Long exposure basics: tripods, ND maths and timings

Tripod up, ND on, and the frame still comes out soft, blown out or black. Usually the order was wrong. Fix the order and what is left is one equation and a table of timings.

A forest stream where a long exposure has smoothed the water into silk

1. What you need

A tripod. A featherweight one in wind is worse than hand-holding, and a fully raised centre column is the wobbliest part of the rig, so keep it down.
A way to fire without touching the camera: cable release, phone remote, or the two-second self-timer. Finger shake hurts most between 1/4 s and 2 s.
An ND filter for daylight. See lens filters explained.
An eyepiece cover on a DSLR, or tape. Light leaking through the viewfinder brightens the frame or adds flare. Mirrorless bodies don't have this problem.

2. Setting the camera

Manual or bulb, lowest native ISO. Skip the extended "L" setting; it costs dynamic range.
f/8 to f/11, where most lenses are sharpest. Stopping down to f/22 to stretch the time buys you diffraction softness. Buy the time with ND instead.
Autofocus once, switch to manual, and leave the ring alone. Behind a dark ND, autofocus often can't lock at all.
Stabilisation off; on a tripod it can hunt. See hand-holding and stabilisation.
Shoot RAW so white balance and shadows stay adjustable. See RAW or JPEG.
Turn on electronic first-curtain shutter or mirror lock-up if you have them.
Check the histogram after every frame. At night the rear screen always looks brighter than the file. See how to read a histogram.

Run the camera from your phone

Zlens turns the phone into the camera console: live view, remote settings, RAW transfer. Android first, coming soon.

About Zlens and supported cameras

3. Converting ND stops

One equation: target time = metered time × 2^stops. Meter with no filter on, then move the shutter by the filter's stops. Each stop doubles the time.

Six steps through one long exposure: Set the tripod, compose, autofocus once, then switch to manual focus, Meter without any filter first, Move the shutter by the filter's stops — each stop doubles the time, Stabilisation off, viewfinder eyepiece covered, filter on, Fire with a remote or the 2-second timer; past 30s switch to bulb and time it yourself, Check the histogram on playback, then double or halve the time and shoot again — Six steps through one long exposure

Filter	Stops	1/125s becomes	1/15s becomes	1/2s becomes
ND8	3	about 1/15s	about 0.5s	about 4s
ND64	6	about 0.5s	about 4s	about 32s
ND1000	10	about 8s	about 60s	about 8 minutes

Most bodies top out at 30 seconds. Past that, use bulb: the shutter stays open while the release is held, and you time it with a phone stopwatch or a programmable remote.

4. How long to expose

Subject	Starting time	Notes
Streams and small falls, keeping texture	1/4–1s	Longer and the water loses its shape
Silky waterfall	1–3s	The usual starting point
Sea smoothed to mist	15–30s	Ten stops of ND in daylight
Vehicle light trails	10–30s	Long enough for a car to cross the frame
Moving cloud	30–120s	No wind, no movement
Making a crowd disappear	60s and up	Only what stays still survives

You find the time by shooting. One frame at the table value, then zoom in. Water melted into a white blob: halve it. Droplets still visible: double it. Two or three frames settle it, because current, traffic and wind are never the same twice.

5. The usual failures

Centre column fully up, or the tripod on a boardwalk where every passer-by shakes it.
Stabilisation left on; DSLR eyepiece left open.
Not re-metering after the ND goes on, or not switching to manual focus.
Pressing the shutter by hand. Exposures around one second are the most shake-sensitive.
A variable ND turned to its limit, leaving a dark X. See lens filters.
Long-exposure noise reduction off past 30 seconds, and hot pixels everywhere.
One duration all evening. Bracket the time from one spot and choose at home.

How this works in Lenscript. Zlens puts most of this on your phone: remote shutter, aperture, ISO and exposure compensation, live view with a histogram and zebras, and interval shooting that works with bulb and carries on after a power interruption. RAW files come straight back to the phone. It replaces the cable release and the crouching beside the camera, not the tripod. Android first, one-time purchase, not released yet.

6. FAQ

What if I have no cable release?

Use the two-second self-timer. Your hand is off the camera and the shake has died down by the time the shutter opens. You only need a lockable release or a phone remote for bulb past 30 seconds.

Can I do a 30-second exposure in daylight?

Only with ND. A sunny scene meters around 1/125s, so eight seconds takes ten stops. For a minute or more, stack filters or wait for the weaker light of early morning or dusk.

Should I turn on long-exposure noise reduction?

It shoots a matching dark frame and subtracts it, which kills hot pixels and doubles the time per shot. Leave it on for an unhurried night. Turn it off when racing the blue hour or shooting timelapse frames, and clean up in editing.

Can a phone shoot long exposures?

Most pro modes reach 30 seconds. Light-trail modes are really multi-frame stacks, not one long shutter: less noise, less control in very dark scenes. Either way the phone has to stay still, and a railing beats your hands.