The Exposure Family: Aperture, Shutter Speed and Metering
Knowing how to affect the exposure on your photographs can make the difference between a great shot and a shot of some unrecognizable white blob with no borders or color. And just like any other aspect of photography, there are many elements coming into play when dealing with exposure. Aperture, shutter speeds, metering and light sources all have to be considered when adjusting exposure. Once you figure out which combinations work best for different situations, you will be able to reference those combinations you have mentally preset instead of fiddling with your camera wasting precious time.
Exposure refers to the amount of light exposed onto the film or image sensor during a shutter cycle. A shutter cycle is the amount of time the shutter is open for a single picture; more commonly known as shutter speed. A light source is anything and (unfortunately sometimes) everything that emits light. Metering is measured three different ways. Each method analyzes what is in the frame and determines how much light is needed in order to properly expose the image.
· Center-Weighted Metering: The image is divided into segments predetermined by the camera. All of the segments are analyzed but the camera exposes what is in the frame to better suit the center section.
· Multi-segment or Matrix Metering: It places reference points across the entire frame and exposes the shot for the average of those reference points. On most cameras, this is the exposure default. It works great in a well balanced image but when there are great variations in light across the frame, it will expose the shot inconsistently.
· Spot Metering: Spot metering gives the user the most control. One spot in the frame is evaluated and exposed to that individual reference point. The photographer can normally adjust and move that spot according to their needs.
With these powers combined, you can be Captain Exposure. Just kidding, you'll still be you but you will have a great picture.
In order to work on your mental presets for exposure, take a look at the pictures below. The first group of shots has the same aperture setting with increasingly short shutter speeds. You can see how they work together or against each other. The ISO for all the pictures is set to 100. They were all taken within 5 minutes of each other so the light is the same as well. This little exposure experiment was conducted so you wouldn't have to (but I completely suggest doing it anyway so you can see for yourself how your camera is going to react to different settings).
Aperture: 4.5 Shutter Speed: ¼
I had to put a border around the image so you knew it was even there. Obviously, this combination can be scrapped for this particular light source.
Aperture: 4.5 Shutter Speed: 1/100
Now that you can see color, you can also see that this image is overexposed. Overexposure happens when too much light hits the sensor due to the shutter being open for too long. The colors appear washed out and the brighter parts of the picture are highlighted too bright.
Aperture: 4.5 Shutter Speed: 1/200
This is getting closer to being balanced on all fronts. The highlights are less...highlighted; you can start to see a door on the shed behind the tree which wasn't apparent in the last version. Still, there is more potential for the color to be richer, deeper and less...bright.
Aperture: 4.5 Shutter Speed: 1/400
You can see a major difference in the color of the sky and the grass with this shutter speed. But it still isn't quite there. Using the real natural elements as a reference point, there is more to see than this shot allows.
Aperture: 4.5 Shutter Speed: 1/1250
Look at that: a blue sky, green grass, pink flowers, a blue door frame, a nice shadow underneath the tree and even some clouds with good texture and depth. If you look hard enough, you can even see the lovely telephone lines strung over the houses. Shadows are just as important in a correctly exposed image as the highlighted parts are. The contrast is what gives your picture depth and interest. Getting the exposure right is like putting a magnifying glass to all the details in the shot, showing off every part of the picture.
Aperture: 4.5 Shutter Speed: 1/4000
Clearly, underexposure can also ruin a shot. Suddenly, eleven in the morning turns into six in the evening. This is handy knowledge if you are in fact trying to get a night shot and only have time to take pictures in the middle of the day, but that technique is not all that practical. Everything is cast in shadow taking away all the gorgeous color.
Another way to alter exposure is to leave the shutter at a particular speed and alter the aperture setting. Aperture, also referred to as an "f/stop", is basically the size of the hole in the lens when the picture is taken. Technically, it's the fraction of the focal length of the lens at its biggest. Explaining what that means in length involves math and I stopped doing math a long time ago. The important thing to remember about aperture is that the smaller the number you see on the screen, the larger the aperture really is. For example: f/2 is the smallest f/stop most cameras will go, but that allows in the most light possible. Conversely, f/22 is the biggest number but allows in the least amount of light. Got it? Good.
In this second set of pictures, you'll see that the shutter speed stays the same at 1250, but the aperture changes.
Aperture: 3.5 Shutter Speed: 1250
Aperture: 5.6 Shutter Speed: 1250
Aperture: 7.1 Shutter Speed: 1250
Aperture: 9.0 Shutter Speed: 1250
Aperture changes more than just how much light comes in. It also dictates how much of the picture will be in focus. An aperture with a large number will mean that most of the picture will be in focus. Vice versa, a small aperture will have a more focused focus. The pictures above are not a very good example of that. Because there isn't too terribly much to look at past the tree, there is no variation of what is in focus. You can see how aperture and shutter speed play off each other. And if you really pay attention, you can see how two different combinations will produce the same effect on the final picture.
Larger aperture settings (smaller number, remember?) work best when you need those nitty gritty details, as in portrait and wildlife photography. That is also a good time to use spot metering; find that spot that has the best light that you want for the whole image and use that. Smaller apertures work best for landscape photography or when you need the whole grand picture to be in focus. Multi-segment or matrix metering is your best bet for that type of photography as well.
Now after all that has been said and done, technology masters out there have successfully made this entire article unnecessary by manufacturing light meters. They tell you everything you need to know about correctly exposing your image; it gives you a read out that you then program into your camera. There are different kinds of meters for different kinds of jobs done by different kinds of photographers, and this article is a great explanation for all of that. So now that you have wasted your time reading this article, go read the explanation of the different kinds of meters and buy one. It will save you time and make you look like an awesome professional who knows how to get the job done.