Will stacked ND filters perform as well as a single 10 stop ND filter?

Question

I have the Tiffen 0.9 ND filter which gives me approximately 3f stops reduction. I like to really slow things down and although it's sufficient in low light it just won't do in daylight.

Should I get a pair of 2 more 0.9 ND filters to stack or should I would a single say 10 stop filter be a better solution? Will the stacked ND filters perform as well as a single 10 stop filter?

I would like to be able to slow things down to the point where I get similar results to this photo which was shot at f4, ISO 200 and 75 seconds.

Asked by Jakub

Answer

From a functional standpoint, yes, you could essentially achieve the same effect with multiple stacked filters as a single high-density filter (say a 10-stopper.) There are a variety of concerns to be aware of, however, regarding stacking multiple filters.

1. Filter quality:
   • The Lee "Big Stopper" 10-stop ND filter is pretty high quality glass filter
   • There are two Lee ProGlass filters (0.6 and 0.9) of the same quality
   • The full range of standard Lee ND filters, (0.3, 0.45, 0.6, 0.75, 0.9) are resin
   • Many third-party filters are resin, a very few are glass
2. Color cast:
   • Stacking lots of resin filters can cause strange color casts
   • With digital, color cast is usually an easy post-process fix
3. Vignetting:
   • Stacking enough resin filters means a pretty thick hunk of junk on the end of the lens
   • At least four resin ND filters (0.3/1stop, 0.6/2, 0.9/3, 1.2/4) are needed to match a single 10-stop filter
   • On wider-angle lenses, you can barely get away with two stacked filters let alone four without vignetting
4. Flaring:
   • Any additional optical device added to the light path adds the potential for flare and ghosting
   • Stacking four ND filters adds a total of 8 additional surfaces which can reflect
   • To my knowledge, no 4x4/100mm filters are multicoated, so their chances of flaring/ghosting are high
5. Versatility:
   • A single 10-stop filter still leaves you some room to also stack on a graduated ND filter to balance contrast

If you want to do some serious ND work, such as 5 minute exposures of sunset along the coast, you should get a 10-stop filter. You'll probably also need at least one GND to normalize scene contrast if you are photographing a sunset/sunrise, which would usually require a setup like the Lee base filter kit with a tandem adapter and an additional independent filter holder (for a total of up to 8 filters)...that is pretty much guaranteed to add vignetting on anything other than a telephoto lens.

Answered by jrista

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Why are doubled polarization filters as continuous ND not popular?

Question

One thing I was always wondering when it comes to ND filters is why there's need for such a large amount of different strengths, when two linear polarization filters would do the trick just fine. Assuming a light loss of 1/sqrt(2) per filter, one could easily create a ND "1 to infinity" filter. Why is that not in everyone's camera bag?

Unfortunately, all the filter companies offer only LCPL (linear, then circular), so I can't simply make one myself. Apart from cases where I do not want to take polarized light into account while having a ND filter (architecture photography maybe), I can't think of a reason not to do this.

Edit: More specifically, I'd like to know why there's nothing like this at all, or how I could get my hands on such a filter.

Answer

Does Singh-Ray's VariND filter not count?

Note, though, that even in their own product advertising, they point out the fundamental problem with this filter arrangement: it's still a polarizer, so it's still going to give unsatisfactory results on skies when used with a wide-angle lens. And there's that little thickness problem as well; vignetting is going to be an issue on wide lenses or on lenses that don't have a lot of wiggle room between the front element's edge and the filter thread.

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What's the result of stacking ND filters?

Question

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

Answer

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.

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Does stacking more lens filters decrease overall image quality?

Question

As stated by @t3mujin in another thread's comments:

Stacking filters will decrease image quality, as it's another piece of glass light has to go through before reaching the sensor.

I was wondering whether this is true, or better yet, how radical the quality decrease really is?

I was thinking about using UV filter on my lens and on that, stacked a polarizer. This combination of filters sounded to me like a great idea on how to shoot the nature, but after I read the aforementioned, I'm really wondering whether I might be wrong.

What would be the best option for this situation?

Answer

Anytime you add something in the optical path you will lose quality. The quality you will lose depends on the filter quality, filter type and lighting conditions.

Most filters are extremely susceptible to flare because they add a flat reflective surface and can take a great image and make it completely unusable. That does not mean you should never use a filter or even more than one, but you should do so for a valid reason knowing the sacrifice you are making at the same time.

Polarizers have specific usage and an effect which cannot be simulated by software, so use them to cut-down reflection, increase saturation in the sky, etc. No need to add it over another filter that essentially does nothing.

UV filters have a use too but are rarely actually needed. They are usually recommended at high-altitude but AWB or custom white-balance takes care of that. Since they are cheap and do not cut-down light by much you can use them to protect your lens when it is in eminent danger such as near salt-water and flying sand.

Against knocks, a lens hood is much more effective protection and may even increase image quality by reducing flare from stray light.

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What is the difference between using an ND filter versus 2 polarizers?

Question

I know what an ND filter does. I know what a polarizer does. I also know what two polarizers stacked together and rotated properly do.

So the question: why should I use an ND filter to achieve a darker image at the input, when I can use 2 polarizers instead and rotate them to exactly as dark an image at the input as I want?

Answer

• Polarizers are often more expensive than ND filters and you need two of them.

• Stacking two filters can cause vignetting with wide lenses.

• You have an extra glass surface with two polarizers which can cause flare and potentially loss of contrast/sharpness.

• This arrangement can cause colour shift toward yellow (but so some ND filters).

• Extreme wide angle lenses will exhibit uneven darkening due to the difference in incidence angle across the polarizers.

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Is it possible to use a polarizer and a ND filter at the same time?

Question

Is is possible to use two filters (one polarizing and one ND) at the same time on a 18-55 kit lens from Canon. I am new to photography and wanted to work with filters. Do you suggest me buy these filters?

Answer

The front element of the 18-55 rotates when focusing, which makes using polarizing filters a bit irksome. But other than that, yes, it should work, although using multiple filters on a lens is something that should be avoided as far as possible on general principles because of the added risk of flare, internal reflections, vignetting and other optical nasties.

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