Lensing

As light passes through a camera lens, it does not stay perfect. The glass stack, aperture, coatings, and matte box all imprint artifacts on the image: soft focus falloff, dark corners, geometric warps, color fringes, flares, haze, and grain (or noise).

Matching those artifacts when you integrate CG or elements into a photographed plate — or adding a coherent lens package on a full-CG shot — is essential to photorealism. One effect alone will not “complete the picture.” A full, ordered lensing setup is greater than the sum of its parts.

Every compositor should understand a little photography: how light interacts with a lens, and in what order those effects happen.

Apply lensing last (and in camera order)

Lensing should be one of the last stages of the composite, after keys, grades that match plate lighting, and creative integration — then roughly in the order light would experience inside a real camera / pipeline.

A practical end-of-stack order (adjust to show standards):

1. Beauty integration (CG / elements over plate, already in a shared working space)
2. Depth of field / defocus (if not already in the render)
3. Optical diffusion (pro-mist, net, mild bloom)
4. Chromatic aberration / fringing
5. Lens distortion (on CG and full-CG; plates are usually already “in lens”)
6. Vignette
7. Flare / ghosting (when motivated by sources)
8. Grain / noise (always last, or last before a delivery ODT depending on show)

Doing grain or a heavy vignette before a key or a big grade makes every retake fight you. Keep the lens package modular and late.

Depth of field

Real lenses only focus a limited depth range. Subjects outside the focus plane go soft; bright out-of-focus points become bokeh shaped by the aperture.

In the renderer vs in comp

A majority of the time DOF is achieved with the zDefocus node in comp. Most render engines do provide ray traced depth of field, however this will often dramatically slow down rendering and is irreversible.

Nuke tools

• ZDefocus — depth-driven blur from a Z channel; workhorse for many shows.
• pgBokeh (and similar) — more photographic bokeh shapes, blades, cat-eye, etc.

What to match

• Focus plane and how it moves (rack focus)
• Blur amount vs distance (not a single constant blur on “background”)
• Bokeh shape (round vs anamorphic oval, blade count)
• Edge quality: highlight bloom inside the bokeh, not just a Gaussian mush

If the plate is soft and your CG is razor-sharp at the same depth, the shot fails a tech check immediately.

Diffraction and “expensive glass”

Diffraction is a wave effect: light bends at hard edges (especially a small aperture). On image, you get a slight overall softening and characteristic rings/spikes on bright points when stopped down hard.

In practice on set:

• Stopping down (high f-number) increases diffraction softener.
• Cheap or dirty glass, cheap coatings, and internal scatter add haze and low-contrast “glow.”
• Cinematographers pay for high-end lenses largely to get controlled aberrations — sharpness where wanted, character where wanted, and less of the junk (flare veiling, weird CA, mush).

For VFX:

• You rarely simulate pure diffraction physics.
• You do match the plate’s micro-contrast, highlight bloom, and overall optical softness.
• Over-sharp CG on a soft plate (or the reverse) is a lensing mismatch, even if “focus” is nominally correct.

Vignette

Vignetting is caused by the physical dimensions of a multi-element lens. Rear elements are shaded by elements in front of them, which reduces the effective lens opening for off-axis light. The result is a gradual decrease in light intensity toward the image periphery.

So: corners go darker than the center — sometimes mildly, sometimes dramatically (wide open vintage glass, heavy matte box, wrong filter stack).

Optical falloff (intuition)

Illumination on the sensor also follows geometric falloff for off-axis rays. A common teaching approximation for natural light falloff toward the edge is related to:

\text{relative illumination} \propto \cos^{4}\theta

(where \theta is the angle off axis). People also remember that intensity from a point source falls with distance:

I \propto \frac{1}{d^{2}}

For a vignette in comp, you usually do not re-derive physics. You build a soft radial (or slightly oval / anamorphic) darkening that matches the plate:

• Strength
• Shape (circular vs stretched)
• How early it starts from center
• Whether it is pure exposure loss or also a slight desat / cool-warm shift

Nuke: Grade with a radial mask, Vignette gizmos, or a soft circular ramp. Match by exposing up the plate and your comp — corner mismatch becomes obvious.

Lens distortion

Lenses map the 3D world onto a flat sensor with geometric distortion. Straight lines may bow. Focal length, lens design, and zoom position all change the warp.

Studio practice

Most large facilities measure every show lens (or lens + zoom position) and publish distortion grids / STMaps. Tracking, layout, and paint all work in a known geometric space.

If you do not have a package:

• Photograph or request a lens grid
• Or use Nuke LensDistortion — manual, grid-based, or line/point analysis
• Or derive from tracked features over a pan

Curvilinear vs rectilinear

Most live-action plates you get were shot on real glass with measurable distortion. A default CG camera is a perfect pinhole / rectilinear model until you tell it otherwise.

Critical pipeline rule

Apply lens distortion to CG (and full-CG packages) to match the plate — do not distort the plate to match undistorted CG as your default.

