Photography Basics – Effects of filters on Exposure [Part III]

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In the Part I, we learnt about the basics of Exposure. In part II, we learnt about how to achieve optimum exposure for various scenarios that we encounter on day to day basis.

In this part, let us understand how to achieve optimum exposure using filters in some of the tricky situations.

Using Filters to Achieve Optimum Exposure

Despite the fact that some cameras have greater dynamic range and also there are tools to achieve High Dynamic Range (HDR), there is still a need for filters.

Filters usually manipulate the light in one way or the other based on their characteristics. Let us have a brief look at essential filters that we can use to achieve optimum exposure in the field.

Circular Polarizer (CPL) Filter

Circular Polarizer filter is one of the most important filters for the landscape photographers. A CPL filter is mainly used in landscape photography to:

  • Cut any reflections from the surfaces like glass, water, windows, etc
  • Darken the skies

When we photograph a reflective surface like glass, water, etc, the camera metering might go wrong indicating a wrong exposure. Since camera metering sensors uses the reflectance light meters, as we discussed in the Camera Metering Modes, it tends to give wrong exposure. This is especially true if the reflective surface takes up a large space in the frame.

Another important reason for using CPL filter is to darken the skies or saturating the colors, in general, by cutting the reflections from the subjects.

Since Circular Polarizing filter cuts the reflection, the meter reading is more close to optimum. However, in the process of polarization, the filter cuts some incoming light that enters the lens.

One should account for this loss of light by compensating for the exposure. For instance, if the CPL filter you are using, loses 1-stop, then you have to compensate for the exposure by adding 1 stop exposure to the meter reading.

Neutral Density (ND) Filter

A Neutral Density filter cuts down the light by the designated number of f-stops without manipulating the color.

A ND filter is named based on how many f-stops of light does it block. Examples of some ND filters:

  • ND2 filter – Blocks 1-stop of light or lets through only half of the actual light for a given aperture setting
  • ND4 filter – Blocks 2-stops of light or lets through only 1/4th of the actual light for a given aperture setting
  • ND8 filter – Blocks 3-stops of light or lets through only 1/8th of the actual light for a given aperture setting

For instance, if the aperture setting needed for an exposure is f/8 and you have added a ND4 filter to the front of the lens, then it will block the light by 2-stops making the effective aperture equals to f/16.

ND filters are very useful when long-exposure is necessary like waterfalls, night scenes and seascapes. If we want to achieve the long-exposure with a required aperture value and slower shutter speed, sometimes it is necessary to cut down the incoming light by several stops.

For instance you are shooting a seascape and you want to use a smaller aperture opening like f/16. The meter reading might show you to use a shutter speed of 1/15th of a second. However, you are interested in a slow shutter speed of 1 sec to smooth out all the waves in the water.

There is a 4-stop difference from 1/15th of a second to 1 sec (1/8 s, ¼ s, ½ s, 1 s).

If you just use the manual mode and try to bring the shutter speed to 1 sec, then the overall image will be washed out since you are exposing the sensor 4-stops more than the meter reading.

Instead, you would need a ND8 filter to achieve a shutter speed of 1 sec leading to an optimum exposure that you are aiming for.

Graduated Neutral Density (GND) Filters

Graduated Neutral Density filter is a ND filter which only blocks the light from a portion of the scene.

If one part of the scene is extremely bright compared to the rest of the scene, then GND filter is useful to block light only from the extreme bright portion thereby reducing the dynamic range of the entire scene.

For instance, a photograph of a Sunrise or a Sunset scene would usually have the upper portion of the landscape extremely brighter than the foreground and the middle ground. By using GND filters you could block the sunlight coming from the upper portion but allow the light from the rest of the scene.

If the upper portion (like Sunrise or Sunset) needs an exposure of f/32 and the rest of the scene needs an exposure of f/2.8 to f/5.6, then by adding a GND8 (3-stop reduction) you can get an overall dynamic range of f/2.8 to f/11.

In a way, you would have to compensate for the exposure manually by adding the required GND filter to reduce the dynamic range and then expose the scene accordingly.

Note: We will discuss about working of Linear and Circular Polarizing, Neutral Density and Graduated Neutral Density filters in-depth in a separate article.

In the next article we discuss about Exposure Triangle comprising of three core pillars of Exposure.

Which filters do you regularly use? Do you want to add anything about UV filter or Diffusion filter?

Nature Photography Simplified. Understanding Exposure, Aperture, Shutter Speed, ISO eBook. Written by Prathap.Buy Printable version of Understanding Exposure (9-Part Series) for $1.99 USD only! Contact for payment details.



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2 Responses to Photography Basics – Effects of filters on Exposure [Part III]

  1. Gerrie Malan April 4, 2014 at 8:05 am #

    Thanks again, Prathap.

    • Prathap April 6, 2014 at 11:43 pm #

      You are most welcome Gerrie 🙂

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