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Really long ex­po­sures, in the or­der of a quar­ter of a minute up to sev­eral min­utes, need strong ND fil­ters of sev­eral-to-many f-stops. Most fil­ter man­u­fac­tur­ers pro­duce fil­ters in a va­ri­ety of strengths, and a set of three or four will give you flex­i­bil­ity, en­abling you to com­bine two or more as nec­es­sary. Here’s what you need for long ex­po­sures:

A set of ND fil­ters rang­ing from around three stops to about nine stops will en­able you cope with most daylight sit­u­a­tions.

Tri­pod (needs no fur­ther com­ment).

Al­ways work at your low­est ISO.

So as not to waste time, it helps to have a rough idea of which strength of fil­ter to reach for first. As a rule of thumb you can start with that old-fash­ioned but still rel­e­vant ‘sunny 16’ rule, which says that if it’s sunny, set the aper­ture to f/16 and the shut­ter speed to the re­cip­ro­cal of the ISO – so 1/100 sec at ISO100. In this ex­am­ple, you’d need a seven-stop fil­ter to get you an ex­po­sure of a sec­ond (1/100>1/60>1/30>1/15>1/8>1/4>1/2>1), and an 11-stop fil­ter to reach a quar­ter of a minute. Stop­ping down to f/22, as you can with most lenses, dou­bles the pos­si­ble ex­po­sure time yet again.

Fo­cus and com­pose be­fore you fit the fil­ter(s), be­cause once they’re at­tached you won’t be able to see a thing. Oh, and dis­able aut­o­fo­cus, too.

ND fil­ters are of­ten avail­able as kits, which can save you money com­pared to buy­ing them separately

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