Why RAW?
March 15, 2009 11:00 PM Filed in: Gadget
I took this photo standing in the building that is the entrance to the Taj Mahal compounds in Agra, India. This was a handheld shot with no flash. I paused just for a moment to capture the image as I was walking with the flow of the crowd. The indoors where I am standing was only illuminated by natural light—there was no artificial light inside. As a consequence the contrast range between the shadowy dark inside and the sunlit outside was huge. As you can see, in this photograph the shadow areas are severely underexposed and the bright areas are somewhat overexposed. However, this is not how it actually looked in person because the human eye can see a much greater contrast range than a JPEG image can show. How best does a photographer handle situations like this?
Let's begin by talking about desired effects. If I wanted a silhouette effect then I would correctly expose for the Taj under the sun making the indoors completely dark—more dark than this photo. I would then see the silhouette of the arched entranceway and the tops of the people's heads framing the Taj. In this scenario, I would crop the photograph on the sides and the bottom because I would not need the vast amount of black areas.
If I wanted a high-key photograph, I would correctly expose for the indoor and let the outside wash out in the sunlight. It would give the feeling of immense brightness from the Taj flooding the indoors where I was standing, with beams of light shining through the doorway.
Both of the above would make great photographs and they would be very doable technically. But suppose I wanted to expose both the inside and the outside correctly what are my options? Conventional wisdom would say use a fill-in flash—would that work? It depends.
If you used a camera mounted fill-in flash firing straight ahead you would get the following: the little girl and the two people standing very close in front of me would be overexposed, almost washed out by the flash; the walls on either side of the archway and the people under the archway would be the same as the above photo (i.e. dark) because they are too far from the flash; the Taj would be the same as the above photo because we are keeping the exposure the same. In short, it will make a very ugly photograph. If you pointed the flash upwards for diffused, reflected light the photograph would look the same as above because the ceiling is some 50 feet above my head and it is dome shaped. The way to make fill-in flash work is to have five wireless remote triggered flashes on light stands—two illuminating the walls on either side of the archway, one illuminating the people in the archway, and two on either side of me reflected off umbrellas illuminating the people and the floor in front of me. This is obviously an elaborate set up that would take quite a bit of time, require permission from the curators ahead of the visit and I would have to carry a lot of equipment with me all the way from the US.
So, any other ideas? Yes, a bit of jiggery-pokery with a 14-bit RAW image could get you to the same place assuming you have initially exposed the photograph "correctly". Let's start by looking at the histogram of the above photograph.
Imagine we break up the horizontal axis of the histogram into four equal quadrants—0 to 63, 64 to 127, 128 to 191 and 192 to 255. Most of the image information resides in the far left and the far right quadrants, with almost nothing in the middle two quadrants. What we want to do is to take the left cluster and stretch it over the left 3 quadrants and take the right cluster and stretch it over the right 3 quadrants. Each of these quadrants have only 64 possible values—so trying to stretch them over 3 quadrants is going to leave some gaps. Visually this will look like a posterized image. [This is the scenario if you tried this maneuver with a JPEG original]
Here is how the final image looked starting with a 14-bit RAW file. Keep in mind that if the exposure was even slightly off, this kind of image adjustment would not be possible. So it is critical that the camera has a good exposure meter and exposure calculation algorithm and that you always look at the histogram on the camera LCD display, especially in situations like this. If you are not sure from looking at the histogram, then bracket the exposure. Now try this with film! It is possible with negative but not slide film—it used to take me hours in the dark room while inhaling nasty chemicals to do what I can now do to a digital image with just a few clicks on a computer.