All about ND Filters

I've been asked about ND filters at the last few workshops we've done, so I figured it's time to go over everything you need to know about ND filters.

What are ND filters?  

A ND filter is a specialty filter used to block a certain amount of light from reaching your camera's sensor.

Why would I want to block light?  

There are multiple reasons, but a few would be to slow your shutter speed down in order to do things like show movement, eliminate moving people from a photo or be able to shoot at a wide open aperture in bright daylight.

How are ND filter strengths labeled?

OK, let's talk about determining the strength of the ND filter.  Different manufacturers label ND filters differently.  Sometimes you may see a filter labeled as a ND4 filter,  sometimes you may see one labeled as a 0.6 ND Filter and sometimes it may be labeled as a 2 stop ND filter.  I'm going to attempt to explain why this is and how to know what strength a filter is by these numbers.

If you see a filter labeled with a ND #, like ND2, that is actually the attenuation factor of that filter.  I know...big word.  However, it's easier to remember it like this.  The number, in this case 2, would be the denominator, or bottom number, of a fraction.  So, if we have a ND2 filter, the fraction would be 1/2, so half the amount of light being allowed through the filter, or roughly 1 full stop.  A ND4 would be 1/4 the amount of light being allowed through the filter, or 2 stops.

Some manufacturers label ND filters in terms of their optical density.  Optical density is just a fancy way of describing the light blocking ability of the filter.  Those numbers will look like this: 0.6 ND Filter.  What you basically need to remember here is the higher the number, the more light that is blocked by the filter.

The most common way ND filters are refereed to are in stops.  Let's face it, as photographers we all talk in stops.  It's our language.  None of us really care about the optical density, we just want to know how many stops of light is our filter blocking. If you have a ND filter that is labeled with the Attenuation Factor or Optical Density, you can use the chart below to find out how many stops of light it is blocking.

Attenuation FactorOptical DensityLight Reduction in Stop #'s
ND20.31
ND40.62
ND80.93
ND161.24
ND321.55
ND641.86
ND10026 2/3
ND2562.48
ND4002.68 2/3
ND5002.79
ND1000310

 

Do ND Filters come in different shapes or sizes?

Yes.  They do.  Typically they will either be a round, screw on type filter or a square or rectangle shaped filter.

The round screw on types are pretty easy.  They screw on to the front of your lens threads and you would buy the size filter that you need for your lens.  For example, you have a lens with a 77mm filter thread, then you buy the 77mm round ND filter.  

The square or rectangular shaped filters come in various sizes.  They require a filter "holder" system in order to mount to your lens.  The "holder" system would consist of a ring that screws onto your lens filter threads and a "holder" that mounts to that ring that would have slots that you would slide your filter(s) into.

I prefer the screw on types myself because there tends be much less chance of light leak using these.

 

Are these the only types of ND Filters?

Not at all.  There are also Variable ND filters, Graduated ND Filters and do not forget in a pinch a Circular Polarizer can be used as a ND Filter.

A Variable ND Filter is a screw on type ND filter that has a variable amount of ND adjustment. So, you would turn it just like you do a circular polarizer to dial in how much light blockage you want.  Typically these filters will range from about 2-10 stops of light blockage. 

 

A Graduated ND Filter is most often in the square or rectangle format.  It is basically a ND Fitler with a graduation of ND effect.  Typically about half the filter has the ND effect.  So if you were in a situation where the sky was very bright and the foreground was dark, you would want to use a graduated ND to darken the sky, but have no effect on the foreground.  These also come in the round, screw on flavor, but they aren't as popular because the round ones limit your amount of control over the filter.  They also come in Hard Edge or Soft Edge.  A Hard Edge Graduated ND filter has an abrupt stop of graduation.  They would be best used in a situation where you would be photographing a flat horizon line, like at the beach. The Soft Edge Graduated ND Filter has the graduation taper off slower.  Those are best used in a situation where your foreground is not level, like mountains.   Graduated ND Filters also some in "reversed".  This means the graduation would be darker at the horizon line and fade out as it goes up.  You would use a Revers Graduated ND Filter when the horizon is much brighter than the rest of the sky, say at sunrise or sunset when the sun is right along the horizon line.

