luxmetered dark glass arithmetic · nothing leaves this tab

Ten stops
of calm.

Meter the scene bare, screw on the dark glass, and the shutter owes you the difference. This does the owing: metered speed times the filter — stacked if you stacked — with film reciprocity folded in. The metering itself happens on the meter page.

The debt, settled

metered shutter, no filter

the filter

stacked on top

film stock (reciprocity)

—

each stop doubles the time · past two minutes, bracket a stop over as well — the fits are extrapolating out there

Every ring, timed

your metered speed behind each common filter — reciprocity included when a stock is picked

filter	stops	give it

Screwed on, worked out

meter bare, then filter — behind ten stops of glass the camera's own meter and this site's are both guessing at noise; take the reading first, do the arithmetic after
compose before, too — through an ND1000 the viewfinder shows you nothing; frame, focus, lock, then screw on
big NDs run warm — most ten-stops lean amber or magenta; digital shoots RAW and shrugs, colour film accepts the mood or filters against it
cover the eyepiece on SLRs — minute-long exposures let light sneak in the back door and fog the frame from behind
film wants extra past 1s — that's reciprocity, folded in above when you pick a stock; the full arithmetic lives on the reciprocity page

What the dark glass is for

Neutral density is exposure's escape hatch: it buys shutter time (or aperture) that the light won't sell you. Waterfalls smoothed to silk at midday, crowds erased from a plaza because nobody stands still for two minutes, f/1.4 portraits under full sun without melting the shutter's top speed. The glass subtracts light equally across the spectrum — that's the "neutral" — and the arithmetic above pays it back in seconds. Meter first with the camera you're holding; the sky-only version of that reading is on the sunny 16 page.

← back to the light meter — where the reading this page multiplies comes from.