Sunday, August 3, 2008

Getting better exposure

I use lots of manual metering but I usually use Aperture priority. One thing that I have learned is that it is really easy to fool the camera’s built in light meter. When I shoot (all modes including manual), I watch two things in the camera, the blinking highlights and my histogram. I try to get as much info to the right as possible without blowing out important details.

The reason to do this is two fold. First it just plain looks good especially when shooting portrait style pics. Making sure that the scene is exposed correctly is what separates the average from the better. Second, due to the way that a camera sensor captures data, the first half of the data bits is used to capture the first upper tier of light information. Confusing?

I will put it this way; say that your camera can capture 8 stops worth of light (see here if you are confused by the word stop of light). The camera captures that 8 stops by dividing its bits (data) into 8 different segments one for each stop. The catch is this, it isn’t an equal division instead the camera keeps ½ of the data for the first stop of light, in this case the brightest stop. Then it gives half of what is left for the next stop of light and so on. Imagine you only had 256 bits of data for your image the 256 bits would be broken up like this:

Stop1 - 128 data bits
Stop2 - 64
Stop3 - 32
Stop4 - 16
Stop5 - 8
Stop6 - 4
Stop7 - 2
Stop8 - 1

First thing to notice is that there is not much data for the last stops (dark areas). Another thing to keep in mind, if you try to bring up the light level in post you will often get a lot of noise, but decreasing light level in post isn’t such a big deal (as long as it isn’t by a lot). Part of the reason for this is that you just don’t have much data in those dark areas. There isn’t any detail to retrieve in the darkest regions.

I don’t know for a fact but I am sure that any DSLR will have at least a luminance histogram. Here is the next secret when using that histogram, or watching the blinking highlights. In order for that luminance histogram to be accurate you need to be at the right white balance setting. If not you may be loosing data that you didn’t know was there.

Last thing to keep in mind is that histograms come in all shapes and sizes. There are times when you want the histogram to be at the very left end of the spectrum. Don’t worry about the shape or how well the histogram is filled out. The histogram is just a helpful feature, not an exact measure but is a guide that requires personal interpretation. Other times you may want a histogram that clips at the right end, like when you have shiny metal objects in the image or something bright like the sun. Just remember to have fun...


Jeremy said...

Very cool post. Usually I'm just looking at my LCD to see if it looks ok, then finding it's not great when opening it on my computer.

Nathan Marx said...

Glad that I shared something helpful!