What I Know About a Camera

Exposure Bracketing

In the last episode of What I Know About a Camera, Uncle George had learned that by moving the power switch for his new digital camera to the on position and pushing the shutter button he could take pretty good pictures. Uncle George also learned that the glass window on the back of the camera could be used to aim the camera at what he wanted a picture of, and that just like his plush Lincoln Town Car, his new camera was equipped with automatic everything.

This week Uncle George is faced with a new photographic dilemma. His niece Maria has just told him about an assignment her sixth grade teacher has given the class.  As part of their graphic arts program, they need to take some bracketed exposures using a modern digital camera and then use these images to overcome some typical imaging challenges.

Maria has just explained to her elderly uncle that exposure bracketing is when you take an image that is correctly exposed and then others that are under and overexposed. Uncle George can’t understand why you don’t just use auto mode, but Maria explains that digital cameras have a limited dynamic range and sometimes the camera can’t capture all the light levels in the scene resulting in lost detail in either the shadows or highlights.

Then Uncle George says to Maria: “that reminds me of my summer vacation to Yellowstone National Park in 1974.” Uncle George explains that he had used slide film to take his pictures that summer, and that sometimes he took multiple shots of the same scene adjusting the exposure time to be sure he got one well exposed image. Uncle George figured digital cameras must be like slide film, they have a limited ability to capture both bright and shadow areas in a contrasty scene.

Maria laughed and said: “slime film in Jellystone Park, you say the funniest things Uncle George!” Just then Maria’s friend Anne popped in for a visit. Maria and Anne had been working on their class project together. They were going to use the software application Bracketeer to process some exposure stacks they had shot the day before at the state park. Maria grabbed her MacBook and was heading to her bedroom, with Anne in tow, when she thought of her Uncle George. She thought about how he was so hapless and slow to learn. As she and Anne were heading out of the room, she laid her assignment handout in Uncle George’s lap and told him “if you read it real slow, you might be able to understand it.” As the youngsters were dashing out of the room, Uncle George shouted: “Won’t you need this booklet for your assignment?” As they disappeared down the hallway, a faint reply came back: “it’s all on the Web.”

Uncle George was more determined than ever to master digital photography. He read the handout from start to finish. He learned that exposure bracketing is just shooting multiple exposures of the same scene. Some cameras have ways to automate the process, but all cameras that can set exposure manually can do it.

He read that digital cameras have about six to eight stops of dynamic range and that isn’t always enough to capture all the light levels in a contrasty scene. For comparison, he read, negative film can have from 10 to 12 or more stops of dynamic range. This means sometimes he will need to use exposure bracketing to make good detailed pictures with his new digital camera.

George discovered that there are several imaging challenges that can be overcome using exposure bracketing. The most simple is optimal exposure, that is, choosing the best exposed image from among several. Exposure fusion can be used to produce a single low dynamic range image from a set of bracketed images. Finally, George discovered that bracketed sets of images can be used to produce high dynamic range (HDR) images.

Uncle George realized that bracketing isn’t all that complicated. You simply take multiple exposures of the same scene and then either pick the best single exposure from among the several images you have, or you create a composite image, using software, that combines the best parts of all the images. He thought to himself that compared to shooting slide film, with its narrow exposure latitude, this is a piece of cake. He mumbles to himself, “the trick to this new-fangled stuff is to just dive in and figure it out.”

George got up and went to the hallway closet where he rooted around until he found his old tripod behind a box of Christmas decorations. He dusted it off quickly as he walked into the dining room looking for his new camera bag. As he walked past the hallway to the bedrooms he shouted: “girls, come on, lets go to the park before the good light is gone…”

Tune in again next time for another episode of What I Know About A Camera.

Interesting Related Stuff

Bracketing: http://en.wikipedia.org/wiki/Bracketing

Dynamic Range: http://en.wikipedia.org/wiki/Dynamic_range

Exposure Latitude: http://en.wikipedia.org/wiki/Exposure_latitude

Bracketeer: http://www.pangeasoft.net/pano/bracketeer/index.html


Exposure Fusion
The first two images above are bracketed exposures that were fused to create the bottom image which has better detail in the highlights than either of the original images.