. . . and your face so dark?”
“Because you’re relying on your camera’s built-in light meter, sweetie,” his exasperated wife explained.
I said when I started this blog that we would be discussing travel, writing, books and movies, and photography. After my first full week in the blogosphere we’ve yet to touch on that last one. So, today’s blog is something called exposure compensation.
Just about every consumer camera made nowadays has a built in light meter that the camera uses to set how much light is gathered to record an image. The three things either the camera or the photographer can do to adjust exposure are: change the sensitivity of the sensor; change the amount of time the shutter stays open to the exposed sensor; change the aperture of the lens diaphragm to either increase or decrease the amount of light striking the sensor. Let’s take these one at a time.
Shutter Sensitivity (ISO): As with its predecessor, the film camera, the sensitivity of the sensor on digital camera is determined by the ISO number. The lower the number the less sensitive the sensor. Standard “stops” (which we’ll discuss later) for ISO are 50, 100, 200, 400, 800, and so on, doubling the number as you go up the scale. Each full-stop increase doubles the sensitivity of the sensor. Conversely, each full-stop decrease cuts sensor sensitivity by half. Numbers falling in between these “full-stop” numbers are called half-stops or, if you have a more advanced camera, third-stops, or perhaps even quarter-stops in some really high-end equipment. One might be tempted to think at this juncture, “Well, heck, I’ll just dial in the highest ISO and then I can take pictures even in dark situations controlling the other two adjustments.” Don’t do it. The higher the sensitivity the more susceptible the sensor is to recording “noise.” What is digital noise? Think of it as electronic interference not unlike television in the days of rabbit ear antennas and poor reception. Noise degrades image sharpness, lessons color quality, and causes all kinds of blotchy junk to appear, especially in the darker areas of your photograph.
Shutter Speed: This one is pretty self-explanatory. It’s how long the shutter remains open, thus exposing the sensor to light. Full stops in shutter speed start at 1 (for one full second). Going faster up the scale you get 1/15 of a second, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, etc. Going the other way, slower, would get you 2 seconds, 4, 8, 15, 30, a full minute, and so on. You will note that halving 1/60 does not get you 1/125, but rounding off that number is the standard so as to make subsequent full stops easier to calculate, and the difference in actual exposure control is very minute. The same goes for the opposite end of the scale when “doubling” 8 seconds gets you 15 instead of the expected 16.
Aperture (f-Stop): Now this one is kind of confusing and a bit counterintuitive, but I’ll attempt to simplify it. The lenses on most dedicated cameras (meaning cameras not built into cell phones, webcams, and just about every Apple Mac iThingy in existence) have a little diaphragm built in that either expands or contracts to increase or limit the amount of light falling on the sensor. Your eye does the same thing. The normal eye’s pupil contracts in bright light and expands in dim unless acted upon by something you really shouldn’t be smoking, but I digress. The important thing to know about f-Stop is that the lower the number the more light that gets to the camera sensor. That’s because f-Stop is a mathematical ratio derived by (if you really care) the focal length of the lens divided by the aperture of the diaphragm. So, full stops in f-Stop-speak are 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 32, 64, and so forth. And, yes, I did all that by memory. So can you. It’s a snap. The magic numbers to remember are 1 and 1.4. Each subsequent full f-Stop starts as a doubling of those numbers in alternating order: 1, 1.2, 2 (1×2), 2.8 (1.4×2), 4 (2×2), 5.6 (2.8×2), 8 (4×2), 11 (5.6×2 rounded off) 16 (2×8) and onward. Easy once you know the magic first two numbers, isn’t it?
So what does all this stuff mean? Let’s say you set your camera’s ISO to 100 to take advantage of a bright, sunny day with a minimum of sensor noise. You press the shutter release button halfway down. The camera focuses the scene then analyzes the light striking the sensor. It does some quick calculations and sets your shutter speed to 1/200th of a second and the lens aperture to f5.6. You press the button further and, voilà, you have a perfectly exposed photograph under most normal conditions. You could also have taken pretty much the same shot (with differences in depth-of-field, which we might cover in a future article) by, say, keeping the ISO at 100, but halving the shutter speed to 1/400th from 1/200th to freeze the action, and then compensating for that by doubling the aperture size to f4 from f5.6 (remember, aperture size increases with a decrease in the f-stop number) to double the amount of light getting to the sensor. Or, you could keep the f-Stop at 5.6 and freeze even faster action by quadrupling the shutter speed to 1/800th of a second, but then you would have to increase the ISO two full stops, taking it to 400 to quadruple the sensitivity of the sensor. Congratulations! Another perfect shot using two totally different sets of exposure numbers. Aren’t you just too clever!
