ND filter calculator

Type the exposure your meter gives you, pick the filter you are about to screw on, and this calculator returns the exposure to set behind it — written as a duration, with a countdown for the ones that have left the shutter dial. Free, no account, nothing stored.

What it does, and why it is here

A neutral density filter takes light away in stops, and one stop is one doubling. The arithmetic is a single line — new shutter = metered shutter × 2^stops — and it is not where the mistakes come from.

They come from three other places. The same glass is sold under three names (ND number, optical density, stops), and the round ones are not powers of two: an “ND1000” is 9.97 stops, an “ND100” is 6.64. Stacked filters add their stops and not their numbers, so an ND1000 with an ND8 on top is 12.97 stops and nothing on either ring says so. And once the filter is on, the viewfinder is dark and the meter is blind: the exposure has to be known before the glass goes on, in front of the scene, with the light moving.

So this page converts once, correctly, and adds the two things the formula leaves out — whether the answer has run past the 30 seconds where camera dials stop, and how much longer to expose on film.

What you fill in

• Metered shutter speed — the exposure without the filter, written the way the camera shows it: 1/125, 2, 30. A text field rather than a decimal one, because that is how a shutter speed is read and remembered.
• Filter — ND2 to ND100000, listed as they are sold. Each entry carries its real density, so you never convert an ND number into stops yourself.
• Second filter, stacked — optional, and where the arithmetic usually goes wrong. Name both and the stops are added for you.
• Extra correction, in stops — tenths of a stop, either way: for glass that is not in the list, or for a filter whose marked density you have measured and found optimistic. Leave it at 0 otherwise.
• Exposure wanted — optional, and it turns the question around: name the exposure you are after and the answer names the filter that reaches it.
• Film — leave it on none for digital. Name the emulsion and the corrected exposure comes back separately, from the maker’s datasheet.

The chips above the form fill in the kits people actually own — three, six and ten stops, and the two stacks that reach the long exposures. You bring the metered speed.

What you get back

• Expose for — the answer, written as a duration and not a count of seconds: 8 min 32 s, because nobody times a long exposure by counting to 512.
• Set your camera to — the nearest speed the dial actually has, while the answer is still on it.
• A countdown that beeps — when it is not. Past 30 seconds you are in bulb mode, holding the shutter open and timing it yourself; the page counts your exposure down and sounds the end, so the device already in your hand is the timer.
• Total light lost, in stops, beside the metered speed it started from — the line that shows the tool read your input the way you meant it.
• The exposure on film, named with the emulsion and fitted to within a third of a stop, when you named one.
• Which filter for that exposure — when you filled in the exposure wanted: the closest single filter and the closest stacked pair, each with the exposure it really gives, so the difference can be made up on aperture or ISO.
• The whole dial behind the filter — every metered speed against its filtered one, the ones past the dial marked bulb. Keep it on screen when the light moves: re-meter, find the new row, do not recalculate.
• A way on to the timelapse planner, opened on an interval no shorter than this exposure — an interval can never be shorter than its frame.

What it does not do

It answers in shutter speed only. Aperture and ISO stay yours to set: a neutral density filter is bought to move the shutter, which is why that is the axis the answer sits on.

It knows nothing about your particular piece of glass beyond its marked density — real filters vary by up to half a stop, and strong ones often push magenta or blue, which is a correction in the raw file and not in the exposure. And it does not apply to a graduated ND, dark at the top and clear at the bottom to hold back a sky: that filter does not lengthen the exposure of the whole frame, so no single number describes it.

ND filter chart: numbers, densities and stops

The conversion the tool does for you, laid out for when you would rather read it — and for deciding what to buy.

Two things a chart cannot show. Stacking adds the stops column and never the ND numbers — ND1000 with an ND64 is 9.97 + 6 = 15.97 stops, and no chart is labelled ND64000. And past thirty seconds every row needs bulb mode, whatever the camera.

Questions

Is this calculator free?

Yes. No account, no e-mail, nothing stored — the shutter speed you type is used to answer and then forgotten.

How do I write the shutter speed?

The way your camera shows it: 1/125, 2, 30. Decimals work too, but nobody thinks in decimals under a second.

Why does my ten-stop filter behave like a nine-stop one?

Because it probably is one. Marked densities are approximate, and half a stop either way is common. Take one frame at the calculated exposure, look at the histogram, and put the difference in the extra-correction field — from then on that filter is calibrated.

What about a variable ND?

Two polarisers in a ring, and convenient. At the extremes they produce a dark cross across the frame and unpredictable colour, so treat the marked range as optimistic and stay inside it. Enter the stops you believe you are using in the extra-correction field.

Which filter should I buy first?

A six-stop is the most generally useful: enough to reach a second or two in daylight, still light enough to see and focus through. Ten-stop filters are for the minute-long exposures, and much harder to work with.

Do I need one for video?

More often than for stills. Video wants a shutter around twice the frame rate — 1/50 s at 25 fps — which outdoors means either stopping the lens down until it looks flat, or putting glass in front of it.

Does an ND filter reduce dynamic range or add noise?

Not in itself: it takes light away evenly, so the longer exposure restores the same signal. Very long exposures do heat the sensor, which is a separate source of noise.

Do I have to fit the filter before focusing?

No — do the opposite. Focus and meter first, then fit the glass without touching the ring: past six stops the viewfinder is useless and autofocus hopeless. On a DSLR, cover the eyepiece too, since light entering from behind can fog a minute-long frame.

Sources and further reading

• Neutral density filter — densities, ND numbers and stops.
• Exposure value — what a stop is.
• Reciprocity and its failure on film — why the film field exists.