by ʇolɐǝz ǝɥʇ qoq

Submit your Photo
Hall of Fame

Please participate in Meta
and help us grow.

Photography Stack Exchange is a question and answer site for professional, enthusiast and amateur photographers. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

If you stack two 3-stop neutral density filters, does it reduce the exposure by 6 stops or 9 stops?

share|improve this question
up vote 33 down vote accepted

It's six.

Remember, the stops are already logarithmic. That is, a 3-stop reduction (as from a 3-stop ND filter) is a 2³× loss of light — ¹⁄₈ of the light gets through. A one stop filter halves light, since 2¹ is just 2 (→ ¹⁄₂), and two stop filter is 2² (→ ¹⁄₄ the light). When you stack them together, you're adding the exponents, so 2³ stacked with 2³ is 2⁶ — or ¹⁄₆₄th the light. That's the same as thinking "three stops is one over 2³, or ¹⁄₈, and ¹⁄₈ × ¹⁄₈ = ¹⁄₆₄ — which is one over 2⁶".

But, fortunately (and in fact partly why it's done that way), you don't have to remember all this. Just remember that 2³ × 2³ = 2⁶ — or, 3 stops plus 3 stops is 6 stops.

Of course, this is just the math. In the real world, there may be other practical effects, like vignetting in the corners (due to the increased thickness) or color casts — you're adding more layers for the light to go through, and that takes a toll on image quality. See the comments below.

share|improve this answer
+1 but you could have summed up as 6! – John Cavan Apr 2 '11 at 0:28
@mattdm - The kernel of your answer always comes in the last sentence or two. Maybe a little bold highlighting or people who skim for the short version? – Sean Apr 2 '11 at 0:32
But the site doesn't take one-character answers. :) – mattdm Apr 2 '11 at 0:45
You forgot to explain the law of stackability failure ;) Two 3-stop ND filters = 6-stops, but three 3-stop filters equals 9-stops in the center and 10-stops along the edges due to vignetting. – Itai Apr 2 '11 at 1:35
Be careful though. Cokin ND filters give a colour cast when you stack them, as I found out to my cost! – NickM Apr 2 '11 at 16:40

In addition to @mattdm's answer - the result of stacking ND filters can also be vignetting.

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.