What a director's viewfinder is, and how a phone does the job

A director's viewfinder answers one question before the camera shows up: from this spot, with this lens, what ends up in the frame? Directors and DPs carry one on scouts and on prep days so that the lens conversation happens on the location, with the people who care about it, instead of on the shoot day with a full crew waiting.

The question it settles

What a shot looks like comes down to two decisions. The first is where the camera stands. Perspective, meaning how big the background looks against the person in the foreground, comes from position alone. The second is how wide the lens sees, which decides how much of that view lands inside the frame lines.

So a viewfinder is used standing exactly where the camera will be, at the height it will be. Hold it up, step through the lenses, and the argument about the 32 or the 40 turns into something everyone can look at. It also gives the rest of the team a reference they can act on. "The 40 from the doorway, at eye height" tells the gaffer where the edges are and tells the designer what will be seen.

Three kinds of viewfinder

The kinds are not exclusive, and an optical finder in the bag with an app on the phone is a sensible pair. The rest of this guide is about how the third kind works, because the method decides how far you can trust it.

How a phone does the job

Step 1: the angle your camera will see

The horizontal angle a lens covers is 2 × arctan(width ÷ (2 × focal length)), where the width is the size of the recording mode you will shoot. That last part matters more than most people expect. On an ARRI ALEXA 35, a 32 mm lens covers 42.5° across in 4K 16:9, which uses 24.88 mm of the sensor's width. Switch the same camera to 4.6K 3:2 Open Gate (27.99 mm wide) and the same lens covers 47.2°. Same body, same glass, a visibly different frame.

The table shows how far the recording mode moves the frame. The three columns are real modes from the gear data.

The first two columns are both modes of the ARRI ALEXA 35; the third is the ARRI ALEXA Mini LF. Figures computed from the recording mode sizes in Cutvey Viewfinder's gear data, counted November 2, 2026, with the app's own rules.

Step 2: the angle the phone sees

Every camera on a phone has its own angle of view, and it changes with the capture format the phone is running. Cutvey Viewfinder asks iOS for that figure on each back camera (ultra wide, wide and telephoto, whichever the device has) at run time. There is no table of phones inside the app, so a new iPhone works without waiting for an update.

Step 3: the zoom, worked out in tangents

To show your lens's frame, the app scales the phone's picture by tan(phone half-angle) ÷ tan(target half-angle). It is tempting to divide the angles instead, and at wide angles that is badly wrong. A camera with the angle of a 13 mm lens on full frame sees 108.3° across; a 24 mm frame is 73.7°. The tangent ratio is 1.85, the angle ratio only 1.47. Use the second and the frame is out by about a quarter, which you can see from across a room.

The app picks the phone camera for each lens: the main camera whenever it can hold the frame, the ultra wide for a frame wider than that, and the telephoto only when the main camera would have to zoom too far. The frame has to fit both ways, so the tighter of the horizontal and vertical fits wins. A delivery aspect such as 2.39 is then cut from the middle of the recording mode, as it would be in the edit.

When your lens is wider than the phone

Wide cine lenses on large sensors see a lot. A 12 mm on the ARRI ALEXA Mini LF in 4.5K LF 3:2 Open Gate (36.7 mm wide) covers 113.6° across. Whether a phone's ultra wide reaches that depends on the phone and the format it is running.

When it does not, Cutvey Viewfinder shows the part the phone can see and draws the rest of your frame as a plain band, in purple, charcoal or black (your choice in Settings). The band is honest about what is missing: it is the part of your lens's view the phone cannot see, and it stays in stills and takes so nobody mistakes the picture for the full frame. A clip-on wide converter over the phone, set under Phone Attachment, fills it. Why the frame can look wider than your camera and lens adapters and clip-on lenses cover both.

What a phone viewfinder does not do

It frames. That is a narrower job than it sounds, and it helps to know the edges:

• The picture is the phone's lens. Your lens's bokeh, flare, distortion and color are not in it. Depth of field is a calculation in its own panel, worked from the focal length, stop and distance, and the background blur on screen is the phone's.
• Focus breathing is not modeled. The focal length is the marked one at every focus distance.
• It is as accurate as its sources. The frame comes from a published figure for the mode size where one exists (958 modes) and from the published pixel pitch times the pixel count for most of the rest (1,245 modes). 108 modes carry a Provisional badge until a second check. Each mode shows a line saying where its size came from.
• Every phone camera is slightly different. Settings, Calibration has a MEASURE step per camera module; it takes a minute and the app uses the correction from then on. See calibrate your iPhone cameras.
• Exposure is the phone's. A look or a LUT on the picture previews a mood. It says nothing about your camera's latitude in a bright window.
• New gear arrives with app updates. The database is part of the app.

Picking one

An optical finder is quick and needs nothing charged. An app knows the exact recording mode, keeps a record of every frame you try, and turns the scout into a shot list. Many DPs use both. If you are comparing apps, our comparison and the director's viewfinder apps, side by side lay out what each one does, with dates on every claim. To try the arithmetic without a phone, the field of view calculator runs the same rule in your browser, and how the app works walks through it on the phone.

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