Anamorphic vs Spherical for 2.39:1

Two ways to a 2.39 frame

A 2.39:1 picture can be made two ways. Spherical lenses record a normal image on a 16:9, 17:9 or 3:2 area, and the top and bottom are cropped off: a 2.39 extraction. Anamorphic lenses squeeze a wide field of view onto a taller area, and the picture is stretched back out in post. Both end in the same container. They differ in how much of the sensor they use, the lenses they need for a given shot, and a set of optical traits that many directors choose anamorphic for in the first place.

Spherical: crop a 2.39 extraction

A spherical extraction keeps the full width and crops the height to width ÷ 2.387, the ratio of the DCI Scope container. The rest of the frame is recorded but not shown, which leaves room to reframe up or down in post. The free aspect ratio calculator finds the largest 2.39 frame inside any recording size, in your browser.

The taller the recording, the more of it a spherical scope extraction leaves unused: from a 3:2 Open Gate area, close to 40% of the height is outside the frame. That headroom is useful for stabilization and reframing, but it is sensor area the picture does not use.

Both routes on one camera

The ALEXA 35 shows the comparison cleanly, since ARRI publishes the size of every mode. Here are a spherical scope extraction and two ways of using a 2× anamorphic lens.

The pixel counts are similar; what differs is the shape. The anamorphic frame is 2790 or 3006 photosites tall against 1930 for the extraction, and narrower, because the lens has already folded the width into it. Remember that the desqueeze then stretches the width, so those horizontal pixels end up spread over twice the width. ARRI's guideline puts it as a budget: deliver no more total pixels than the active area recorded. The desqueeze guide covers that in detail.

Focal length and field of view

An anamorphic lens sees vertically like its marked focal length and horizontally like that focal length divided by the squeeze. Because the anamorphic frame is taller on the sensor, you need a longer lens than the spherical camera for the same shot.

The factor here is 1.445, from the two frame heights on the ALEXA 35. On another camera, work it out from that camera's own mode sizes, or put the numbers into the Field of View calculator, which takes a squeeze factor and gives both angles. The free web version does it in your browser.

What looks different

• Out-of-focus highlights. The squeeze compresses the round blur from the aperture horizontally, and the desqueeze stretches the image back, so the blur ends up as a vertical oval. The degree depends on the lens design and is often strongest in the background.
• Flares. The cylindrical elements in many anamorphics spread a bright point into a horizontal streak across the frame. Coatings decide how strong and what color it is.
• Perspective at the edges. Many anamorphics show barrel distortion and softer edges and corners than modern spherical primes; for some productions that is the point.
• Depth. The longer focal length for the same frame and the shallower depth of field that comes with it give the larger-format feel described above.

None of these are fixed numbers: they vary a great deal from one lens series to another. Test the actual lenses with the actual camera mode before you decide.

Practical trade-offs

Anamorphic lenses are often bigger, heavier and slower than spherical primes of similar focal length, and many focus less close, which matters for handheld, Steadicam, gimbals and inserts. Check the specifications of the series you are considering against the shots you need.

Choosing

• Choose anamorphic for the look: the flares, the oval bokeh, the larger-format depth. Use a tall sensor mode so the 2× squeeze fills it, or a 1.33× lens on a 16:9 camera.
• Choose a spherical extraction when you need flexibility: a wider choice of lenses, room to reframe, a simpler post pipeline, or a 16:9 version cut from the same footage.
• Either way, shoot a framing chart with the frame lines and send it to post, and work out the delivery sizes before the shoot.

Next steps

• Work out the desqueezed ratio and the horizontal-equivalent lens for any squeeze with the free anamorphic calculator.
• Squeeze factors, sensor modes and 2.39 delivery sizes are in the anamorphic desqueeze guide.
• Match a lens across two spherical sensor sizes with the Focal Length Match calculator.
• Size the letterbox for a 16:9 deliverable with the Aspect Ratio calculator.

CineMath's free Anamorphic calculator gives the desqueezed ratio and the horizontal-equivalent focal length for any lens and sensor area, so you can plan the lens list before the camera test.