Chapter 4
Snow and Fill Flash
In the last chapter, I discussed the golden hours and using the graduated neutral density filter to even out the exposure between sky and foreground. You will; however, find yourself in other situations where your camera's automatic exposure calculations may be insufficient and the graduated ND filter will be of little use. A prime example is a scene that consists of a great deal of white such as snow.
Your DSLR's electronic
metering system is fairly accurate in non-challenging and/or evenly
lit situations. Throw a lot of white into your scene and you'll find
your camera will consistently underexpose your scene.
The below images will illustrate this effect.
The image on the left and it's associated histogram (below the image) is a default metering result and as you can see by looking at the histogram, the brightest areas on the right side of the histogram are showing about ½ of a stop of underexposure. The image on the right shows how it and the histogram should look if properly exposed. Remember my earlier statement with regard to exposure. “Expose to the right”.
A ½ stop underexposure becomes a serious issue when you have to correct it in post processing by increasing the exposure at the computer as you're also increasing the brightness of digital image noise. Exposing properly in the camera keeps the noise levels of the image lower. This noise will show up in the darker areas of the scene, such as the shadows beneath the log and in the nooks of the ice. This noise problem becomes an even bigger issue as you shoot at higher ISO levels.
The solution is to recognize that the scene contains a lot of white (in this case snow) and and purposely overexpose the image by about + .5 stops of exposure by dialing in exposure compensation with your camera before taking the shot. This becomes even more critical if you are shooting JPG files, as your ability to make post-processing adjustments to JPG files is much more reduced than when shooting RAW files. If you've been following what I've already said, you already know to shoot RAW formated files though, right? RAW files have more room for correcting exposure problems.
Here's a sample where I overexposed the scene by 1 full stop by adjusting the camera's exposure compensation to +1. The end result for the RAW file was an over-exposure of about .2 stops, which was easily adjusted in Photoshop. End result, a perfect exposure here would have been 3/4 of a stop of over exposure. I'm displaying a histogram for the luminosity channel. When configuring your camera to display a histogram, the luminosity channel will generally be a more accurate representation of the color intensity for each color channel in the image.
The reason for underexposure is easily understood. The metering system evaluates the scene and attempts to calculate the exposure of the scene based on what it thinks is a neutral gray level. Neutral gray is defined as 18% gray. Your camera can only measure reflected light. When calculating a proper exposure, your camera can only make an accurate measurement if the amount of reflected light is evenly distributed across the three primary colors; red, green & blue. Your camera will give higher significance to the color white when metering and it will tend to ignore darker colors and black all together. If your scene has a predominance of reflected white, your camera will make the assumption that this white represents a neutral value and adjust your exposure based on that assumption. The net effect of this misjudgment, is your camera is going to reduce the exposure of the white to that of neutral gray. In my experience with Canon and Nikon DSLR's, the snow scene metering is usually between .5 and 1 stop underexposed as a result.
Fill Flash.
Another common photographic problem scenario is when you have a bright background and dark subject in the foreground. A common situation where this occurs is when encountering wildlife.
Let's be honest. We don't always have a plan in mind when we are out and about with our camera. I almost always keep a camera sitting in the seat of the car when I'm driving around Colorado. Reason being, I never know when I'm going to see some wildlife near the road and I want to be able to get a shot. May not be National Geographic material, but what-the-hey. If you don't have a camera, you can't take a picture.
If you're in a similar situation you are certain to come across is a scene that is back-lit. A back-lit scene is when there is a bright source of light behind the main subject of the photograph and the main subject is darker than the background. There are several ways to cope with these situations, some easier than others.
Here's a shot of opportunity where I ran into this exact situation. While driving through a small mountain village one afternoon I spot a group of deer standing at the side of the road. Wow, pull off and get a picture.
In the below image and histogram, you'll see that the deer are properly exposed but the sky is blown out. The sky portion of the image is about 2 stops over-exposed. Even shot as a RAW file, the detail in the sky can't be recovered. If this shot had been made with the sky properly exposed, the deer in the foreground would be too dark. I can't fix this. Nothing I do will make the sky look right. The way to fix a shot like this is to not take it the way I did.
Here's what I did wrong. I was using a Canon 1DsMKII with a telephoto zoom and shooting through the passenger side window from the side of the road. I didn't have my camera configured to meter the scene correctly. I had previously configured the camera for spot metering and never changed it back. I didn't pay attention to the bright sky. I focused on the deer and the camera metered the scene using only the deer. It was a spur of the moment thing and with a little better foresight on my part, I could have done a better job with this. That foresight would have involved insuring that my camera was set to the correct metering mode for the scene and putting a speed-light (flash) on my camera so I could light up the deer from 20 feet away and still keep the sky exposed properly.
Many times we don't think of using a flash when we're roaming outdoors. Extra bulk, extra batteries, just another thing to break. What I've learned, time and time again, is that having a speed-light handy can give you a better chance of getting a shot in some situations. If I had been thinking ahead when I put my camera on the car seat, I'd have put the camera in evaluative metering mode and placed one of my speed-lights on the camera (my camera doesn't have a built in flash.) If you own a camera that has a built in flash, learn how to use it when you need it. There's a little button on the side of the camera near the flash and when you push it the flash will pop up.
When using a flash, most cameras will automatically attempt to analyze a scene and attempt to determine how much flash to emit in order to properly light the scene. If the camera estimates the background is bright enough, it will emit a lower amount of flash for the purpose of lighting the objects in the foreground. If the camera meters the background light as being too low, it will fire at a higher intensity and attempt to light the entire scene with flash. In the above scene, the camera would have metered the background as being bright enough and adjusted the flash level so that it was only filling in the foreground objects. This is called “fill flash.”
Here is a sample fill flash shot, done properly.
In this back-lit situation, I have a bright blue sky with a few hazy clouds in the background. The foreground is actually in dark shadow due to shade from the surrounding trees. The trick to the fill flash on this shot is to expose for the background (sky) and light the foreground enough to render the detail of the wood planks of the upper portion of the house without killing the shadow of the trees. The Sun is directly behind me. I'm using the camera's evaluative metering and allowing the camera to calculate the correct amount of flash using ETTL. The fill flash was a Canon 430EX and is mounted on a stand about 10 feet to my left, aimed at the doorway upper side of the house and fired remotely. Without the fill flash on this scene, the upper half of the front of the house would be completely dark from the shade of the trees. You can get a hint of how dark the shadows were by looking at the shadows on the far right side of the house, an area that wasn't lit with the flash.