There are two filters that almost any professional photographer always has in his or her gear bag: a neutral density (ND) filter and a graduated neutral density (GND) filter.
These two filters are useful because they allow you to control exposure. By using these, you can reduce the amount of light reaching your camera's image sensor either over the entire scene of your image (in the case of the standard ND filter) or over just a portion it (as with a GND filter). This reduction in light allows for leeway in terms of the shutter speed and aperture settings that are required for proper exposure of an image and this, in turn, gives the photographer the opportunity to use these settings more creatively.
So let's talk about the basics of neutral density filters.
A neutral density (ND) filter reduces the amount of light that enters the camera lens. This type of filter is neutral gray in color so that the amount of light entering the lens is reduced, but the color of the light isn't changed. The darker the gray color, the more the light is reduced.
Reducing the light entering your lens gives you added flexibility in terms of exposure. A neutral density filter can allow you to use a large aperture opening to create a shallow depth of field, or a slow shutter speed to create the blur of motion, even when the actual scene lighting won't allow for these exposure settings. By using a neutral density filter, you make the scene darker so that the slow shutter speed or the wide-open aperture work to give your image proper exposure.
ND filters comes in varying strengths. And, not surprising, different manufacturers use different systems for specifying the strength of their filters. There are several different methods for labeling neutral density filters.
But, first, in order to understand these different labeling methods, you need to be familiar with the term stop. In photography, light and exposure settings are discussed in terms of stops. A stop represents a relative change in the brightness of light. If you increase light by one stop, you're doubling it. Conversely, if you decrease light by one stop, the amount of light is halved.
Okay, so with that in mind, let's talk about how neutral density filters are labeled and what that label means relative to how much light the filter blocks.
Some filters are labeled by the number of stops the filter blocks.
So under this labeling method, a filter would be labeled as one stop, two stops, three stops, etc. One stop means half the light is filtered out; two stops cuts the light in half again, to give just a fourth of the light that would otherwise be coming through the lens.
Some filters are rated based on the amount of light they allow through on a fractional or percent basis.
As an example, consider a filter that reduces incoming light by one stop. When light is reduced by one stop, the amount of light entering your camera lens is halved. So the amount of light that makes it into the lens is ½ or 50% of the original light. If your filter reduces light by two stops, the amount of light is reduced by half again, leaving just ¼, or 25% of the amount of light reaching the camera lens. A filter that reduces light by 3 stops is reducing the amount of light to just 1/8 or 12.6%.
A filter labeled using this method would be marked with the amount of light that is transmitted through the lens, notated either as a fraction or a percentage. So a filter that reduces the incoming light by 1 stop would be marked with either ½ or 50%. A 2-stop filter would be marked with either ¼ or 25%.
By the way, for the math nerds (like me) among us, the transmission fraction equals ½ raised to the number of stops the filter blocks.
This method is similar to the fractional method above but here the labeling is based on just the denominator (the bottom part) of the fraction. With this method, a filter that reduces the light by 1 stop, or by ½, is labeled as a 2 with this digit either followed by an x or preceded by an ND. So a 1-stop ND filter (that reduces light to ½) would be labeled either 2x or ND2. A 2-stop filter (that reduces the light to ¼) would be labeled either 4x or ND4.
This last method rates filters based on their optical density (OD). Optical density is a physics terms that describes how much light moves through a lens or filter and it is defined as the base 10 logarithm of the inverse of the filter's transmission fraction:
I know that this may seem complicated. Don't worry about that. It's not as complicated as it seems. And, being the certified math nerd that I am, I can tell you that this all boils down to a pretty simple relationship between filter density and stops. It's this. To convert the density number to the reduction of stops across the ND filter, divide the density number by 3 and then multiply the result by 10. Voila!
So, if you have a filter rated with a density of 0.3, that filter will reduce the incoming light by one stop (.3 ÷ 3 x 10 = 1); a filter with a density of 0.6 reduces the light by 2 stops.
