POST PROCESSING. EXPOSURE BLENDING
11/06/12How blend exposures.
Modern digital cameras and marvels of technology but one limiting element of the digital camera is dynamic range. This is the ability to capture all shadow and highlight detail in one shot. Film does have a wider range but how many of us relish film and developing costs when digital is effectively free.
Think of it this way. You are standing on top of a tor looking across the moor, the sky is perfect to your eye and so is the darker foreground. Then you take the shot with your camera. More often than not you will have either a perfect sky and very dark underexposed foreground or the well exposed foreground and blown out sky. Look on flickr, you will see thousands of landscape shots like this. Here is another scenario. You are standing in a church inner doorway. The inside of the church and the sky look exposed correctly to the naked eye but when the same scene is captured with the camera you will have a correctly exposed sky but the inside of the doorway will be just a dark silhouette. This phenomenon is not operator error but is due to the fact that the sensor in your camera has a far narrower dynamic range than your eyes.
So how do we ‘help’ the camera out?
Traditionally, especially with film, graduated filters were used. These are clear, optical grade resin or glass and have a darker upper half and are held in a holder on the front of your lens. The darker area of the filter is then positioned over the brighter area of the sky to assist the camera in balancing the exposure. These work very well and I still use Lee graduated filters that are generally considered amongst the best available
There is another way that is also cheaper as you have no need for the expensive filters. This is to take two different exposures of the same scene with a tripod, as they have to be identical and then blend them together on the PC.
The first and second images can be blended to give the third image. The end result is very similar to using a two stop hard edge graduated filter over the sky.
TAKING THE SHOTS
Set your camera to Aperture Priority and an aperture of f/8. Use the exposure compensation to adjust the exposure on the light meter to -1stop (usually three dashes down from the centre) Take the shot. Now without moving the camera at all (you should be securely on a tripod) adjust the exposure to +1 stop. Take another shot. You will now have two identical images one 2 stops darker than the other. One will be about right for the sky and the other for the foreground. Ideally these would be taken in RAW for finer adjustments but high quality Jpeg is ok.
Dark image for the sky taken at -1
Light image for the foreground taken at +1
NOW WE NEED THE COMPUTER....
Blending in Elements 9/10 shown as an example.
Open both the images in Elements.
Find the small square symbol at the top of the editor directly to the right of the ‘help’ tab. Click on it and then on the symbol on the left of the second row. This will separate the two images as below.
Select the ‘move’ tool, top left (arrowed) and then left click on the lighter image to select it.
Then do this exactly. Hold down ‘shift’ then left click on the lighter image and holding left click down move the lighter image over the darker one and let go of the left click. Then let go of the ‘shift’ key. The lighter image should now perfectly aligned over the darker one as shown on the right in the layers pallet. If it doesn't work first time, click on undo (the reverse blue circular arrow labeled 'undo' at the top of the editor) or press 'Ctrl-Z'. Then try again.
Then close the original light version as you no longer need it. You should now have the same as below. If you look at the layers pallet on the right you will see two images. The darker one is hidden under the lighter one.
Next you add a layer mask to the top image.
Now the clever bit. Blending them together….
Click on the gradient tool as shown below (arrowed) and then on the gradient at the top left of the window just below the blue PSE symbol. The window shown should open. This looks just like a graduated filter on its side, which is exactly what it is. If the gradient goes from black on the left to white on the right then click on ‘ok’ and it will close.
Now select a spot on the image where you wish the blend to start, for example just above the horizon and holding down the left click draw a vertical line down the page and let go just below the horizon.
When you have done this the images will be blended together. If you are not happy with the blend simply draw another line down the image in a slightly different position. There is no need to undo the previous blend as it is overwritten by the next.
When you are happy with the result simply click on ‘layer’ at the top of the editor and then on ‘flatten’ at the bottom of the menu that appears. The image is now one and can be adjusted normally as any other.
Here is another extreme example where the range of light as far too great for the camera to cope with. I have bracketed two images of the scene. As you can see the lighter one is perfectly exposed for the darker foreground. The darker one is perfectly exposed for the cottage and sky. The camera would not have been able to capture this scene correctly in one shot.
If you would like to practice this technique on the image shown as the first example please contact me and I will email you the two bracketed images to try for yourself.
Adrian Oakes 2012