After saying all of that, I will tell you I do not regularly use Graduated ND Filters.   I typically take separate exposures while I'm in the field and blend them together using Photoshop.  This requires more post processing work, but it keeps me from lugging around extra filters and I feel it yields better results.

I typically keep Variable ND Filters in my bag at all times.  They are very convenient to use and eliminate the need for several filters in several different strengths.

Another thing to keep in mind is a circular polarizer can block a certain amount of light, too.  Depending on the brand of polarizer you have, it could block from 1-2 stops of light.  It can be used in a pinch to slow down your shutter speed, or if you have forgotten or do not own a ND Filter.

How much will an ND Filter slow down my shutter speed?

Let's take a look at how many stops of ND will have an effect on your shutter speed.

ND Filter In StopsShutter SpeedShutter SpeedShutter SpeedShutter SpeedShutter SpeedShutter SpeedShutter SpeedShutter Speed
0 Stops1/4000 second1/1000 second1/250 second1/60 second1/15 second1/4 second1 second4 seconds
1 Stop1/2000 second1/500 second1/125 second1/30 second1/8 second1/2 second2 seconds8 seconds
2 Stops1/1000 second1/250 second1/60 second1/15 second1/4 second1 second4 seconds16 seconds
3 Stops1/500 second1/125 second1/30 second1/8 second1/2 second2 seconds8 seconds30 seconds
4 Stops1/250 second1/60 second1/15 second1/4 second1 second4 seconds16 seconds1 minute
5 Stops1/125 second1/30 second1/8 second1/2 second2 seconds8 seconds30 seconds2 minutes
6 Stops1/60 second1/15 second1/4 second1 second4 seconds16 seconds1 minute4 minutes
6 2/3 Stops1/30 second1/8 second1/2 second2 seconds8 seconds30 seconds2 minutes8 minutes
8 Stops1/15 second1/4 second1 second4 seconds16 seconds1 minute4 minutes15 minutes
8 2/3 Stops1/20 second1/5 second1.6 seconds6 seconds25 seconds1 minute 40 seconds6 minutes26 minutes
9 Stops1/8 second1/2 second2 seconds8 seconds30 seconds2 minutes8 minutes30 minutes
10 Stops1/4 second1 second4 seconds16 seconds1 minute4 minutes15 minutes1 hour

You can see from the chart above that if you have a /4000th of a second shutter speed and add a 10 stop ND Filter to the front of your lens that will result in a shutter speed of 1/4 second.  Likewise if you have a shutter speed of 1 second with no filter and you add a 10 Stop ND, then you get a 15 minute shutter speed.

What kind of effect does this have on my images?

Let me show you a few examples.

ISO 400,  51mm,  F/8 @ 244 seconds

This is an image I made at sunrise on Folly Beach, SC.  I added a 10 stop ND filter to the front of my lens, which yielded a shutter speed of 244 seconds.  Without the filter my shutter speed would have been somewhere around 1/4 second.   The longer exposure here allowed the sensor to capture all the movement of the water  as a smooth, dreamy look.  My original shutter speed of 1/4 second would have frozen the water compared to this. 

ISO 100, 70mm, F/13 @ 25 seconds

This is an image that I took in the middle of the day in San Leon, TX.  After I added the 10 stop ND filter it gave me a resulting shutter speed of 25 seconds.  Although it isn't as long a shutter speed as the previous image, it was taken midday in much more light.  Without the filter I would have had a shutter speed of around 1/30th second.  The water was pretty choppy this day and the longer 25 second shutter speed created a much more visually appealing image.

Besides having ND Filters, you will also need a sturdy tripod since your exposures will be so long.  I use Sirui Tripods because of their stability and durability.  I would recommend them to anyone in the market for a good tripod.

That's all I have today.  I hope you learned a little something and I didn't take up too  much of your time. Happy Friday!