Stacking ND filters

Two grey filters on top of each other make a stronger one. The arithmetic is the most common mistake in the whole subject: the stops add up, the filter factors do not. The calculator takes both filters.

The arithmetic

ND8 is three stops, ND64 is six. Together nine – and nine stops are a factor of 2⁹ = 512. Add the factors instead and you land at 72, which is more than two and a half stops wrong.

Multiplying works too, as it happens: 8 × 64 = 512. Adding applies only to stops, multiplying only to factors. Mix the two and you get nonsense.

Common pairs

Starting from 1/125 s, the speed a camera often suggests at f/8 on an overcast day.

What stacking costs

• Flare at every surface: Two filters are four extra glass-to-air transitions. Against the light that shows as a veil over dark areas which flattens the contrast.
• Colour casts add up: If one filter pulls magenta and the other green, they may cancel. Usually they do not, and then the cast sits in the frame twice.
• Vignetting on wide lenses: Two rings on top of each other stick out. Below 24 mm full-frame equivalent the corners go dark, and a step-up ring in between makes it worse.
• No variable ND in the stack: A variable ND is a pair of polarisers. Two of them behind each other produce patterns you cannot predict. Stack fixed filters only.

When a single filter is better

As soon as you need a combination regularly. If you screw ND8 and ND64 together every time, buy an ND512 instead: one piece of glass rather than two, no thinking about the order, no added colour cast.

Stacking solves the one-off case – when the strength you need is missing and the shop is far away. As a permanent arrangement it is a loss of quality with nothing in return.

Two filters are workable, three rarely are. The rule holds: add stops, multiply factors, and never mix the two.

Every value is in the ND filter table, the FAQ covers common questions, and Which ND filter? takes you from the shot to the number of stops.