Fixed or variable ND filter? Choose the strength your shot needs

Quick answer: choose the light reduction, then the adjustment

Choose a fixed neutral-density (ND) filter when you want one documented strength for a planned shot. Choose a variable ND when changing the strength without swapping filters is more useful. Before either purchase, check your lens's filter thread and work out how much light you need to remove. A wide advertised adjustment range is not a promise that every setting will give an even image with every lens.

This guide covers ordinary photographic ND filters for controlling exposure. It does not cover solar photography or viewing: do not use these filters as eye protection or point an optical viewfinder at the sun. HOYA's solar-filter cautions explain why even a specialised very dark photographic filter is not safe eye protection.

What problem are you trying to solve?

An ND filter reduces light entering the camera, giving you room to use a longer exposure or a wider aperture when the scene is too bright for your chosen settings. It is worth checking whether your camera can already make the exposure you want without one. HOYA's explanation of fixed and variable ND filters describes both approaches and their exposure uses.

• You want moving water to blur: decide on an exposure time, then calculate the reduction needed to reach it.
• You want to keep a particular aperture: establish whether excess light is actually preventing that setting before adding a filter.
• You regularly move between brighter and darker scenes: an adjustable filter may be convenient, provided its usable range covers your needs.

Fixed versus variable: compare the working method

Variable filters use rotating polarising elements. HOYA warns that its Variable Density II can produce a dark cross near maximum density, with the result depending on the setup. Treat that as a reason to inspect the intended filter's instructions and practical limits, not as a measured verdict on another brand. Its instruction sheet also describes the greater risk with wider-angle lenses. This does not establish a universal safe focal length for other filters.

Read ND numbers without confusing the scales

ND8 normally names a filter factor of eight, which corresponds to three stops, rather than eight stops. A stop halves the transmitted light; keeping aperture and ISO unchanged means doubling exposure time for each stop. Optical-density labels such as 0.9 are a third notation. NiSi's terminology guide explains these scales.

These are calculated examples using filter factor × starting exposure time, not measurements of a product. Camera shutter steps may be rounded. ND2–ND400 therefore describes a nominal factor range, not “2–400 stops”. The practical range and transmission of a variable filter need separate confirmation.

A five-line exposure worksheet

Use this before comparing listings. Keep aperture, ISO and scene lighting unchanged for the calculation.

1. Starting exposure: record a suitable unfiltered exposure, for example 1/125 second.
2. Target exposure: write the time you want, for example 1/2 second.
3. Required factor: divide target time by starting time. Here, 0.5 ÷ 0.008 = 62.5.
4. Shortlist strength: a nominal ND64 is close to that target. At the same settings it calculates to 0.512 second. This is a planning estimate; check the actual exposure.
5. Selection branch: compare fixed ND64 options for that planned shot, or a variable filter whose usable range includes the required reduction. If the documentation cannot establish that range, record it as unknown.

A second example shows why the maximum matters: reaching four seconds from 1/250 second requires a factor of 1,000. A filter advertised only up to ND400 does not reach that calculated target at unchanged aperture and ISO. Do not buy it on the assumption that turning it further will supply clean extra range.

Match the lens thread before choosing a listing

Use the lens's filter diameter, often marked beside the Ø symbol, or check the exact lens specification. Do not choose by camera brand alone. PolarPro's filter-size guide explains the marking and the use of step-up rings. If an adapter is involved, verify both its sizes and the complete setup rather than treating a diameter label as the entire fit check.

If your lens has a compatible 67mm screw-in filter thread and an adjustable filter suits your worksheet, compare the Cuely 67mm ND2–ND400 variable filter at Buy2Fix. The listing identifies that diameter and nominal range and describes a rotating frame. Check the current specification against your lens and required reduction before ordering.

The listing alone does not establish measured colour accuracy, the usable maximum on your lens, or freedom from vignetting. If one of those points is essential, ask for model-specific documentation or test evidence before choosing it. No price, stock availability or hands-on performance is claimed here.

Make the final choice with four checks

• Exposure: the chosen strength reaches the target you calculated.
• Fit: the exact lens thread, any adapter, and access to the filter's controls are confirmed.
• Image limits: colour casts, corner darkening and any variable-filter cross pattern are checked for the intended setup.
• Working method: changing filters or rotating one is practical for the photographs you actually make.

Research scope: manufacturer guidance and the linked retailer listing, checked 6 November 2026. Calculations and the worksheet are planning aids. No filter was tested for this guide.