 3Defocus the CG with an oval
- 4Redistort with the real lens (see LA6)
Oval bokeh says anamorphic even with no streak in sight
4 In real work
In Nuke
- Anamorphic footage carries a pixel aspect (for example 2) in its format; the Viewer desqueezes it for display unless you turn on ignore pixel aspect.
- Defocus or ZDefocus: set the aspect ratio so the bokeh is the oval you measured in the plate.
- Think of the streak as what the cylindrical element does to light: every point brighter than about 1 is dragged out along the frame’s horizontal. So start from the drag, not from the source. The drag is a spread that is very long and almost zero tall: a Blur with a large x size and next to no y, or a Convolve whose kernel image is one long horizontal stroke. Feed it only real light: pull the values above about 1 out of the picture and leave out any light that shouldn’t streak. Tint the result to the coating colour you see in the plate. It is light, so add it with Merge plus; above 1, screen returns max(A, B) and throws the streak’s energy away.
- Convolve is slow: precomp and crop the bbox first.
Common mistakes → the fix
- ✗ Round CG defocus on an anamorphic plate✓ Set the defocus aspect ratio to the bokeh you measured.
- ✗ Blue streaks on every light, every shot✓ Take the colour and length from the plate, and pick the lights that serve the story.
- ✗ Diagonal or vertical streaks✓ The cylindrical element stretches horizontally; streaks lie along the frame’s horizontal.
- ✗ Forgetting the anamorphic lens’s distortion when redistorting CG✓ Use the real lens solve from a grid or the matchmove.
Before you send the shot
- CG bokeh has the same proportions as the plate’s
- Streak colour, length and which lights streak match the plate
- CG distortion matches straight lines in the plate
- Judged on the desqueezed picture, not the squeezed one
5 Check yourself
In the desqueezed picture, what shape is an anamorphic lens’s bokeh?
🔒 Answer the question above and this card opens
1. The plate’s bokeh measure 16 px wide × 24 px tall. Roughly what squeeze is it?
2. Why is an anamorphic streak horizontal?
Where the numbers come from: The principles (cylindrical element, vertical oval bokeh, horizontal streaks, streak colour from the coating) come from published lens references and Sky’s flare/anamorphic book. Bokeh aspect ≈ squeeze is an approximation; real lenses can differ (rear-anamorphic designs, for example). The horizontal-angle comparison assumes the same sensor width. The distortion here is a simple barrel for teaching, not any lens’s profile. The real lens bokeh is ray-traced through a published Double-Gauss prescription (Smith, Modern Lens Design) by our own tracer: 2 million rays per shape, a round iris, a light at infinity with focus at 1.5 m, then scaled to the size you set. At 90% of the half diagonal at f/2 the cat’s-eye is 0.61 as wide (radially) as it is tall and 36% as bright as at the centre. The anamorphic mode squeezes that traced shape horizontally, a teaching approximation rather than a traced anamorphic lens. The plain disc mode is computed at 320×180 and scaled up; the sparks are our own HDR Blender render.