Part 2 - Four More Ingredients in the Perfect Picture Recipe

By now it should be evident that if exposure is everything and if you can't rely on the camera to determine this most critical component of picture taking, then your vision and that computer inside your head are more important than any equipment in your camera bag.

Hopefully, based on my last two posts, you have a good understanding of the purpose and functioning of your camera's light meter. But what is the light meter or the manual setting on your camera controlling? To continue the exposure discussion, I'll introduce the mechanical light valves that work together to restrict the light entering your camera lens to allow just the amount needed to produce the exposure -- the aperture and shutter.

Like the rest of this photo primer, aperture and shutter will be discussed in just enough detail to get you up and running. More information about these two controls will be incorporated into this blog as a follow up to this first series on basic photography.

Aperture

The camera’s iris. As shown below, the aperture can be seen through the front of your camera lens. It's made up of overlapping "leaves" that open and close in a circular fashion similar to the iris of your eye:



The aperture (normally located inside the camera lens) is best compared to the iris of the human eye. The aperture not only looks like our iris, it also performs the same function. The smaller the opening of the iris, the less light enters the camera. Unlike the human iris, the camera aperture is controlled by a graduated series of settings called f/stops. These f/stop settings control precisely how much light passes through the aperture and into the camera.

Unfortunately the f/stop numbers don’t appear to be logical at first glance. A typical camera might have whole f/stop numbers of 2, 2.8, 4, 5.6, 8, 11 and 16. These are traditional f/stop designators. (There are many more.) The basis for these numbers is in trigonometry, and the smaller the number the larger the physical iris opening. You don’t need to know the trigonometry. However, the following paragraph is extremely important to your understanding and is fundamental to photography.

Each whole f/stop (as in the list of f/stops above) is set apart from its adjacent f/stop number by exactly one-half OR twice the amount of light. If an aperture is “opened” one f/stop (the iris is physically opened to a smaller f/stop number) then twice as much light enters the camera lens. The opposite is true if the lens is “stopped down” one f/stop. In this case, the iris of the lens is being made physically smaller to allow exactly half the amount of light to enter the camera. For example, f/4 lets in twice as much light as f/5.6 and half as much light as f/2.8. F/stops are a continuous scale. So, yes there are f/stops of 4.3, 8.2, etc.

Just to make the math perfectly clear, let's say that an f/stop of f/4 allows one thousand units of light to enter the camera. Then:

• f/4 = 1,000 units of light entering the camera
• f/5.6 = 500 units of light entering the camera (aperture opening is physically smaller)
  
• f/2.8 = 2,000 units of light entering the camera (aperture opening is physically larger).
  

Shutter

The camera’s front door. Just like a door, the shutter opens and closes to allow light to strike your camera's image sensor:




The camera’s shutter (usually located in the body of the camera between the rear of the lens and the camera's image capturing sensor) functions like the front door of your house. The shutter speed control on your camera determines how long this “door” is open to allow light to enter the camera. As the shutter speed increases, the amount of light entering the camera decreases. (Like your front door, if the door is open for a short time, fewer people can enter.) As the shutter speed decreases, the amount of light striking the photo sensor increases. (The door is left open longer and more people can enter.)

A typical set of shutter speeds would include these numbers: 1, 2, 4, 8, 15, 30, 60, 125, 250, 500, 1000. In this case the shutter speed number "1" stands for one full second -- meaning the shutter will remain open for one full second. The remaining numbers in this example are fractions of a second. So, the number "2" equals one-half of a second. 250 represents 1/250th of a second. And so on. So the "doorman" in your camera has pretty quick reflexes. Some cameras can open and close the door (shutter) in 1/2000th of a second or quicker. (This numeric scheme does appear more logical than the aperture values, as the numbers double and halve. Don't ask me about 1/8th of a second and 1/15th of a second. It's one of photography's little mysteries and, in this case, close counts.)

Shutter light control and the amount of light entering the camera can be thought of in much the same way as the aperture. The faster the shutter, the less the amount of light entering the camera. The slower the shutter, the more the amount of light entering the camera. In the example shutter speeds above, each speed setting has EXACTLY the same relationship as the series of f/stops previously discussed. Each shown shutter speed lets in exactly twice as much OR half as much light as the adjacent settings. Example: 1/60th of a second lets in twice as much light as 1/125th of a second but only half as much light as a setting of 1/30th of a second. Similar to the aperture, the shutter speed settings in modern digital cameras are on a continuous scale. For example, on today's cameras a shutter speed of 1/600th is possible, although this speed is not an option on traditional film cameras where shutter settings were manually selected from a series of factory determined speeds.

As with the aperture, let's say, for example, that 1/60th of a second allows one thousand units of light to enter the camera. Then:

• 1/60 sec. = 1,000 units of light entering the camera
• at 1/125 sec. = 500 units of light entering the camera (shutter speed is twice as fast)
  
• at 1/30 sec. = 2,000 units of light entering the camera (shutter speed is half as fast).
  

