Filters are pieces of glass or plastic that are inserted in front of the lens to produce photographic effects that are often impossible to re-create in post-processing.

As good as our digital cameras may be, they cannot change the physical properties of the light entering the lens. Post-processing using software such as Photoshop or Lightroom can go a long way in improving the look and feel of our photographs, but it is not without its limitations. One example of this would be the removal of reflections in water using a polarizing filter.

There are four main types of filter:

  • Neutral Density
  • Graduated Neutral Density
  • Polarizer
  • Ultra Violet (UV)

Below I’ll walk through each one, explaining what they are and how to use them.

Neutral Density (ND)

Neutral density filters do nothing more than reduce the amount of light that passes through the lens. They are sunglasses for your camera. Ideally, light reduction should be equal across the spectrum and the colour cast of the image should not be altered (this is true of the more expensive filters). This is why they are called “neutral” density filters (neutral colour cast). They are used to increase exposure time without the need to adjust ISO or aperture and come in a range of densities; typically, 1-stop, 2-stop, 3-stop, 4-stop, and 10-stop. These ranges are usually expressed in terms of the density of the filter:

  • 1-stop = 0.3ND
  • 2-stop = 0.6ND
  • 3-stop = 0.9ND
  • 4-stop = 1.2ND

And they look like this—a square piece of glass with a grayish hue:

When do I use them?

ND filters essentially help you keep your desired depth of focus. Their most common application is to smooth our flowing water. For instance, say you take a shot of a waterfall with the following settings…

f/16, 1/4, ISO 100

… and want to take a longer exposure to smooth out the flow of the water. Well, you can’t drop the ISO setting because you’re already at ISO 100. Aperture is already at f/16 and so even if your lens went beyond f/22 (unlikely) diffraction would become evident in the photography. So you use a ND graduated filter (2-stop, say). By attaching this filter to the front of your lens you can change the settings to…

f/16, 1″, ISO 100

… to give you the image that you wanted!

It’s worth giving a special mention to the 10-stop filter, if only because I couldn’t possibly write about ND filters and pass it over. The principle is exactly the same as explained above. But whereas a 2-stop filter increases exposure time by 4x, a 10-stop filter increases it by 1024x! They can transform some of the most plain daytime scenes, almost magically turning water to mist, clouds to streaming streaks of white, and vanquishing passerby to a binary hidden deep within your image files…

Graduated ND

One of the most useful filters for a landscape photographer. These are half-and-half graduated filters—half graduated filters, half clear plastic. There are two types: soft and hard, referring to the “speed” with which the filter transitions from neutral density to clear. Here is what they look like:

Like ND filter cousins, each comes in a range of densities, typically: 1-stop, 2-stop, 3-stop, and 4-stop (again, often expressed in terms of stops like the table for ND filters above).

When to use them?

Unlike ND filters, ND graduated filters are not designed to help you increase exposure time; they are to help maintain contrast in a scene with a high dynamic range.

As you can see, the sky is “blown-out” or clipped (it’s just white in some places with no detail)—as is evidenced by the histogram shooting off to the right. So although technically the camera has calculated the exposure “correctly”, it really doesn’t look too good. Now this problem occurs whether you have a £100 point-and-shoot or a £2000 DSLR (the above image was taken on a Canon 5D Mark III). Fortunately, our ND graduated filter can help us by reducing the light in the top half and creating a more balanced scene. By doing this you can retain the detail and make the image more impactful:

How to use them?

1. The technical way

(Note: It helps to have an understanding of metering for the below)

Let’s take our scene above—a very typical composition in landscape photography, especially if shooting during daylight. First turn your camera to “spot metering” or “partial metering”. Now with your desired ISO and aperture, aim the centre of the viewfinder just above the horizon and note down the shutter speed. Here are the readings for the above scene without a filter.

f/8, 1/250, ISO 100

Now, without adjusting anything, point the camera at the ground, halfway between the horizon and the bottom edge of the desired frame. Again, take the exposure readings. Readings:

f/8, 1/60, ISO 100

You will use this second set of exposure settings for the photograph. But first note the differential between the shutter speed in the first and second composition (1/250 vs 1/60).

This means that the lower half of the image is 2-stops away from the top half (because 1/60 x ½ x ½ = 1/250). So we need a 2-stop ND Graduated Filter to balance out the light.

Now, you can either:

a) Press “auto-exposure lock” (AE lock) whilst pointing the camera down for the second composition; or,

b) Switch to manual mode and configure the settings yourself (this is better whilst you’re getting the hand of this technique).