Typical flow:

1. Undistort plate only when the pipeline requires it (e.g. for tracking/paint in a linear space), with a documented STMap.
2. Integrate / render CG in the same geometric convention the show uses (often “undistorted working space”).
3. Redistort CG (or the whole stack) with the show lens map before delivery so it sits on the plate’s geometry.

If you warp the plate “because CG was straight,” you fight editorial, matchmove, and every other shot on the lens. Prefer CG → plate geometry.

Lens flare

Flares are internal reflections and scatter between lens elements, coatings, and the sensor stack. Bright sources (sun, practicals, speculars) kick ghosts, veils, and colored blobs along the optical axis.

• Not random decoration — they should be motivated by something bright in frame or just out of frame.
• They move with the source relative to the lens center.
• Anamorphic flares streak; spherical flares ghost differently.

Tools

The common production hammer in Nuke / After Effects is Optical Flares (Video Copilot). Shows also use proprietary tools, CG flare cards, or plate-shot practical flares.

Match:

• Color temperature of the source
• Aperture shape in the flare structure
• Intensity under the show LUT / ODT (easy to overdo in linear)

A huge flare on a plate that has none (or the wrong shape) looks like a plugin demo, not a camera.

Fringing

Fringing usually means a bright or colored halo along high-contrast edges — not always full longitudinal CA, but related.

Causes / looks:

• Optical chromatic effects at contrast boundaries
• Sensor / demosaic edge artifacts
• Over-sharpening in camera or plate processing
• Bloom from highlights bleeding into darks

In comp, mild edge fringing can help CG sit in a plate that already has it. Heavy fake fringing reads as a bad YouTube tutorial. Sample the plate at similar contrast edges and keep it subtle.

Chromatic aberration

Chromatic aberration is wavelength-dependent focusing and magnification. Different colors do not land on the same sensor position → color fringes along edges (often cyan/magenta or red/blue).

Common where:

• High contrast (bright sky against dark silhouette, metal edges)
• Wide open or cheap wide lenses
• Image corners more than center (lateral CA)

Reference overview: photographylife.com — What is chromatic aberration

In VFX

• Split RGB slightly radially or directionally; or use a dedicated CA / fringing tool.
• Match the plate — amount, direction, and which edges show it.
• Full-CG shots: a little CA sells “glass”; too much looks like a phone panorama gone wrong.

Chromatic aberration is usually a late, subtle layer after defocus, not a substitute for good focus matching.

Matte box effects (pro-mist and friends)

The matte box holds filters in front of the lens. Filters are part of the optical path, not a grade.

Pro-mist (and similar diffusion)

A pro-mist (or black pro-mist, pearlescent, etc.) softens speculars, lifts shadows slightly, and blooms highlights. Cinematographers use them for skin and “cinema” texture.

For VFX:

• If the plate was shot with diffusion, clinical-sharp CG will never sit.
• Match with mild glow / bloom on highlights, slight global softening, or a dedicated diffusion tool — after integration, before or with grain depending on show taste.
• Strength should match distance and stop: diffusion looks different wide open vs stopped down.

Other matte-box / filter looks: polarizers (sky/reflections), ND (exposure only, not a “look” by itself), streak filters, nets.

Grain (and digital noise)

Grain should be among the final steps of any composite (often the last creative step before or after a delivery transform, per pipeline).

Why last:

• Grain is high-frequency; blurs and reformats after grain destroy the match.
• Keys, warps, and grades should not cook grain differently per layer.
• CG is usually too clean; plate grain (or matched synthetic grain) unifies the stack.

See the Tech Check lesson for grain matching and a high-pass grain check. Link the interactive checklist: /tech-check.

Film grain ≠ digital noise, but the job is the same: match the plate’s frequency, amount, and channel balance.

Full-CG vs plate integration

Full-CG without lensing looks like a render. Full-CG with random uncorrelated flares, grain, and distortion still looks like a render. One story for the lens.

Practical checklist

• Lensing is late in the graph (not buried under keys)
• DOF matches focus plane, amount, and bokeh character
• Micro-contrast / diffusion matches glass + filters (pro-mist if present)
• Vignette matches plate corners (check with exposure up)
• Distortion: CG follows plate / show STMap — plate not casually warped to CG
• CA / fringing subtle and plate-matched on high contrast
• Flares motivated and timed to sources
• Grain / noise last and matched
• All of the above viewed under the show OCIO view / LUT

Quick reference

Summary

• Lensing is the set of optical fingerprints a real camera leaves on an image.
• Match them for plate work; design a consistent package for full-CG.
• Apply late, in an order that respects the camera path.
• Distort CG to the plate, not the plate to perfect CG.
• Grain last. DOF, vignette, CA, diffusion, and flares only work if they agree with the plate under the show viewer.

Individually, each effect is a small lie. Together, they are how CG becomes photography.