Archive for the ‘Photo of the Week’ category

Sunset

August 22, 2011

Snow

January 19, 2011

 

 

Adam

July 29, 2010

“That’s my brother!”  —Bob Morales, La Bamba.

POTW #13: Safe

May 25, 2010

POTW #12: Chipping Sparrow

May 11, 2010

Chipping sparrow (Spizella passerina), Durham, NH. 7 May 2010. 72mm, f/2.8, 1/50 sec, distance 3 meters.

POTW #11: Miniature Beth Flying a Kite

March 31, 2010

We have a pretty involved model railroad setup in our basement…

Photo of the Week #10: UCLA in High Dynamic Range

February 10, 2010

click image to view larger

I was at UCLA a few weeks ago for a plasma physics winter school, a week long workshop for grad students and post-docs. In the evenings we had homework sessions on the roof of the physics building, and one evening I took several shots of a building across the street. The above photo is a “tonemapped”, high dynamic range (HDR) image compiled from a stack of three bracketed photos with different exposures.

The light was really amazing because the sun was setting to my left as I was taking the photo. However, I knew that no one exposure could capture both the detail in the clouds, and the details in the shadows on the right side of the picture; the scene had too large a dynamic range. So I took three pictures using the auto exposure bracketing  feature in my camera. These pictures (seen below) were all taken with the aperture set at f/8, but the shutter speeds were 1/6, 1/10, and 1/15.

What is Dynamic Range?

Static dynamic range refers to the difference between brightest and darkest things you can see at the same time without moving your eye around. The static dynamic range of the human eye is generally around 100:1.  So, the dimmest thing you can really see when looking at, say, a campfire is about 100 times dimmer than the fire itself. When talking about photography, differences in brightness are typically discussed in terms of “stops.” A stop is a factor of two difference in brightness. So, a ratio of 100:1 corresponds to about 6 and 1/2 stops (26=64 ; 27=128).

Of course the total dynamic range of your eye is MUCH bigger than that. In total, your eye can resolve an impressive 20 stops, or about a 1,000,000:1 ratio of luminosities (brightnesses). That means if you move your eyes around, they can adapt to see a much wider range of luminosities (just not all at the same time).

My camera, however, can only resolve a modest 5 stops in a single scene (stored as an 8-bit per color channel jpeg file; this post really deserves its geek tag, doesn’t it?). Most cameras have a similar limitation. Consequently, when you look at a photo taken with basically any camera (digital or film), and displayed on a typical monitor or on photo paper, the luminosity information has possibly been heavily truncated. This why skies often look white in photos even though they looked blue in person.

What is Tonemapping?

One way to convey more of the luminosity information from the original scene is by combining multiple exposures into an image that contains a wide dynamic range. The luminosity data can then be compressed to a range that can be displayed in a single scene. That compression process is called tonemapping. In the example above, I took 3 photos, each spanning 5 stops and separated by 1.5 stops and combined them to yield a single photo that retains local contrast information in both the highlights and the shadows. Here are the original images:

While the local contrast information has been better retained everywhere in the tonemapped image, the total dynamic range has not been increased and is still limited by the maximum dynamic ranges of the file format and the display device. I used software called Photomatix Pro to do the tonemapping. The free trial version can make images as large as the one above, or larger ones that have a watermark on them.


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