How it works

The math

The angle a lens covers is 2 × arctan(width ÷ (2 × focal length)). Width is the active area of the recording mode you pick, so a 4K crop and open gate on the same body give different frames, the way they do on set. A lens adapter changes the focal length first.

angle = 2 × atan(mode width ÷ (2 × focal))

mode width 27.99 mmfocal 35 mm43.6°

Your phone's own field of view is read from iOS while the app runs, for each back camera and each capture format, so a new iPhone works without an update. Viewfinder picks the ultra wide, wide or telephoto camera that fits, then zooms until the phone's angle matches the target. The frame has to fit both ways, across and up.

When your lens sees wider than any camera on the phone, the part the phone cannot cover is drawn as a plain band. You can measure each of your phone's cameras in Settings, Calibration, and Viewfinder applies the correction from then on. The frame is as accurate as its sources.

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Frame lines, looks and stand-ins

• 13 aspect presets from 2.40 to 9:16, plus any custom ratio from 1:10 to 10:1, cut from the mode's center. A second frame line can sit inside, such as 2.39 inside 1.85.
• Shade outside the frame, center cross, action and title safe, thirds, golden ratio and a grid, in white, amber, purple or black.
• Looks: lift, gamma and gain wheels, temperature, tint and saturation, your own 3D .cube LUTs and Photoshop curves, applied live and burned into stills and takes.
• Six stand-in figures, sized by distance and height, so you see how big the actor sits in frame and whether they are in focus.
• Anamorphic: 108 lens series with squeezes from 1.3x to 2x, desqueezed on the frame.

The sun and depth of field

The sun's path is computed on the device with NOAA's solar algorithm and drawn over the live picture, with sunrise, sunset, golden hour (the sun from 4 degrees below the horizon to 6 above) and blue hour (8 to 4 degrees below) for any day of the year. Where it is drawn depends on the phone's compass, so the app shows its accuracy and lets you nudge the heading.

Depth of field uses the exact thin-lens formulas: near, far, total and hyperfocal, in meters or feet, with a circle of confusion chosen by the mode's size. It reads the marked T-stop, so it shows a little more depth than the true f-number would, and it says on screen that it does not apply an anamorphic squeeze.

How depth of field is worked out

On your wrist

The Apple Watch app is a remote for the iPhone app open on the viewfinder. It shows the frame at about one and a half frames a second with the focal length and angle, steps through the kit with the Digital Crown, and fires the shutter or starts a take.

On the iPhone itself, the volume buttons and Camera Control fire the shutter too, and on iPad a hardware keyboard has a Camera menu of shortcuts.

On the monitor

Connect a monitor with a cable, such as HDMI through a USB-C display adapter, or pick an AirPlay screen such as an Apple TV, and Viewfinder sends it a clean feed while the iPhone or iPad keeps every control. The monitor shows the frame on black, or the whole picture with the frame marked and the outside dimmed for the operator. Your look is on it too.

A lower third names the camera, mode, lens and aspect, with the field of view, depth of field and hyperfocal on a second line. A red tally shows while a take records, and overscan steps of 2 to 8 percent keep it all inside a monitor that crops.

Show a clean feed on a monitor