Next, frame the image as you want, and once focused switch the lens to “Manual”. Finally, drop the 2-stop ND Graduated Filter down so that the point the filter dissipates into the darker grey meets with the horizon. Now you’re ready to press the shutter for one perfectly exposed photograph:

2. The easier and quicker way

Ignore all of the above and with the filter in place just take a few shots in manual mode, adjusting the exposure value each time until you get shot with balanced contrast (still advisable to switch to manual focus once the scene is composed and to keep an eye on your histogram). By the time you’ve messed around using the first method chances are that you’d have already captured the scene as you want it after few trial-and-error snaps.

Polarizers

Along with graduated ND filters, a polarizer is an indispensable part of a landscape photographer’s kit. Polarizers can mean the difference between a lifeless shot and vivid colours with a beautiful, azure blue sky. Moreover, you cannot simulate the effects of a polarizer using post-processing techniques, and it is for this reason that if a photographer could only carry one filter many would choose a polarizer. Here are some before and after shots to illustrate the effect a polarizer can have on your image:

Without a polarizer
With a polarizer

As demonstrated at the top of this page, they can also be used to kill reflections in water and glass (so can come in handy when you’re trying to get that city night shot through the thick glass of the viewing deck!).

The building to the left is clipped and the dynamic range is impossible to control. On the right a polarizing filter has been used to kill these reflections.

Note that there are two types of polarizers: linear and circular. For digital cameras, you should buy a circular polarizer.

The technical stuff on polarized light can be found here.

How do I use them?

Once attached (either to the lens thread or a filter adapter) circular polarizing filters will spin through 360 degrees enabling you to twist it around until you have the desired effect. Polarizers work best when the scene is at an angle of 90 degrees to the sun. If the sun is directly behind or in front of your scene it will have little or no effect on the sky.

Polarizers will reduce EV by 1-1½ stops so you should compensate accordingly when using one. This means you should:

  1. Set up your shot without the filter attached
  2. Note the exposure, aperture, and ISO settings.
  3. Switch to manual mode on your camera and turn the lens to manual focus (important as the camera may have difficulty focusing through the polarizer);
  4. Either increase the ISO, stop down or reduce the shutter speed by 1-2 stops.
  5. Attach the polarizer and twist until you see the desired effect through the viewfinder
  6. Viola! Take the photograph

It is not unheard of to see photographers walking around with their polarizers permanently attached to the lens. This is not advisable. Unlike a UV filter, a polarizer can dramatically change your images (often for the better but sometimes for the worst) and, in any case, the fact that they reduce light entering the lens itself should be enough of a deterrent, unless you want photographs that are always slightly under-exposed.

Polarizers and wide-angle lenses

At focal lengths less than (roughly) 24mm use of a circular polarizer can create a wave-like darkening of the sky of questionable aesthetic value so you need to be extra cautious when using a polarizer on a wide-angle lens.

The result of using a polarizing filter on a wide-angle lens.

Trouble-shooting

If you’re not getting the results you expect from your polarizer, remember the following points:

  1. Polarizers will have almost no impact when the sun is directly behind or in front of the camera.
  2. Effects are equally minimal when you have a thin mist of cloud (light from clouds is not polarized in any direction).
  3. Polarization through reflection (from a horizontal surface, e.g. water) is at a different angle to polarization from the sky and so you cannot darken the sky and kill reflections in a lake at the same time.
  4. Check it’s been fitted correctly at the factory. Yes, amazingly enough, it’s worth making this simple check. I can tell you from personal experience: a £250 ($400) LEE circular polarizer filter had the thread attached the wrong way round, which says nothing for their basic quality controls. Checking that your circular polarizer has been fitted the right way round takes 10 seconds and it’s a worthwhile check (a cursory glance through some internet forums seems to suggest I’m not the only unfortunate recipient of a dud). To do so, simply point your circular polarizer at your computer screen so that you are looking at the screen with the thread closest to your eye (i.e. as if your eye is the camera with the filter attached). Twist it and you should see it go from clear to black (blocking the screen entirely). If it does not (and instead just changes the hue of the screen) then you need to make a phone call to the seller or manufacturer…

Ultra-violet (UV) filters

Effects of the UV filter are not visible to the human eye but can reduce contrast on a hazy day. These are basically a remnant from the days of film—digital sensors are far less sensitive to UV light. Their primary use nowadays is to protect the front of your lens and as such many people (including myself) have them as a permanent attachment.

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