ND and Polarizing Filter Guide: Which Filter Does What
Photo: Unsplash
Contents
Screwing glass onto the front of your lens can feel old-fashioned — if software can fix almost anything, why bother with a physical filter? The answer is simple: some physical effects can’t be faked in post. A reflection disappearing off a water surface, or clouds streaking silk-smooth across the sky, are events tied to the light itself; if the pixel data never captured it, Lightroom can’t bring it back. This guide covers three filter families — ND, polarizing, and GND — and shows exactly when each one actually makes a difference.
ND (Neutral Density) filters: cutting light in stops
An ND filter works like sunglasses for your lens — it darkens the image uniformly without shifting its color, only its brightness. Its strength is measured in stops, and each stop halves the light:
- ND2 → 1 stop → 1/2 of the light passes through
- ND8 → 3 stops → 1/8 of the light passes through
- ND64 → 6 stops → 1/64 of the light passes through
- ND1000 → ~10 stops → 1/1000 of the light passes through
The formula that actually matters in the field: new shutter speed = old speed × 2^stops. Example: if your correct midday exposure is f/11, 1/60s, ISO 100, and you want to stretch the shutter to 30 seconds to render water as silky, the difference between 1/60 and 30 seconds is roughly 11 stops (30 ÷ (1/60) = 1800, log₂(1800) ≈ 10.8) — meaning a ND1000 filter (10 stops) plus stopping down one more click (to f/16) gets you there. This is the equivalent-exposure logic from our exposure triangle guide in direct practice: an ND cuts light so you can hold shutter speed open for minutes without touching aperture or ISO.
This is the physical precondition for the “silky water, streaked clouds” look we describe in our long exposure guide — without an ND, a 30-second daytime exposure isn’t possible; the sensor blows out instantly.
Fixed vs. variable ND: Fixed NDs are manufactured at a single stop value — optically cleaner and more predictable. Variable NDs work by rotating two polarizing elements against each other, giving a wide stop range from a single filter; but on wide-angle lenses, approaching maximum density can produce a characteristic “X” darkening pattern across the frame. For dedicated landscape/seascape work, a fixed ND1000 is the better tool; for varied everyday use, a variable ND is more practical.
Polarizing (CPL) filters: physically cutting reflections
A polarizer doesn’t darken the image much (aside from a 1-2 stop loss) — it filters the plane of vibration of light. Sunlight normally vibrates in every direction, but once it reflects off a flat surface (water, glass, leaves), it becomes largely polarized in a single plane. Rotate a CPL to cut that polarized component, and what’s underneath the surface — rocks at the bottom of a lake, a product behind a shop window — becomes visible.
The effect is strongest in sky 90° from the sun, and on water surfaces viewed at roughly a 35-40 degree angle (close to Brewster’s angle). One caution on wide-angle lenses (16-24mm): if part of the sky sits at 90° to the sun and part doesn’t, you get an uneven darkening band across the frame — which is why, as we note in our autumn landscape guide, it’s usually better to rotate the polarizer partway rather than to maximum for a more natural result.
The polarizer’s second major benefit is saturation: once stray reflections off leaves and sky are cut, colors read more saturated straight out of the sensor, not through software. This is the actual reason we recommend a polarizer in our landscape sharpness guide.
GND (Graduated ND) filters: closing the gap between sky and ground
At sunset, the sky is often 3-4 stops brighter than the ground — more range than a sensor can capture in one frame; either the sky blows out or the foreground goes black. A GND filter is a glass or resin plate, dark on one half and clear on the other (or the reverse), physically balancing that dynamic range by darkening the sky while leaving the foreground untouched. Measuring a scene’s range, and the alternatives to a GND (fill flash, exposure blending), are gathered in what is dynamic range.
There are three types: soft edge (a gradual, forgiving transition — good for uneven, mountainous horizons), hard edge (a sharp transition for a flat horizon — good for seascapes), and reverse (darkest right at the horizon line — good when the setting sun itself sits in frame). Square/rectangular filter systems (mounted in a holder) let you slide the GND up and down, giving more flexibility than screw-on filters, at the cost of more expense and bulk.
The quality question: color cast
The most common problem with cheap ND1000s is color cast — usually a reddish-purple tint across the whole frame. High-quality glass filters minimize this but rarely eliminate it entirely; shooting with an ND, using manual white balance rather than trusting auto white balance is the key to consistent results, as we cover in our white balance guide.
The second price of cheap glass is not colour but contrast: every filter adds two more glass-air surfaces in front of the lens, and weakly coated ones throw ghost blobs when you shoot into bright light. The mechanism is in what is lens flare.
A tripod is not optional
Once a 10-stop ND gets you to a 30-second exposure, the slightest vibration blurs the entire frame. The heavy, wind-resistant tripods from our tripod buying guide are mandatory here; using a remote release or a 2-second timer instead of pressing the shutter by hand removes the last bit of shake too.
Common mistakes
- Attaching the ND and trusting autofocus: With a 10-stop filter, the camera often can’t see enough light to focus. Focus manually without the filter first, lock it, then attach the filter.
- Twisting a variable ND to maximum: Produces the X pattern on wide lenses; stay just under maximum density.
- Rotating a polarizer to full strength for the sky and forgetting it: Causes an uneven darkening band on wide-angle shots — check through the viewfinder and rotate partway instead.
- Misaligning a GND’s transition line with an uneven horizon: On a mountainous, irregular skyline, a hard-edge GND leaves a visible line between sky and terrain — use soft edge instead.
