What Is a Neutral Density Filter?
A neutral density filter (ND filter) is designed to reduce the amount of light getting into the lens. The task of an ND filter is to darken your image, so you can use wide apertures or slow shutter speeds.
You can use a neutral density filter (ND filter) for a ton of creative purposes and improving the quality of your photos or videos.
What Can You Use an ND Filter For?
ND filters are traditionally a tool of filmmakers.
To capture the nicest looking video possible, you want to have motion blur, but not too much. A great rule of thumb is to use a shutter angle of 180°. This means that your shutter speed is roughly half of your frame rate.
For example, if you’re shooting a clip in 25fps, your shutter speed should be 1/50s. If you’re shooting in 120fps for slow-motion purposes, your shutter speed should be 1/250s.
This means your sole variable remains the ISO.
Now, it’s rare that you find ideal exposure circumstances on the location, especially if you shoot outdoors. You either have to use higher ISOs to brighten, or an ND filter to darken your exposure.
In videography, it’s ideal to use a variable strength ND filter. Otherwise, you would need a lot of neutral density filters.
If you’ve ever played with slower shutter speeds at night, you’ve also thought about how cool this could be in daylight.
Fortunately, an ND filter is perfect for that.
You can be creative with an ND filter and remove people from photos just by taking a long enough exposure. You can create light trails with moving cars, and much more.
Photography With Fast Lenses
If you want to shoot with a wide aperture lens utilizing its full capabilities, you may struggle to get a correct exposure in harsh sunlight.
Your ISO only goes down to 100, or a little below, and your shutter speed maxes out at 1/8000s.
An ND filter comes handy in this situation.
For this purpose, your best option is a 3-5 stop ND filter. That is enough to bring up your shutter speed to 1/2000s even at f/1.4. We also recommend getting a non-variable ND filter, because generally, they are cheaper and of better quality.
Photography With Flash in Daylight
Do you want to use a flash in daylight? This technique undoubtedly delivers eye-catching results, but it also requires very low exposures.
Even if you have High-Speed Sync it might be necessary to use an ND filter. And if you don’t have it, it’s inevitable.
Our suggestion is the same as the previous one here. If you get a 3-5 stop non-variable ND filter, you’ll be good to go.
How Do I Choose the Right ND Filter?
In addition to the previously discussed viewpoints, you have to consider a few things.
Screw-On or Slot Filters?
There are multiple arguments for both types.
Generally, screw-on ND filters are cheaper. They don’t have to be as big as slot ND filters.
If you’re only buying an ND filter for one diameter (most commonly 58mm or 77mm), it’s the more convenient option. If your diameters differ only slightly, you can use a step-up ring and use ND filters for the bigger one.
You can’t find variable ND filters for slot systems. You have to go for a screw-in ND filter if you need that.
On the other hand, if you need several different types of ND filter, we recommend investing in a slot system.
Standard or Gradual?
Are you planning to shoot a lot of landscapes? A gradual ND filter might be great for you.
Use a gradual ND filter if part of the scene (the sky, for instance) is too bright, or the foreground is too dark.
The strength of an ND filter can be written in two forms.
It can be in stops. A 5-stop ND filter reduces your exposure by five stops, so you can use five times slower shutter speeds.
Or, it can be the absolute amount reduction. A 5-stop ND filter lets in 32x less light, so it can be indicated as a 32x ND or ND32.
This type of ND filter allows you to change the strength by the turn of a ring.
These are called variable filters.
If you’re not looking for the best image quality, but rather flexibility, consider choosing this type of ND filter. They are usually more expensive.
The best use of a variable neutral density filter is video production.
What You Need to Be Careful About
First, you need to pay attention to the thickness of the ND filter. On wide-angle lenses, a thick ND filter might cover the corners of the frame. So, to avoid severe vignetting, look for a thin ND filter.
Also, think about the lens that you’re buying the ND filter for. ND filters, just like any other tool in photography, differ in quality. Buying a cheap, softer ND filter might make sense for a kit lens, but certainly not for a top-notch prime lens.
Sharpness is only one factor. The other thing is color shifting. Most, especially cheap filters have a color cast. You can correct that in editing. If you’re not familiar with color correction, keep that in mind when choosing your ND filter.
Do not confuse an ND filter with a circular polarizer filter. CP filters are used to cut reflections by filtering light rays in one axis. They also reduce the amount of light, but that’s not their main purpose.