How do I meter correctly for a long exposure?
Meter your scene at your chosen aperture and ISO without any dense ND filter attached, then multiply that shutter speed by the filter's stop factor to get your actual long-exposure shutter speed. A 1/60s base exposure with a 10-stop ND becomes roughly 17 seconds (1/60 × 1024) — a real, calculable number, not a guess.
Why You Can't Just Meter Through a Dense Filter
Most camera meters and autofocus systems stop working reliably once a 6-stop-or-denser ND filter is attached, simply because too little light reaches the sensor for the metering system to read accurately. The standard workflow is meter-then-filter: compose and focus, meter the scene normally, calculate the corrected shutter speed for your filter's density, then switch the camera to manual mode (or bulb, for exposures beyond your camera's normal shutter-speed range) and dial in that computed value.
Where Digital Cameras Make This Easier Than Film Ever Was
Once you're in a long exposure, a digital camera's rear screen and histogram let you check exposure immediately and adjust before moving on — a genuine advantage over film, which offered no real-time feedback and demanded the reciprocity-failure correction get right the first time. For exposures beyond about 30 seconds, most cameras need bulb mode plus either a physical remote or an intervalometer to hold the shutter open for a precise duration.
Work out your own base metering and corrected shutter speed in the Exposure Triangle Solver.
Frequently Asked Questions
What if my calculated shutter speed is longer than my camera's built-in maximum?
Switch to bulb mode, which holds the shutter open for as long as you manually keep it triggered — pair it with a wired or wireless remote (or a built-in intervalometer, if your camera has one) to hold a precise, repeatable duration rather than timing it by hand.
Should I trust my camera's live histogram during a long exposure with a dense ND filter?
Yes, once the exposure is far enough along to register meaningful light — the histogram remains an accurate real-time check regardless of how the exposure was calculated, and it's the most reliable way to catch a metering or calculation error before the shot finishes.