Let's Talk Neutral Density Filters

The most popular use of the ND filter is introducing motion blur

Hello photo neighbours!

I received a question today from a really talented photographer. She has a great eye but does not, to her credit, get all wound up in tech and gear. She will be travelling to the Arctic later this year and is concerned about highly reflective subject matter amongst other shooting scenarios.

She already has a polarizer that fits one of her lenses, but not for the larger front element of her long zoom. She hopes to deal with wildlife including sealife and knows that the ocean and bays will be very reflective at that time of year because the sun is up most of the time.

When speaking with a local retailer, I am not sure what happened but it got on the topic of neutral density filters and the seller wanted her to buy a variable ND. She didn’t think that sounded right (she is correct) and reached out to me.

Neutral Density Filters - What Are They Exactly?

A neutral density filter, or ND filter reduces light transmission. That is all. Nothing more. Think of it as plain sunglasses for your camera, with no tint and no fancy logo etched frames sold for a fortune and made for pennies in southeast Asia.

I wander.

There are two primary uses for Neutral Density filters so let’s be clear on both of them.

Slow Shutter Speed Needed

You are out to photograph waterfalls. You make a light meter reading and with your camera set at ISO 100 recommends 1/125 of a second at f/16. You do a quick test exposure to confirm that the meter is not being fooled by highly reflective surfaces and are happy. Except the waterfall is frozen. You know that you need a slower shutter speed, but you have a challenge. Your camera does not go below ISO 100 which would allow for a slightly reduced shutter speed if ISO 50 was supported. Your lens only goes as small as f/22 so even if you set that, your shutter speed according to the meter is still going to be 1/60th of a second and that’s not slow enough to blur the water.

You are pretty sure that you need to get that shutter speed down to at least ¼ of a second. You do some quick photo math in your head, or consult an app or chart and discover that you need to reduce the amount of light entering the camera by four stops. Thus you know if you use a 4 stop ND filter you could get what you want, and if you used a denser ND filter, like a 6 stop version, you could get the shutter speed to one second and that would deliver really creamy looking water. You’re smart enough to know that means a tripod to prevent camera shake, and you are smart enough to have one with you. In fact, that’s my first rule of Neutral Density. Have a tripod with you.

Large Aperture, Shallow Depth of Field Needed

You have been voluntold that you will be making a portrait of a person and as is typical, you get little notice and it has to be made during the day and made outside. You get lucky and you can make the image just after a rain shower, so the light is soft and pretty. However, you find when you meter the scene, with your camera at ISO 100 that to shoot with the lens at f2.0 you need a shutter speed of 1/2000th of a second, which your camera will do but you would prefer to get a little bit of movement in the hair due to wind. You’d really like your shutter speed to be around 1/60th of a second but that means in this example that your aperture would be f/11. And that’s not the aperture you want for the shallow depth of field you know that you want to deliver. In this case, f/11 to f/2.0 is five stops, so a 5 stop Neutral Density filter will do the job for you. You can probably handhold 1/60th of a second, but maybe you are going to shoot this portrait using your 70-200 zoom and you are smart enough to know that there is a risk of camera shake at 1/60th of a second if your focal length is at 180mm for the look and composition you want. Remember the first rule. You have a tripod.

Thus whether you need to have a longer shutter speed or a wider lens aperture for less depth of field in a given shooting situation, a Neutral Density filter is going to be your problem solver. You consider these examples and go into a store and explain your desires, and the person suggests that a variable neutral density filter will give you flexibility. This would be true if most variables weren’t such pieces of crap.

Why Are Variables So Awful?

A variable neutral density filter is actually two polarizers mounted together. The inner one screws to the lens filter mount and is fixed. The outer one rotates like a regular polarizer and by rotating it you get varying levels of light cut. Sounds brilliant, however consider the cost of a really fine polarizer. Now double that and that is where you would need to be for a good variable. Hmm, the variable the store has is actually less expensive than a single polarizer. What does this mean? It means that the variable in question is make with two really lousy polarizers and you should avoid it like you would a pit viper.

Fixed Density Neutral Density Filters

Ok then, variables are out, so that leaves fixed density Neutral Density filters. Where do you start? First remember that a polarizer is the most useful add on filter that there is and a decent one cuts about two stops of light transmission so you already have a 2 stop ND in your bag. Think about the kind of things that you will need to do where an ND filter will be beneficial. You will likely discover that a 5 or 6 stop ND is a good starter, and if you want to really cause slow shutter speeds, your second ND should be about 10 stops.