The table below shows the four labeling methods side-by-side:
|Stops||Transmission Fraction||Transmission Factor||Filter Density|
|1||½ or 50%||2x or ND2||0.3|
|2||¼ or 25%||4x or ND4||0.6|
|3||1/8 or 12.6%||8x or ND8||0.9|
|4||1/16 or 6.25%||16x or ND16||1.2|
|5||1/32 or 3.125%||32x or ND32||1.5|
|6||1/64 or 1.563%||64x or ND64||1.8|
|7||1/128 or 0.781%||128x or ND128||2.1|
|8||1/256 or 0.391%||256x or ND256||2.4|
A graduated neutral(GND) density filter, sometimes called a split neutral density filter, is a lot like a standard ND filter, but the graduated version only blocks light from one-half of the lens area. Graduated ND filters are neutral gray on one side and fade to clear close to the middle of the filter. Where a standard ND filter reduces light over the entire area of your scene, a graduated filter only blocks light on one side of the scene. That allows you to expose different parts of your scene differently.
Graduated ND filters are great in situations where the brightness of your scene changes along the horizon, like when the sky is brighter than the ground below it. A graduated ND filter allows you to darken the sky area so that you can get an exposure that works for both the land and the sky. For this reason, this is a filter that is particularly helpful for landscape photographers.
For example, consider trying to photograph a bright sunset with a darker area of grass in the foreground. If you set your exposure for the sky, the grass in the foreground will be dark and underexposed. But if you set your camera to properly expose the grass, your sky will be washed-out and overexposed. Using a graduated ND filter, with the darker side of the filter covering the sky area, allows you to achieve a balanced exposure for both parts of the scene.
As with standard ND filters, the strength of the graduated filter depends on how dark the gray side is. And you'll find that these filters come in two varieties, featuring a transition from the gray area of the lens to the transparent area that is either hard-edged and abrupt, or soft-edged and gradual. I prefer the gradual transition myself for a more subtle and realistic effect.
Now that we've talked about the two kinds of neutral density filters, there's one more thing you need to think about when considering a purchase: the filter shape. Both ND and GND filters come in two basic shapes: circular and square/slot.
The simplest ND filters are the circular, threaded variety. These circular models screw onto the front of your camera lens, making them quick and easy to mount since no other accessories are required. And since these lenses screw directly onto the front of the lens, there isn't a problem with light leaking around the edges and you don't have to worry about them moving or sliding off.
But there are disadvantages to using a circular filter. The first is that you can run into vignetting problems if you attach more than one filter to your lens at a time and use a wide-angle lens. (Vignetting is the term used when the edges of the filter show as dark corners along the outer border on your image.) Another problem is that it can be difficult lining up the horizon with your composed image when using a graduated ND filter.
Finally, circular filters are sized based on diameter, typically measured in millimeters, and so will only work on a lens of the same diameter. So you need to have a filter that matches the diameter of your lens. Lens diameters can vary widely, usually ranging from 46 to 82 mm. So it's possible that you will have to purchase a different filter for each lens you own. And that can get expensive.
By the way, you'll find the diameter of a lens printed on the front or along its barrel. It's usually the number preceded by a circle with a line going through it (Φ).
On the other hand, a square type filter (actually, they're usually rectangular) is held in place via a filter holder that is attached to your lens. This type of filter requires a bit more set up—you have to attach the filter holder first—but once that's done, you can slide one or more filters in place quickly and easily. That's a big advantage when you want to use more than one filter at a time or when you want to switch between filters frequently.
Another advantage to using square filters is that they aren't dependent on the lens size. Most square filters can be used on lenses of any diameter.
But, even though the filters themselves are flexible in terms of the size lens that they work on, the same isn't completely true for the lens holder that keeps the filter in place. These holders screw onto the front of the lens via an adapter ring. So you will need to purchase different adapter rings for different size lenses.
The biggest disadvantage of using square filters is that they can be awkward to use. The set up and form factor of a square filter is simply more cumbersome than that of a circular one. Square filters can move, slip around and fall off. And, since they aren't attached directly to the front of the lens, you can have problems with light leaking into the lens from around the top and bottom of the filter.
We've given you a lot of info on neutral density filters. But there's a lot more to say. So if you are interested in more information on neutral density filters, here are some links and a video from Gavin Hooey that I think you'll find helpful.
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