Combining 16-bit auto exposure bracketed images

When using Autoexposure Braketing, available on most digital prosumer and professional cameras, the camera automatically changes the exposure level within the set range for three successive frames, providing an image at the correct exposure, one underexposed and one overexposed. This tutorial will show how to combine the three frames in Photoshop to create one final image. We have also chosen to capture our images at 16-bits per channel.

This technique is extremely useful when the lighting conditions are tricky and photographing the right shot is proving difficult. We have provided the three test images that you can download to use while following this tutorial. To open click on each of the following links img_1217.jpg, img_1218.jpg and img_1219.jpg and then right click and select download placing them in an appropriate location that you can find easily.

Before we begin let's first understand the term 'bit'. 'Bit' depth (also referred to as pixel or colour depth) measures how much colour information is available to display or print each pixel in an image. A single bit can store two possible values - black and white, whereas an 8-bit pixel has 256 possible tonal values and a pixel with a bit depth of 16 can store up to 65,536 tonal values per channel.

The benefit of capturing and using 16-bit images is simple; when making any tonal or colour correction changes the amount of detail lost through editing means that the image after manipulation will still have more tonal values per channel than if we'd started with an 8-bit file.

8-bit files provide excellent results, but containing only 256 tonal values per channel it is important that we capture them correctly and minimize the amount of correction that is required.

16-bit images can only be captured at present with digital cameras set to use the RAW file format. Unlike the familiar formats TIFF and JPEG that capture only what the camera sees, RAW files capture much more information than is displayed by the camera and many options are available to improve the exposure or brightness, add or remove the sharpening, adjust the image colour temperature and more using Photoshop's Camera RAW plug-in. Editing files in this way is an opportunity to perfect the image without losing any detail from the original shot.

1. Using either the 'File Browser' or the 'Open' command, navigate to the supplied images and open all three. 2. Choose... 'Window' > 'Arrange' and select 'Tile', to organise all the images to view on the screen side by side. Click onto the image named 'img_1218', this will be our base file, it was captured using the correct exposure. The file named 'img_1217' is the lightest image and 'img_1219' is the darkest. By combining elements of these two images into the base file, we will end up with a new image that is completely unlike any of the source files tonally. 3. Choose... 'Window' and select 'History'. The 'History' palette should now be visible.

4. Select the lightest file 'img_1217'. From the 'History' palette, drag and drop the documents 'Snapshot' history state thumbnail (small image icon at the top of the palette) onto the base image ('img_1218') document window. Click onto the base file to see two history states present in the 'History' palette. 5. Double-click on the history state 'img_1217' in the base file and rename 'lighter'. 6. Now, select the image 'img_1219' document window, and drag the documents 'Snapshot' from the 'History' palette onto the base file's document window.

7. Return to the base file and you should now have 3 history states in the 'History' palette. Double-click on the 'Snapshot' named 'img_1219' and rename to 'darker'. History states should be ordered from the top: img_1218, lighter, darker. 8. Close the files 'img_1217' and 'img_1219'.

9. In the 'History' palette, click on the snapshot 'img_1218' to load the original state of the image into the document window. The selected thumbnail is identified with a small marker to the left of the 'Snapshot'. From the 'Toolbox', select the 'History Brush Tool'. This tool uses a 'Snapshot' from the 'History' palette as the source when painting. Click to the left of the 'lighter' thumbnail to activate it as the source for the 'History Brush Tool'. Note: There should now be an icon of the 'History Brush' displayed next to the 'lighter' snapshot. The small marker identifying the 'img_1218' thumbnail as being displayed in the document window should still be pointing to the snapshot 'img_1218'.

10. The key to making this work successfully is by altering the opacity of the brush as you paint detail onto the base image. Unless you are using a Wacom tablet and pen which is pressure sensitive, then I suggest that you use the keys 1 - 0 along the top of the keyboard to quickly change brush opacity. 1 = 10%, 3 = 30% and 7 = 70%, etc.
With the 'lighter' thumbnail still selected as the source for the 'History Brush', paint over the lightest areas of the image to lighten them further. You can also paint over areas that you would like to be lighter.

11. In the 'History' palette, change the source of the 'History Brush' to the 'darker' snapshot. The 'History Brush' icon should be displayed next to the 'darker' snapshot. The small marker should still point to the 'img_1218' snapshot.

12. To emphasize the flower we need to darken the background.
Increase the size of the 'History Brush' using either the 'Brush Options' in the 'Options' bar or by using the keyboard shortcut right square bracket ']' and paint over the background.
I have also chosen to reduce the size of the brush and paint over some of the dark areas of the flower to increase the contrast throughout the image.

13. Click on the small camera icon at the bottom of the 'History' palette to create a new 'Snapshot' of the finished image. Double-click and rename the new thumbnail 'Final'.
Finally, click through all snapshots to see how the different versions vary. The 'Final' image should appear much more dynamic than any of the bracketed versions.

About the Author
With more than 12 years' experience working with photographers and their images, Terry Steeley is recognised as a world-class Photoshop specialist, having for the past four years represented Adobe as an authoritative speaker at worldwide trade shows and seminars.

His passion for photography and enthusiasm for sharing his knowledge has led him to develop his highly-acclaimed courses catering for photographers of all abilities - from the novice and enthusiast to the amateur or semi-professional.

Terry's relaxed style ensures that people can easily understand and absorb his knowledge. Covering all aspects of capturing and editing digital images, his teaching blends traditional film techniques with today's modern digital equivalents providing essential viewing for all interested in exploring digital photography.

Chris Kitchener - Adobe Systems added:
"One of the most natural and gifted presenters of Adobe Photoshop… the audience never fails to be engaged."

Photoshop4Photographers
The 'Photoshop4Photographers' seminar is an essential day's training specifically tailored by photographers, to meet the needs of the novice, enthusiast, amateur or semi-professional photographer, capturing and editing full colour RGB images digitally.

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