Open Gate vs 16:9 vs Anamorphic: Field of View

The recording mode sets the view

A camera's headline sensor size is the most it can record. Each recording mode reads a particular window of that sensor, and the window, not the sensor, decides what a lens sees. Change from open gate to a 4K 16:9 mode and the same 32mm frames tighter; switch on a 2× anamorphic mode and it frames wider across but no taller.

All of this follows from one formula, angle = 2 × arctan(recorded dimension ÷ (2 × focal length)), explained in the field of view guide. The examples below use the published recorded areas of current ARRI cameras; the sensor size chart has the same figures for other makers.

One lens, every mode

The ALEXA 35 Xtreme has a Super 35 sensor 27.99 × 19.22mm across and a choice of sensor windows. Here is what a 32mm lens sees in six of them. The last column is the lens that would give the same horizontal view in open gate, which is how much each mode "lengthens" the lens.

The 32mm stays a 32mm: what changes is how much of its image the camera keeps. Shooting 2K S16 on this camera is optically the same as putting the 32mm on a Super 16 camera, a 72mm-equivalent view by open gate standards, with Super 16's depth of field as well.

Open gate vs 16:9

Open gate records the whole sensor. On the ALEXA 35 Xtreme, 4.6K 3:2 Open Gate and 4.6K 16:9 have the same 27.99mm width, so their horizontal angle is identical; 16:9 simply drops 1.74mm from the top and bottom of the sensor each. What open gate buys is height, and height is useful in three ways:

• Reframing and stabilization room above and below a 16:9 or 2.39:1 frame, for VFX tracking, tilt corrections and repositions in the grade.
• Vertical deliverables. A 9:16 crop from the full 19.22mm height is 10.81mm wide; from the 16:9 mode's 15.75mm height it is only 8.86mm wide. Open gate gives the vertical version a 22% wider view from the same take.
• Anamorphic and taller aspect ratios, where the squeeze needs more height than 16:9 leaves (see below).

What open gate does not change is a 2.39:1 spherical extraction across the full width: 27.99mm wide by 11.71mm tall fits inside both modes, so the scope frame, and its field of view, is the same either way. If 2.39:1 is the only deliverable and nobody needs the headroom, 16:9 records the same picture with less data.

Crop modes on large-format cameras

Large-format cameras offer smaller windows so you can use Super 35 lenses, record less data, or reach further with the same lens. Each is a crop factor against open gate. On the ALEXA Mini LF (open gate 36.70 × 25.54mm), a 40mm gives these views:

Two things are easy to miss. First, the "LF" UHD mode is already a 1.16× crop of the open gate, so switching from open gate to UHD mid-scene changes the frame noticeably. Second, the "S35" modes on this camera are not the same size as Super 35 on the ALEXA 35 Xtreme (27.99mm open gate) or the 35mm film gate (24.89mm), so a lens chart made for one does not carry over exactly. Match on the recorded widths, as in the crop factor guide.

Anamorphic: wider, not taller

An anamorphic lens compresses the scene horizontally by its squeeze factor before it reaches the sensor, and the picture is stretched back out in post. The sensor records the same width as ever, but each millimeter of it now holds more of the scene, so the horizontal angle is that of a sensor squeeze-times wider. The vertical angle is that of the lens's marked focal length.

The free field of view calculator takes a custom recorded area and a squeeze, so you can run these angles for any mode.

For comparison, the same 2.39:1 framing in spherical comes from a 34.6mm on the 4.6K 16:9 mode with a 2.39:1 extraction (27.99 × 11.71mm): 44.0° across and 19.2° up. The angles match, but the anamorphic shot is made with a 50mm, so it has a 50mm's depth of field at the same stop and the anamorphic lens's own character: oval bokeh, horizontal flares and its particular distortion.

Which mode suits which squeeze

The desqueezed aspect ratio is the recorded aspect times the squeeze. To land near 2.39:1 without cropping, pick the recording shape that the squeeze turns into scope:

So 2× lenses want a 6:5 mode, 1.8× a 4:3 mode, and 1.3× lenses suit 16:9. A 1.5× squeeze on a 3:2 open gate gives 2.25:1, which crops a little top and bottom for scope. When the desqueezed image is wider than you deliver, the side crop narrows the horizontal angle you keep, so work out the angle from the extraction width.

Choosing a mode

• Start from the delivery frame. Decide the aspect ratio you deliver and any secondary versions (vertical social cuts, 16:9 TV versions of a scope film), then pick the mode whose window holds all of them.
• Check lens coverage. Bigger windows need bigger image circles. The image-circle diagonal of each mode is in the sensor size chart; compare it with the lens maker's stated coverage.
• Plan lenses per mode. A crop mode lengthens every lens in the kit by the same factor. A 1.16× UHD crop turns a 40mm into a 46mm view, which may leave a gap at the wide end.
• Keep modes consistent within a scene. Switching modes between setups changes the frame as much as changing lenses does, and the continuity notes should record it.

Questions

Working it out in CineMath

In CineMath's Field of View calculator, type the recorded width and height of your mode in place of the sensor presets, then the focal length and distance. Turn on Anamorphic and choose 1.25, 1.33, 1.5, 1.8 or 2.0× (or type a custom factor): the horizontal angle and frame width widen by the squeeze, and the vertical angle and frame height stay those of the marked focal length. The diagonal angle it shows is for the sensor area as entered, before desqueeze.

To carry a framing from one mode or camera to another, use Focal Length Match with the two recorded widths (for anamorphic, enter the width times the squeeze). To desqueeze the pixels and fit the result to a delivery resolution, the Aspect Ratio calculator does the rest.