If you buy good ND filters, you can even stack them, just watch for vignetting at the edges of the frame.

Mounting Systems

Most lenses can take a front mounted screw in filter. Some super long telephotos offer a drop in tray near the rear of the lens to add filters. Some wide angle lenses don’t take front mounted filters at all because of ultra wide angle of view or a very bulbous front element. For lenses like that you need a special mounting frame for that lens and appropriate density Neutral Density filters to fit in the carrier for that frame. There are also filters that are magnetic attachment. Interesting I suppose, but I’m not sure that the time saving is worth the extra cost, and the taller rings make me worry about vignetting.

What Makes a Good Quality Neutral Density Filter

You are making an image through this filter. Thus it should be made from optically correct glass whose faces are perfectly parallel. The glass can be dyed, or can actually be two pieces sandwiching an ND gel that you cannot take apart. Either work. Some of the filters that go in the frame mounts are made of a polymer. I have not had great success with these myself, but your mileage may vary. Whatever you choose should be multicoated for best transmission and some filters have nano coating for durability. While I have heard good things about different brands, I only recommend that which i have used and chosen to purchase. For screw on filters, I recommend the B=W MRC Neutral Density filters or the Heliopan multi coated units. I have bought and own both brands. For screw in filters buy the filter to fit your largest from diameter filter mount and then get stepping rings to use it on smaller diameter lenses. If you are like me and go out with two bodies each with its own lens, just as I will have a polarizer for each lens, I will have an ND for each lens. In my case the largest filter mount for lenses that I own is 82mm so that’s the size that I buy, along with step rings to use on smaller front element lenses.

For the frame type mounts, such as I required for my 14mm or the 11-24mm I use filters from Lee Filters and Haida. Both have proven to be excellent. Haida even makes rear drop in filters for my 11-24 and my fisheye zoom. Good news that. Some pundits have complained that they get a green tinge from some Lee Filters. If I am shooting a long exposure, I shoot in RAW and so there is no actual colour cast consequently this complaint has no value. I don’t care about a colour cast in the JPEG on the camera LCD, that’s just pointless.

I looked to B&H Photo Video to gather pricing for this article and 6 stop screw ins or 10 stop frame mount filters all came in around $200 USD. Not cheap, but a good filter, well cared for lasts a long time. If I spent $2K on a lens, I’m not going to screw up the image quality with a cheap ass filter.

Challenges In Nomenclature

One of the most frustrating things about buying ND filters is how they are named. A 6 stop ND is the same thing as an ND 1.8 as an ND 64x. This naming confusion is just stupid so be sure you understand what the filter is going to do. Buy from a reputable shop and if going into the store, know your stuff going in, because the person behind the counter will have received limited if any training and will be motivated to sell on margin, not on your needs. Sorry if that seems harsh, but it’s proven true every day. Also plan on ordering ahead because most retail stores don’t stock this kind of thing if you want quality. Consider ordering through a trusted online shop if you cannot find what you want easily locally. For me in Canada, B&H is typically about three business days after they ship and they handle all the customs nonsense as an option.

What About Graduated ND Filters

The use for grads diminished with the arrival of high quality post processing. I dumped all my grads while I could still get some coin for them because I can do graduated filter effects with more precision and control in Lightroom or Photoshop. I’m not shooting film for the most part any more and so I am dealing with a native dynamic range of 12-14 stops, not 6 as I was with film. I make exposure decisions and can always pull down whites and highlights.

Some cinematographers still use graduated ND filters but as I shoot only video, I can do the same thing in post processing using my video editor. I prefer to keep things as simple as possible in the field and most professional video cameras even have built in ND filters if you need to cut the incoming exposure in camera.

Final Thoughts

After the polarizer, the neutral density filter is the next most useful filter. If you can only afford one, get a 5-6 stop unit that will fit your desired lens. Buy quality and don’t skimp, And remember that an ND filter cuts light transmission, nothing more. it won’t manage reflections or deal with colour issues.

Do you have an idea for an article, tutorial, video or podcast? Do you have an imaging question unrelated to this article? Send me an email directly at ross@thephotovideoguy.ca or post in the comments. When you email your questions on any imaging topic, I will try to respond within a day.

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I'm Ross Chevalier, thanks for reading, watching and listening and until next time, peace.

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