There are many photographic situations where the range of luminosity values between shadows and highlights is too great to be accommodated in a single exposure. This is just as true with modern digital imaging as it always has been with film.
With film the solution is to use graduated filters to lower the luminosity range, typically using the upper dark area of the filter to reduce the luminosity of the sky while leaving the foreground unaffected, resulting in an image which holds detail throughout. It many landscape images this approach yields good results, provided of course you have the correct filter strength to hand. In some cases however it is difficult to use a grad filter without its use being obvious, this is particularly true where the transition between dark and light is not a reasonably straight line. Shots using grad filters which have an uneven horizon, such as mountain ranges, or which have trees or other elements poking up into the sky (like the image below) can be impossible to achieve with a grad filter without the telltale darkening in the upper part of the image.
Digital imaging pretty much does away with the need to use graduated filters to hold detail in skies, and allows any scene with a very high contrast range to be visualised by combining separate exposures which individually carry information on different parts of the total image. There are several ways of achieving the blending of two (or more) images to encompass a large contrast range and there are a number of web pages devoted to discussion of image blending. The summary on Luminous Landscape is a good place to start if you are new to this topic.
|shadows exposure (1/8 @ f16)||highlights exposure (1/30 @ f16)|
|The two images above of Llandaff Cathedral were taken at 8 o'clock in the morning at the end of September. I had previously taken one exposure at the matrix metered setting of 1/15sec @ f16, but after checking the camera review screen and histogram it was clear that the contrast range was too great for a single exposure. Taking two images at one stop above and one stop below the meter reading gave much better results for the shadow and highlight regions respectively. These two images were then blended together in Photoshop to give the first image below.|
|blended image||final image|
The blending process is a relatively simple procedure,
which can be carried out manually, but is much easier using an automated
action. Blending is achieved by placing the two images on separate layers
(light image below the dark image) and then using a layer mask based on the
lower image to control how the two images are blended. When the lower image
is applied as a mask to the upper image the colour image is converted to a
greyscale mask with the brightest areas being white and the darkest areas
black. Since layer masks control the transparency of their associated image
by making black mask areas transparent and white areas opaque the two images
are blended to produce an image with details in the highlights and the
shadows. In the example above the mask made from the shadows exposure is
white in the sky areas so the sky in the upper image remains opaque and is
seen in the final image. Conversely the darker foreground area of the mask
renders the upper image transparent in these areas so that the shadow detail
present in the lower image shows through. To prevent any artefacts arising
during the blending process at sharp edges between shadows and highlights,
the layer mask is softened using a small amount of gaussian blur. Once the
two images have been blended, either the upper or lower image layers can be
fine tuned using levels or curves to fine tune the final image, as shown
The Exposure Blend action automates the basic stages described above for merging two exposures. The action guides you through opening two images in the correct order, converts both images to 8-bit (16 bit files cannot be layered in most versions of Photoshop), and then creates a blurred layer mask to blend the images. To download the action click the link below. If you are not familiar with loading and running actions, read the actions guide.
See the comments on the technical intro page if you are using a Mac.
|Exposure Blend (1.3kb)|