Ooops. Not so fast. It’s been snowing outside. Bright sunlight is reflecting off that snow. Now your built-in light sensor is having trouble, because it wants to set the scene thinking that all scenes reflect approximately 18% of the light striking them. In snow conditions (or if you’re atop a sand dune at the White Sands National Monument at noon on the 4th of July) that’s just not the case. There’s a huge amount of light bouncing off that snow (or gypsum “sand”) back to your camera’s sensor. But the camera doesn’t know that it’s snow, so it takes a scene that should be set to 1/200th of a second and f5.6 and instead sets in 1/400th of a second at f8. You take the shot and discover when you get back home that all your snow-bunny pictures (or macho sky-instructor photos for the ladies) are way underexposed. In this example the camera underexposed by two whole f-Stops (first by halving the shutter speed from 1/200th to 1/400th, then again by halving the amount of light by changing the lens aperture from f5.6 to f8). Horrors! What to do?
Well, let’s just say that compensating isn’t just for men seeking bigger . . . um . . . aspirations. It’s also for photographers of both sexes. For the average user, switching the camera to manual mode and dialing in your own numbers is a pretty complex thing to do requiring a lot of technique and knowledge. Happily, many cameras can do the compensating for you but, even if they can’t, I have a nifty little trick for you to use toward the end of this lesson.
First Technique: If you have one of those great little point-and-shoot, fully-automatic, do-everything-save-wash-your-undies cameras, check to see if you have a built-in scene setting for snow. If you do, you’re in luck. Dial that in and the camera will calculate an exposure compensation, probably in the neighborhood of around +1.5 to +2 stops.
Second Technique: Does your camera have a control for “Exposure Compensation”? Then use that. Try increasing exposure by +1.5 to +2 to start. Check your first picture on the LCD and go from there. If you also have exposure bracketing, set exposure compensation to +1.5 and bracket the exposure a half-stop either side (this will take three consecutive pictures in rapid succession at exposure compensations of +1, +1.5, and +2, allowing you to keep the best of the lot). A word of warning: Don’t forget to set your camera’s exposure compensation back to zero when you get off the ski slope or all your subsequent pictures will be vastly overexposed.
Third Technique: Many of the more advanced cameras on the market today allow you to change the metering area, which by default is usually set to something called Evaluative Metering and which takes in far too much of the snow background if you’re trying to photograph a subject surrounded by snow. Changing to Center-Weighted Averaging makes things worse because the entire scene is metered for light with emphasis on the center of the scene. That leaves Partial Metering and Spot Metering. Both meter only a small portion of the scene based around the center, with Spot Metering being the more precise of the two. Set you camera to Partial or, if the subject is relatively small or far from the camera, Spot Metering. Aim directly at the subject and press the shutter release button halfway down to “lock” the exposure settings. Then, while still holding the button down, recompose the scene and take the shot by pressing the shutter release fully.
When All Else Fails (one final trick): What if you don’t have something close to 18% reflectivity off which to meter? What if your camera doesn’t have different metering modes, scene modes, or exposure compensation? Well, then it’s time to get a new camera. But in the meantime I have one more little trick for you to place up your sleeve. You have one last fallback if your scene is under relatively blue skies. Meter mostly on the sky with the sun to your back (I say “mostly” because your camera still has to “focus” on something, especially if there’re no clouds), then recompose the scene and take the shot. Blue sky meters very closely to that magical 18% reflectivity number we keep mentioning. This technique works best if you’re taking a picture of something distant rather than close up because the camera will be focused for a subject at or close to the lens’ infinity setting. If you’re using this technique to photograph something relatively close to the camera then zoom the lens to a wide-angle setting and set the aperture to as small (high f-stop number) as possible, say around f8, f11, or even f16 if you can get away with it. If you’re unable to set the lens aperture, then set the shutter speed to a slower setting (but try not to go much slower than 1/60th to 1/30th of a second if you’re hand-holding the camera) so the camera adjusts the aperture smaller for you. This will give you the deepest field of sharp focus, something called “Depth-of-Field.”
I look forward to hearing from you as to whether you find this brief tutorial helpful, too technical, too basic, too long-winded, or whatever else you’d like to sling my way in the way of constructive criticism. I’d also like to hear ideas for future lessons on problems you may be having getting that “perfect shot.” So, please feel free to leave a comment.