Today I want to take a step or two back to discuss a fundamental concept of photography – exposure.
We’ve all been there – bad exposures happen. Sometimes those bad exposures can be fixed after the fact while other times we are stuck knowing that we almost had an amazing photograph. The key is to understand what it means to make a good exposure and then to take the time to dial in the right settings for the image. But what are the right settings?
It’s fair to say that the exposure meters built in to cameras today are VERY sophisticated. When pointed at an ‘average’ scene the camera will correctly determine the exposure needed to make a good image. The problem is that the camera doesn’t know what you are trying to achieve in any given image and there is no one absolute correct setting for any given scene. Learning how to adjust the settings on your camera will allow you to help your camera use the right settings to help you achieve your vision.
So before we dive into the three parameters that affect exposure, I want to take a moment to discuss focal points. On fully automatic mode, your camera will determine what it thinks should be in focus and then it will base the exposure settings on correctly exposing the element in the frame that it decided should be in focus. By learning how to change where your camera is focusing, you’ll help your camera expose properly for the object that you think is most important. But of course, that’s just the tip of the iceberg. And that’s an oversimplification but making sure that your camera is focusing on the right part of an image (like someone’s eyes instead of the tree in the background) is an excellent starting point.
A useful metaphor here is a bucket of water. Imagine your exposure is a bucket and that you want to fill the bucket exactly to the top. You know that you need a set volume of water to achieve that goal. Too little and you have an under filled bucket. Too much and, well, you get the point. A properly exposed image is like that – the digital sensor needs a set amount of light to ‘fill’ it. Too little light and you’ll have an underexposed image, too much light and it will be overexposed. Seems simple enough, right?
There are three tools that a photographer can use to set an exposure: shutter speed, aperture size (often referred to as an f-stop), and ISO setting. When any one of the settings is modified, then one or both of the other two need to be adjusted accordingly to balance out the exposure.
I think ISO is the least interesting of the three. The ISO (International Organization of Standardization) is something that you want to change with decreasing light. Basically, the darker your scene, the higher ISO setting that you’ll want to use (if you are hand holding your camera) but that higher ISO setting comes with a cost – digital noise. This noise will manifest itself in one of two ways – luminance noise (gray pixels) and color noise (red, green, and blue pixels). This noise will reduce the fine detail and sharpness of an image so you want to keep your ISO setting as close to the default setting as possible (using 100 or 200) for the best possible image quality. Check out this photo (and zoomed in detail shot) to see of an example of what the high ISO noise looks like. It’s important to note that had I not turned up the ISO, I would have had a blurry mess of a photo so sometimes it’s important to crank up the ISO to get something rather than missing the shot all together.
The next setting is shutter speed. This one’s pretty obvious I think. The longer your shutter is open to the light, the brighter your image will turn out. A relatively long shutter speed will cause anything moving in your scene to appear blurred in the photo whereas a relatively short shutter speed will freeze motion. Relative is the key term here. A person standing for a portrait might be able to hold still for 1/15 sec while a cat batting a paw will appear streaked at 1/200 sec or faster. You should choose your shutter speed in consideration to what you are shooting AND how you are shooting it. If you are using a tripod, then any shutter speed is likely fine. If you are hand holding without any stabilization methods, then a rule of thumb is 1/the focal length so if you are using a 100 mm lens, then 1/100 sec should be relatively sharp. Are you with me so far?
The final setting, and perhaps the most important for a majority of subjects is the aperture size. The aperture is the hole in the lens that allows light into the camera. The larger the hole, the more light that will hit the sensor at any given shutter speed. So it might seem that a wide open aperture is the way to go then, right? Not necessarily. Aperture also affects Depth of Field (DOF). The wider the opening in the lens, the smaller the plane of focus is on your image (the confusing part is that these settings are ratios so f/1.2 is really a large opening while f/64 is a very small opening). This is a great tool for helping to focus the viewer’s attention exactly where you want it in an image. Take a look at this still life series to help you understand.
So there you have it. Three different tools that you can use to help achieve a good exposure. My recommendation is that you experiment with each of these three settings. Some might be available as scroll wheels on your camera while others will be menu level adjustments. Once you feel you have a good handle on that, I recommend that you use aperture priority mode for most of your shooting. This will allow you to manage the depth of field (area in sharp focus) while letting your camera figure out the right shutter speed and/or ISO. Practice with it a bit and let me know how it goes.