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HDR, what it is and how to do it. Part 1.

12/23-2009 at 18:38

Jens Stolt Jens Stolt writes::  We have all encountered the limitations of our cameras compared to our eyes vision. The camera simply cannot capture the same range of light as the naked human eye and therefore photographers are always somehow limited when they set the cameras exposure for a frame they want to shoot.

The usual result of capturing a frame in a range of light that is wider than the camera can capture is burned out skies or blacked out shadows. We are used to that and have learned to adapt with tweaking the raw file in photoshop with shadow/ highlights or even better to shoot at the right time of the day when shadows and ambient light is within the cameras dynamic range.

However HDR (High Dynamic Range) photography can deal with those problems by capturing multiple exposures and merging them to a single file so that the whole spectrum of light in the scenery is captured and stored digitally for the photographer to explore further.

Below is an example of a photo that is exposed as a compromise. This is how much of the light the camera can capture: approximately 7 steps. The sky is blown out and the shadows are completely black.
Photo direct out of the camera, not postprocessed.
Author: Jens Stolt

The camera can only capture a narrow range of the available light, the human eye can capture more (7 steps vs 11 steps, I think).

Below is an example of how the light is in the deep shadows.

Take a look at the foot of the tree and see the textures that were hidden in the deep shadow in a midrange exposure:

Author: Jens Stolt


And below again how the light is in the lighter areas of the frame:

Author: Jens Stolt


So we have a problem.

We cant have those under or overexposed images. We want exposure nailed just like we can see it with our eyes. But the cameras of our time cannot do that. YET.

Luckily since we work digitally, nerds and geeks have provided us with a postprocessing solution: HDR.

So that if you take a series of bracketed shots covering the full dynamic range of the scenery, it is possible to merge them to a single file that holds an intrepretation of the image that can compare to what your eye can see.

So to get started with HDR, you first need to capture the light with as many exposures as the scene requires. That can be 1, 3, 5, 7 or 9.

Below is 5 exposures of the same scene. They are one step apart and almost covers the dynamic range.

It is important to get a correct exposure for every light situation in the frame, both the deep shadows, the middtones and the highlights.

Fx in a bright sunlit situation you will need 3 steps alone to cover the lighter tones around the sun if the sun is in the frame, then comes the middtones and the shadows.

I use the following rules of thumb:
9 steps...If the sun or a direct sun reflection is in the frame.
7 steps...If there is something completely white in the sunshine.
5 steps ..In sunshine.
3 steps...If overcast and the sky is in the frame.



Author: Jens Stolt



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