How to Use Film Spot Meter

Film Spot Meter turns your iPhone into a reflected-light spot meter for film photography. You aim at one part of the scene, and the app tells you the exposure to set on your camera. It also handles the zone system, bellows extension and reciprocity failure.

Getting started

Open Film Spot Meter and tap Allow when it asks for camera access. The camera is used only to measure light. No photos are taken or saved.
Choose your film speed on the Film ISO wheel.
Choose how you want to work:
- Set aperture: turn the aperture wheel to the f-stop you want, and the app gives you the shutter time.
- Set shutter: turn the shutter wheel to the time you want, and the app gives you the aperture.
Aim at the area you want to meter and tap it in the viewfinder. The amber spot circle moves there.
Read the result under Expose for (or Set the lens to) and set it on your camera.

Taking a reading

The spot circle measures only the area inside it, not the whole scene. Aim it at the tone you care about.
The reading is shown as EV at ISO 100, in thirds of a stop.
Tap Hold to lock the reading. You can then lower the phone and set your camera without the reading changing. Tap again to release it.
Too bright or Too dark means the spot is outside the range the phone’s camera can measure. Meter something nearby and adjust, or use Manual EV.

Reading the result

In Set aperture mode, the result is a shutter time. Times shorter than a second are shown as standard camera settings (for example 1/125). Times of one second or longer are shown exactly (for example 1m 47s), so you can time them by hand.
In Set shutter mode, the result is an aperture rounded to the nearest third stop, with the exact value beside it when they differ (for example f/9 with f/9.4).
If an answer is beyond what a camera can set, such as faster than 1/8000 or wider than f/1, the app shows a warning.
When bellows or reciprocity corrections are on, the result shows each step: the metered value, then the bellows correction, then the reciprocity correction.

Manual EV

Switch from Camera to Manual EV to enter a light level yourself with the slider, for example EV 15 for bright sun. Everything else, including the zone system, works the same. This is also how you use the app without the camera.

Calibration

A phone camera isn’t a calibrated light meter, so check it once:

Meter a gray card or an evenly lit wall with a light meter you trust.
Meter the same surface with Film Spot Meter.
Use the Calibration + and − buttons until the two readings agree.

The calibration is saved and applied to every camera reading.

Turning features on and off

The Zone System, Bellows Extension and Reciprocity sections each have a switch in their heading. Turn off the ones you aren’t using and they fold away. A switched-off feature has no effect on the exposure, and your settings inside it are kept for next time.

The zone system

The zone system divides tones into eleven zones, from Zone 0 (pure black) to Zone X (pure white). Zone V is middle gray.

Placing a single reading

A plain meter reading puts whatever you meter on Zone V. Use Place spot on to choose another zone. For example, put light skin on Zone VI, or snow with texture on Zone VIII.

Shadows and highlights

Aim at the darkest shadow you want to keep detail in, and tap Record shadow.
Aim at the brightest highlight you want to keep texture in, and tap Record highlight.
The exposure is now set from the shadow, placed on Zone III unless you change Place shadow on.
The zone scale shows S and H where the shadow and highlight will print. The white arrow under the scale shows where the spot you’re aiming at right now will fall.

Development

With both recorded, the app shows the brightness range in stops, which zone the highlight falls on, and the development to bring the highlight to its target (Zone VIII unless you change Highlight target):

N means normal development.
N+1, N+2 mean develop longer (expansion) for a low-contrast scene.
N−1, N−2 mean develop less (contraction) for a high-contrast scene.

With only a highlight recorded, the exposure is set from the highlight instead. This suits slide film, where highlights matter most.

Tap the ✕ on a reading to clear it when you move to a new scene.

Zone view

Tap Zones on the viewfinder to color the live picture by the zone each area will print on, using your current exposure. The colors match the strip under the zone scale: blues for the shadows, teal and green for the lower midtones, gray for Zone V, then yellow, orange, red and magenta up to white for Zone X. Areas left uncolored are too dark or too bright for the phone’s camera to judge.

Bellows extension

For close-ups and large-format work, light is lost as the lens moves farther from the film.

Turn on Bellows Extension.
Enter the lens Focal length and the Bellows draw, measured from the lens to the film, in millimeters.

The app shows the exposure factor, the stops of correction and the magnification. At infinity focus, the draw equals the focal length and there’s no correction. At twice the focal length (life size) you need 2 stops more.

In Set aperture mode the time gets longer. In Set shutter mode the app tells you how far to open up.

Reciprocity

Film loses sensitivity in long exposures, so a metered 10 seconds may need 20 seconds or more.

Turn on Reciprocity.
Choose your film from the list.

Built-in films: Ilford Pan F+, FP4+, HP5+, Delta 100, Delta 400, Delta 3200, SFX and XP2; Kodak Tri-X, T-Max 100 and T-Max 400; and Fujifilm Acros II. The panel shows example corrections for 1 second, 10 seconds and 1 minute.

For another film, choose Custom exponent and enter the value p from its data sheet, where corrected time = metered time^p.

Corrections are applied in order: metered, then bellows, then reciprocity.

Tips

Meter the important part of the scene, not the brightest or darkest thing in view.
For the most accurate readings, hold the phone steady and give the reading a moment to settle before tapping Hold.
Manufacturer reciprocity figures are a starting point. For important long exposures, a test roll is worth doing.
The ? button in the app opens this guide at any time.

Still stuck? The support page has answers to common questions and a form for reaching me directly.