Help Me Buy an… ND filter

Explore this exposure-altering accessory

Neutral-density or ND filters have a very simple job to do. They cut the light entering the lens so that you can use a wider lens aperture in bright light, or a much longer exposure. It’s the long exposures that are most tantalising: these can be used to blur surf and skies in landscape shots, or turn boring, choppy lakes and rivers into a silky sheen.

Although this sounds simple in theory, there are some practical differences between different types of ND filter, which you need to get to grips with. Do you want a circular filter that goes straight on your lens, or a square filter you can use in conjunction with other filters? Should you get a resin filter or a more expensive glass filter? What do you get for your money when you choose a premium filter over a budget version? We aim to answer all these questions and more as we help you decide which ND filter is right for you…

1 Square vs circular

ND filters come in two types: circular filters that screw directly on to your lens’s filter ring and have to be bought in the correct size; and square filters that slot into holders. These come in a series of standard sizes, and can be used on any lens via an inexpensive adaptor ring.

2 Glass vs resin

Cheaper ND filters are made of resin, which tends not to offer the same clarity, sharpness and neutrality as upmarket glass filters. Over time, the dyes used in resin filters can degrade or shift in colour, and resin filters may warp or scratch more easily. Glass filters offer better optical quality, light transmission and colour fidelity, although they can break if they are dropped or bent.

3 Filter size

This sounds like an easy question to answer: surely you just check the size of the filter on your lens? That works for screw-in circular filters, although with wide-angle lenses you may need to seek out special ‘wide-angle’ filters with recessed attachment rings, to avoid vignetting.

Square filter holders can fit different lenses via adaptors, but you need to get a holder large enough to match your cameras and lenses. 100mm is the usual size for square systems.

4 Filter strength

Confusingly, different filter manufacturers quote the strengths of their ND filters in different ways. For example, Hoya and Cokin use exposure ‘multiples’, so ‘ND8’ is three stops. Others use decimals, where a three-stop reduction is ’ND0.9’. Both systems have a sound mathematical basis, but fortunately most makers also quote the EV/f-stop equivalent too, which makes shopping for filters a little easier.

5 Filter coatings

Filters need coatings for the same reason lenses do – to reduce reflections and improve light transmission. With ND filters, coatings also offer the benefit of filtering out unwanted infra-red and ultraviolet light, which can cause unwanted colour shifts in long exposures. The best filters also have water-, oil- and scratch-resistant coatings that make them easier to clean.

6 Colour fidelity

Filter materials and coatings can all help reduce colour shifts with long exposures, but ultimately it’s down to the filter maker’s own manufacturing processes, coating, dyeing technologies and choice of materials (glass or resin). This is why Digital Camera always tests colour fidelity when we review ND filters.