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How ND Filter Math Works

A neutral density (ND) filter is essentially sunglasses for your lens: it blocks a fixed, uniform amount of light across the visible spectrum without shifting color or contrast, letting you use a slower shutter speed even in bright daylight. Light loss is measured in stops, and every stop halves or doubles the amount of light involved — which means the relationship between filter strength and exposure time is exponential, not linear. The formula is stops = log2(filter factor), so an ND8 filter (8× light reduction) equals 3 stops, an ND64 equals 6 stops, and an ND1000 — the filter most landscape photographers reach for — equals 10 stops, because 2^10 = 1024.

To convert an unfiltered meter reading into the correct exposure with the filter attached, multiply by 2 for every stop: new shutter speed = base shutter speed × 2^stops. Concrete example: you meter a scene at 1/100 s with no filter on the lens. Screw on an ND1000 (10 stops) and the math becomes 1/100 × 2^10 = 1/100 × 1024 ≈ 10 seconds. That single jump — from a hundredth of a second to ten full seconds — is what turns choppy water into silky mist, scattered clouds into painterly streaks, and a crowded plaza into an empty, ghostly frame. It is the entire premise of long-exposure photography, and it only works if the number is right.

What You Should Know

This is not a calculation you can do by eye. Because the scale is exponential, being off by just two or three stops in your mental math means an exposure that ends up four to eight times too short or too long — the difference between a beautifully blurred waterfall and a frame that is either frozen solid or hopelessly overexposed. The practical workflow is: meter and compose before screwing on the filter (through a 10-stop ND the viewfinder is nearly black, so autofocus and metering both struggle), calculate the converted shutter speed here, attach the filter, switch the camera to manual or bulb mode, and start the exposure.

• Stacking filters adds their stops — an ND8 (3 stops) plus an ND64 (6 stops) equals 9 stops total — but stacking two pieces of glass also raises the risk of vignetting on wide-angle lenses and can compound a color cast, especially with budget filters.
• Very long exposures behave less predictably. Past roughly 30 seconds, ambient light itself can drift mid-exposure — clouds thinning, dusk deepening — so bracket by about ±1 stop and lean toward extra time as the light fades.
• Bulb mode and a countdown timer are essential once exposures exceed your camera's normal shutter ceiling, usually 30 s; a phone stopwatch works, but a timer that counts down and alerts you removes one more variable from a technique that already has several.