Together, these two precision light valves determine the amount of light entering the camera to create the image captured by the photo sensor.

That’s it. With one exception, this is absolutely as technical an understanding that’s needed to tackle the subject of exposure. That exception is a question exposed in my explanation: If the aperture and shutter both do exactly the same thing (control the amount of light entering the camera) then why are both controls included in a camera?

Good question. Here's the answer.


Stopping the Action (shutter speed dependent)

Aside from controlling the amount of light entering the camera, the shutter also determines the motion-stopping capability of the camera. As shutter speeds increase, the camera is capable of stopping faster and faster motion. It makes sense when you think about it. If you are taking a picture of a baseball batter’s swing at a shutter speed of one second, the reality is that the entire swing took less than one second. So, you’ll record the entire swing with a one second shutter speed. The resulting image will be motion “blurred”. If you used a shutter speed of 1,000th of a second, then you would only capture that part of the swing that took place in that 1,000th of a second. Visually, you have “frozen” the batter’s motion.

To illustrate this, the picture below was taken with a shutter speed of 1/60th of a second. The airplane was moving too fast for this shutter speed to "stop" the action:



In the second picture, the shutter speed was increased to 1/500th of a second and the motion of the plane was "frozen":




Depth of Field (aperture dependent)

Changing the aperture has no effect on the camera’s ability to stop motion. It does, however, have another major impact on your final picture - besides controlling the amount of light that passes through the lens. The smaller the aperture (iris opening) the more depth of field. Depth of field (often abbreviated as DOF) is an important and measurable distance. This measurement is the physical distance from the point closest to you that’s “in focus” to the point farthest from you that’s “in focus”. This distance varies depending on the f/stop selected. It’s no accident that the background in a professional portrait is “out of focus” while the person is very sharp. It’s also no accident that a landscape photographer has everything in focus from the nearest blade of grass to that distant mountain. As the aperture size decreases (smaller physical opening, but larger f/stop number), the more of the total picture “becomes in focus”. Shutter speed settings in themselves have no impact on depth of field.



In this example of depth of field, the aperture setting was f/2.0. This setting resulted in the subject being sharp while the background is "out of focus". This is an example of "shallow depth of field".



This exposure used an f/stop of f/16 to expand the range of focus and bring both the subject and background "in focus" at the same time. This is an example of "extreme depth of field".

This is why cameras have two light limiting controls – aperture and shutter. The two work together to provide exactly the amount of light needed for a proper exposure as well as the motion stopping and depth of field control needed to capture the image you envisioned.

So important is the visual impact of each of these light controls to your final image that today’s camera manufacturers provide you with optional settings that allow you to determine if motion or depth of field is more critical to the image you are about to shoot. You may have glanced at the section of your camera manual covering “aperture priority” and “shutter speed priority” modes. Professionals seldom set their cameras to auto exposure mode. They will likely use either “aperture priority” or “shutter priority” mode. Both of these modes will automatically monitor the available light to expose the image, but each has a different visual purpose in mind.

In “aperture priority” mode, you manually select the f/stop (aperture setting) that produces the amount of depth of field required in your picture. The camera will adhere to that f/stop and automatically adjust the shutter speed to accommodate that specific f/stop. For example, you are taking a picture of a flower and want the background to be “out of focus” to isolate the subject. You determine that a wide aperture setting is needed to create “shallow” depth of field, say f/2.8. Using “aperture priority” mode, you can manually dial in an f/stop of 2.8 and the camera will select the appropriate shutter speed to allow just the right amount of light to strike the photo sensor.

The opposite is true of “shutter priority” mode. In this case, you are shooting a soccer game where the action is fast and furious. You decide a shutter speed of at least 1/500th of a second is required to “freeze” the action of the players. By choosing the “shutter priority” mode, you can manually dial in 1/500th of a second, and the camera will automatically make the necessary f/stop adjustment to give you the proper exposure.

We’ll revisit these shooting modes later.

For the photographic purist, there is only one correct exposure for every scene. This correct f/stop and shutter speed combination is determined by exactly how you, as the photographer, want the image to appear in its final form (how light, how dark, motion, depth of field). That’s quite a challenge given the thousands of exposure possibilities available on your camera.

But I promised a common sense approach to photography. So, in reality, there are several exposures (f/stop and shutter speed combinations) that are “good enough” in most photographic situations. For any given photograph, there may be several shutter speeds that stop the motion you are recording, or more than one f/stop that will provide sufficient depth of field.

So, now we have a multitude of variables that the photographer must bring together to produce the best possible picture -- aperture, shutter speed, depth of field, motion, and what the exposure meter is saying. Bringing order to the chaos that these ingredients add to our recipe for the perfect picture will be the subject of my next post.

If you are confused by any of the information in this discussion, please read the material again. If you still have questions, you know how to reach me.