Exposure calculator

Exposure — pick the light from the scene list (or type the EV your meter gives at ISO 100), then move any two of aperture, shutter and ISO; the one marked Solved follows to keep the picture correctly exposed. Tap Solve on another row to lock that one instead. The sliders step in full, ½ or ⅓ stops and show the values cameras actually mark (f/5.6, 1/125), with the exact solved value under it. Equivalent exposures lists every aperture/shutter pair that lets in the same light at your ISO. Meter from these settings takes the EV from a combination that worked.

EV (exposure value) at ISO 100 is log₂(N²/t) − log₂(ISO/100): EV 0 is f/1 for one second at ISO 100, and each EV halves the light. Scene values are the typical ones from the ANSI PH2.7 / Kodak exposure tables, as collected in Wikipedia’s Exposure value article; real scenes vary by a stop or more.

Stops — how many stops lie between two apertures, shutter speeds or ISOs, with ⅓ and ½ stop nudges and the full scales. One stop doubles or halves the light; f-numbers go by √2 because the light goes with the area of the opening, 1/N².

ND & long exposure — meter without the filter, type the filters (stops, ND1000, or density 0.9; stack them with +), and read the long shutter time. Start the countdown when you open the shutter on Bulb: it beeps for the last three seconds and at the end, keeps the screen awake, and survives a reload. ND1000 is 9.97 stops and optical density 3.0; density is stops × log₁₀2.

Reciprocity — film loses speed in long exposures. Ilford films use Ilford’s formula T_c = T_m^p for metered times over one second, with p = 1.33 (Pan F+), 1.26 (FP4+, Delta 100), 1.31 (HP5+, XP2 Super), 1.41 (Delta 400), 1.33 (Delta 3200) and 1.43 (SFX), from Ilford’s Reciprocity law failure technical information (HP5+ at 10 s → 20.4 s). Kodak films follow Kodak’s published tables, interpolated on log–log axes: Tri-X (F-4017) 1 s → 2 s, 10 s → 50 s, 100 s → 1200 s, with 10, 20 and 30% less development; T-Max 100 (F-4016) 1 s → +⅓ stop, 10 s → 15 s, 100 s → 200 s; T-Max 400 (F-4043) 1 s → +⅓ stop, 10 s → 15 s, 100 s → 300 s. Fujifilm Acros II needs nothing under 120 s and +½ stop from 120 to 1000 s; Provia 100F nothing to 128 s and +⅓ stop to 8 minutes (Fujifilm data sheets). Beyond the published times the tool extrapolates and says so — bracket. Custom takes your own exponent.

Motion & shake — the longest shutter that keeps a moving subject sharp: its image crosses the sensor at v·f/(d − f), so the shutter is the blur allowed divided by that. Pick a subject (walker 1.4 m/s, cyclist 8 m/s, car at 50 km/h…), its distance and the angle to your line of sight (90° = across the frame). The blur allowed is for full frame and is divided by the crop factor. The handheld limit is the reciprocal rule, 1/(focal length × crop factor), relaxed by the stabiliser’s stops and halved for high-resolution sensors — a rule of thumb, not physics. Units (km/h and m, or mph and ft) are remembered across the photography tools.