Help Me Buy an… ND fil­ter

Ex­plore this ex­po­sure-al­ter­ing ac­ces­sory

Digital Camera World - - CONTENTS -

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.

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