A light meter built for large format

Why large format needs more than a basic meter

Large format adds three things that a simple smartphone light meter struggles with: bellows extension reduces the light reaching the film and requires correction, the Zone System is the canonical exposure method that the format encourages, and the long exposures that close focusing and small apertures make routine push the film into reciprocity failure. Each one is a separate stop or more of compensation. Together they are the difference between a usable negative and an unusable one.

Zone Light Meter handles all three as first-class workflows so you are not doing four pieces of math under a dark cloth.

Bellows extension, handled

Bellows extension reduces effective aperture as the image distance grows past the focal length. The correction is two times the log of the image distance over the focal length, in stops, but in practice you do not want to calculate that on the back of a ground-glass card.

Zone Light Meter has a dedicated bellows extension input. Enter the image distance (or measure it once and save it per camera) and the meter folds the correction into the recommended exposure. For a 4x5 portrait at 1:2 reproduction, that is about a full stop. The app will say "add one stop for bellows" and the recommended exposure already includes it.

You can save common camera+lens combinations to reuse, which matters for monorails where you can pre-measure the rail position for typical compositions.

Zone System placement, on the viewfinder

Spot-meter the brightest important highlight in the scene. Place it on zone VIII (textured highlight on negative film, or zone VII on slide film). The app shows where every other tone falls on the 0-10 scale, live over the viewfinder. Spot-meter the deepest shadow you care about; if it lands below zone II, you know you have a contrast problem before you load the film holder.

Multi-spot biasing lets you take several readings and weight them. Three readings on the face for a portrait, one on the background, averaged with bias toward the face: this is how studios actually work and the app ships it.

Reciprocity, per stock

At long exposures every film stock fails differently. TMax 100 fails mildly. Fomapan 100 fails dramatically. Velvia 50 fails in colour as well as exposure. Zone Light Meter ships per-stock reciprocity curves for 693 stocks across 70+ brands and accepts custom curves for anything you shoot that is not in the catalogue.

Tell the app your stock and your meter reading. If the metered exposure is a few seconds or more, the app applies the right curve automatically. A one-minute meter reading might become 90 seconds on TMax 100 or eight minutes on Fomapan. The recommended exposure already includes the correction.

Custom cameras and lenses

Add as many cameras as you own. Custom shutter ranges (including pneumatic and self-cocking shutters that go to T or B). Custom aperture ranges per lens. Custom flash sync speeds where they apply. Custom bellows extension factors per lens. Each frame in the shot log records which camera and lens you used, so when the contact sheets come back you can tell which frames were on which body.

A shot log for film holders

Each frame stores: the camera and lens, the film stock and holder number, the metering choices (which zones, which corrections), the notes you typed, optional geolocation, and the timestamp. Export to CSV with a Lightroom-compatible metadata preset. When the lab returns the negatives, every sheet maps to a frame, and every frame maps to the decisions you made under the dark cloth that day.

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