Your phone is already a light meter.
The iPhone in your pocket has a calibrated CMOS sensor and more compute than the Hasselblad that flew to the Moon. stop. turns that sensor into a spot meter. Six steps. Two minutes. Then you shoot.
It opens and it reads light.
Tap the icon. Grant camera access once. The meter is running before the splash screen finishes. If your camera is busy or denied, stop. falls back to a manual EV input automatically, so the roll is never blocked by the phone.
Save the page to your home screen and it behaves like a native app. Works offline. No tracking, no login, no notification asking for your email.
Point at whatever you want correctly exposed.
The centre reticle is your spot. Move the phone so the reticle sits on the tone you want rendered as middle grey. A face. A wall. A patch of grass. The big number below the reticle is the exposure value for that patch, updated ten times a second.
Swipe across the bottom row to switch between spot, centre-weighted, and matrix. Spot reads a 2 percent circle. Matrix averages the whole frame. Centre-weighted biases the middle and is the sensible default when you are shooting fast.
You do not need to understand the maths. You need the number.
Pick the film you are actually shooting.
Tap the film name at the bottom of the app. Pick your stock. The aperture, shutter, and ISO suggestions rebias for how that emulsion handles over and under exposure. Portra 400 gets pulled a third of a stop to protect shadow detail. Velvia 50 gets metered tight because it has no latitude. Cinestill 800T is rated at 500 for clean shadows.
The Pro stocks (Portra, Tri-X, Velvia, Cinestill, Ektar, HP5, Provia, Fuji C200) also have their reciprocity curves baked in, so long exposures are corrected automatically without you touching the datasheet.
Lock one, the other two rebalance.
The three numbers along the bottom are a correct exposure for the patch in the reticle. Tap any dial to lock it. The other two rebalance in real time to keep the exposure correct.
Shooting wide open for a shallow depth of field? Lock aperture at f/2. The shutter will run up to 1/4000s. Shooting a tripod scene at f/16? Lock aperture, the app gives you the slow shutter. The arithmetic is the same arithmetic your camera's internal meter is doing, we just show you the working.
One calibration. A whole season's worth of accurate readings.
Calibrate if you need to.
Two iPhones a metre apart can read a third of a stop differently. For colour neg that is invisible. For Velvia or a paper print, that is the difference between a punchy frame and a muddy one. You calibrate once.
Tap the settings icon. Point at a Sunny 16 reference scene on a clear day. The app reads your phone's offset, stores it locally, and applies it to every reading after. If you have a dedicated meter you trust, you can enter its EV directly as a reference instead. Reset it any time.
Now go shoot the roll.
That is the whole app. Open it, point it, pick a stock, read the triangle, calibrate if the light is fussy, and shoot. No subscription. No login. No notifications. Offline after first load.
Add it to your home screen. It launches fast, it remembers your last stock and calibration, and it never asks you for anything. If the field guide on the Learn page made you curious about reciprocity or the exposure triangle, read that next. If you just want the meter, it is one tap away.
Open the meter.
The web version is free. The install takes a single tap. Either one runs offline after the first load. Pick one and start metering.