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MANAGING EXPOSURE WITH SPLIT NDS, EXPOSURE BLENDING, AND HDR 1

MANAGING EXPOSURE WITH SPLIT NDS, EXPOSURE BLENDING, AND HDR

Often when taking landscape photographs, you'll find that the sky is much brighter than the landscape. This can make it difficult to keep detail in the sky without the landscape being too dark, and to keep detail in the landscape without the sky being too bright.

Split Neutral Density (ND) filtersOne solution to this problem is the split neutral density filter. This is a filter that can be placed in front of the lens. One half of the filter is clear, while the other half is dark.

Tokina fisheye with Cokin P filter by Dave Kennard on flickr

Typically, you will place the dark section of the filter over the sky to darken the sky, while the clear section is placed over the landscape. You can then expose correctly for the landscape and don't have to worry about the sky being too bright.

ND Grad Filter Comparison by Jez B on flickr

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Split neutral density filters come in two varieties - graduated edge and hard edge. Graduated edge means that there is a soft transition between the dark and soft sections of the filter. Soft edge or graduated split neutral density filters are normally preferred for landscape photography because the transition looks more natural than a hard edge.

Hard edge and soft edge graduated split neutral density filters

Hard edge split neutral density filters are more useful when taking a photo with a well defined horizon that forms a solid line. The split line of the filter can then be placed along this line. They are especially useful when the sun is on or just above the horizon.

Both hard edge and graduated split neutral density filters are available in a range of strengths:

• ND0.3 = 1 stop• ND0.6 = 2 stops• ND0.9 = 3 stops

Graduated split neutral density filters – ND0.3, ND0.6 and ND0.9

The larger in difference between the brightness of the sky and the landscape, the stronger the filter you will want to use.

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Although split neutral density filters are available as glass screw-in filters (see second image below), most photographers prefer a Cokin P or similar 'slot-in' based filter system (see image immediately below). This allows you to easily rotate the filter and move the filter up and down, placing the dark section of the filter exactly where you need it.

Cokin Baby! by Dave77459 on flickr

The brighter and the darker side by Aditi-the-Stargazer on flickr

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The main benefit of split neutral density filters over software methods of equalizing exposure in a photo is that you can see the result as soon as you take the photo. It also saves on the work involved with software based solutions.

The disadvantage, though, is that split neutral density filters work best where the horizon is clearly defined. If you have objects, such as trees in your image, that are both below and above the horizon, you'll find that the top of the object will be darkened while the bottom will not. This is the main problem with split neutral density filters, and in cases like these a software solution to managing the exposure may be preferable.

In the example below, you can see that while the split ND filter does a good job at keeping the sky exposed properly, it also quite visibly darkens the top of the house:

ND8 grad filter comparison by zaphad1 on flickr

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MANAGING EXPOSURE WITH SPLIT NDS, EXPOSURE BLENDING, AND HDR 5

Exposure blendingExposure blending involves the process of taking a series of under exposed and over exposed photos of a scene, in order to capture the full range of details. This is usually done using auto bracketing to take the under and over exposed photos.

Using software such as Photoshop, the photos are aligned and layered on top of each other. Using layer masks you can then decide which area of each image to keep. By keeping the sky area from the underexposed image, and the landscape area from the overexposed image, you can get a well exposed image with details kept in both the sky and landscape.

Under exposed image Correctly exposed image Over exposed image

Images exposure blended together

Final image with additional processing applied

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You can also use software that will automatically blend the exposures together for you, such as tufuse and enfuse.

Unlike split neutral density filters, exposure blending can be used to manage the exposure of images where items extend across the horizon from the bottom of the image. You have precise control over which parts of the image should be bright, and which should be dark.

Petronas Twin Towers by rmlowe on flickr

The main problems with exposure blending is that it can be a long and fiddly process, and requires extra work. If you have moving items in your image, such as people, cars, or anything blowing in the wind, this can make blending the exposures difficult.

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High Dynamic Range (HDR) processingHigh Dynamic Range processing is similar to exposure blending, initially you must take a bracketed photo with under exposed and over exposed images to capture the full dynamic range of a scene.

EV-2 EV-1 EV0

EV+1 EV+2

You then use software to automatically blend these images into a single High Dynamic Range Image. The High Dynamic Range image can't actually be viewed on most monitors, but you can apply tone mapping to the image, which takes the tones of the HDR image and maps them into an 8 or 16 bit color space, so the image is viewable.

Tonemapped HDR

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There is a wide range of HDR software available, some of the more commonly used HDR software includes:

• Photoshop CS5 • Photomatix • EasyHDR • FDRTools

Most HDR software will give you a range of options when tone mapping an HDR image, allowing you to carefully define the look of the finished image. Some HDR software even includes 'de-ghosting', which eliminates problems with moving objects between photos.

Are there any disadvantages to HDR imaging? It may take you a while to get used to the software and tweak the settings to consistently get results you are happy with. Most HDR software is not free, but neither is it prohibitively expensive either.

The time taken to process images is longer compared to taking a single image with a split neutral density filter, but most HDR software offers batch processing. So you can get on with something else while the HDR software works away in the background.

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