Wednesday, December 10, 2008

Exposure is just like filling a bucket

How do you get the right exposure and take control of your camera?
I decided to write this after I met a few, new camera-owners who complained that they completed a photography course through an online photography "college" but still did not know how exposure relates to aperture and shutter speed settings.
First off it is important to understand there is no single "right" exposure. For any given picture-taking situation there are a number of different exposure combinations, each of which is "right" but only one that will achieve exactly the effect the photographer wants.
If you truly want to master the craft of photography you must go back to the basics, understand the fundamentals and take control of your camera.
A good analogy is flying an aircraft. A modern, commercial jet has so much sophisticated electronic wizardry it can almost fly itself. The pilot may use the technology available to make flying easier and more convenient but he or she still needs to know how to override the systems and take control. Ultimately the pilot must be in command of the aircraft -- not the other way around!
The same principle applies to you and your camera and, if you can grasp the principles that follow, you will be able to cope with almost any photographic situation.
The foundation of all photographic technique is the triangle or pyramid of exposure.
On a blank sheet of paper draw a triangle. Above the top point write "ISO," then, beside the bottom left-hand corner write "shutter speed" and next to the right-hand point write "Aperture (hole size)".
You have just drawn the Pyramid of Exposure, but what does it mean and how is it applied?
We must step back to the days of film and film cameras. When you bought a box of film it was (and still is) marked with an ISO number, for example, ISO100, ISO400, ISO1600 etc (sometimes the label read ASA100 etc but ISO or ASA ratings are the same thing.

ISO400 film

From a completely simplistic point of view, film is made up of light-sensitive chemical crystals laid down of a strip of celluloid (don't write and tell me about about dyes and other components, the purpose of this article is to present an easily-understandable point of reference.)
These crystals are laid down in different grain sizes to allow the film to react to light in a certain way. Larger grain sizes let the film accept and react to light and fast film has many advantages. So why do not simply always use a "fast" film such as ISO800 and be done with it?
Because the down-side of a larger-grained film is when the image is enlarged, grain becomes more visible and sometimes unattractive.
Slow film on the other hand has smaller crystals that, when enlarged, do not show up to the same extent.
The obvious question then is why not always use slow film? Because slow film requires light be allowed to fall on it for longer period. That sometimes makes it difficult for the photographer to hold the camera steady while taking the picture.
There are ways to work around the problem that I'll discuss later.
What does all this have to do with digital photography? Scroll through your camera's menu you will see an option to set the ISO speed rating. Your camera's sensor -- in practical terms -- works much the same way as film in as much as the lower you set your ISO rating, the finer the "grain" will be. Higher ISO settings bring more "noise", the digital equivalent of film grain. I have no idea what the process is behind the sensor. Don't worry about it. All you need to do is remember the principle outlined above.

Shutter speed

The next point on the triangle you drew you marked "shutter speed". Earlier I talked about the amount of time the film crystal need light from the image to shine upon them and that time is called "shutter speed."
When you press the shutter button on your camera it causes a little "door" -- the shutter -- to open to allow light to fall upon the film or the sensor. One of the ways you (or your camera if is automatic mode) contol the quantity of light allowed in is by deciding how long the door will be left open before it is shut again. That is shutter speed. Typically your camera can open the door for a period as little as 1/8000th of a second to as long as a few minutes or even hours.
It is important that I point out that all the corners of the triangle of exposure are linked and changing any one, will affect the other two.


Inside the barrel of an old Sun lens showing different aperture settings and a reflection of the photographer.

The final corner of the pyramid, "aperture" is often the most difficult for students to understand.
Inside a camera lens is a series of "blades" arranged to form an adjustable hole through which light passes on its way to the film or sensor. The blades can be set to form a smaller or a bigger hole (aperture). This hole is called the "f-stop". On some cameras -- many digital camera lenses won't let you do this -- you can see what I am talking about by removing the lens from your camera, setting it to its manual setting and then moving the ring on the lens with markings that read 22, 16, 11, 8, 5.6, 4 etc. Your lens may have more or fewer graduations. As you twist the aperture ring look into the lens and you should clearly see the aperture size change.
What you will also notice is, the bigger the number on the aperture dial, the smaller is the hole (aperture) size. The hole visible when (f)16 is selected is much smaller then the hole that appears when (f)4 is selected. In fact, each aperture size is either double or half the size of the setting on either side of it.
Let me explain: the hole when f11 is selected is twice as large as the hole when f16 is chosen but only half the size of the aperture at f8.
Do not worry now about why or when you choose a particular aperture -- that will be covered in another post. Now it is enough to simply grasp the idea of what goes on inside your camera's lens. On a digital camera you set the lens aperture with a thumb-wheel on the camera's body while apertures on mechanical cameras are set manually on the lens.
To properly understand how the three points of the exposure pyramid how they relate to each other we must once again step back to film and the ISO rating.

Sunny 16

When you buy a roll of film that, for example, is marked as being ISO100 that number has a specific value and is not simply arbitrarily-chosen. ISO ratings tell the photographer how to set the camera in order to produce a properly-exposed picture. ISO speeds are linked to a rule known as the "Sunny 16" rule that holds, on a bright, sunny day set your aperture at f16 and your shutter speed at the speed closest to the ISO rating of the film (or sensor) for correct exposure.
An example will better explain. Let's pretend you have bought a roll of ISO400 film -- or set your digital camera to ISO400 -- the sun is bright and clear so, using the "Sunny 16" rule, you can set your shutter speed at the nearest speed to 400 (a digital camera will most likely allow you to set your shutter speed at exactly 1/400th of a second but on a manual camera you will need to set it at 1/500th of a second), set the aperture at f16 and go out and take pictures. Do this and you pictures, in those specific lighting conditions, will all be properly exposed.
But what happens in this example if you want to set your shutter speed at 1/1000th of a second? What must your aperture setting be?
It's really quite simple. When you use a shutter speed that is twice as fast (1/1000th is twice as fast as 1/500th of a second) it stands to reason you will only be allowing half the light to fall onto the film or sensor -- remember the principle of how fast you open and shut the door. In this case then, if the door is going to held open for only half the period, you need to use a bigger door (aperture) to let in the same amount of light.
Remember, aperture settings are double or half of those adjacent to them. Therefore, if I half the time the shutter stays open and still want to allow in the same quantity of light I must then double the size of the hole in the lens. F11 is twice the hole-size of f16. So, in this particular example, to keep the amount of light that strikes the film or the sensor constant, if I want my shutter speed to be 1/1000th rather than 1/500th of a second, I must open the lens aperture up to f11.

Filling a bucket

An easy way to grasp this concept is to liken the photograph you plan to take to an empty bucket you intend to fill from a tap that has an attached hose.
If you increase the diameter of the hose (the aperture) to allow more water to flow, you will have to turn the tap off sooner.
Conversely, if you make the hose diameter smaller you are going to need to let the tap run for longer.
On your camera, full aperture stops are f22, f16, f11, f8, f5.6, f4, f2.8. Digital cameras show an intermediate value that is one half or third stop between the two nearest full stop values. So between f16 and f11 you will be presented with an intermediate value of f13. Do not be concerned about intermediate values. At this point, all you need to remember is full f-stops allow in half or double the amount the light let in by adjacent full f-stops values.

Amaze yourself with a field outing

To see this process in action a field outing is in order.
Choose a sunny day and follow these steps and it will all become clear:

1. Set your camera to "manual".
2. Choose and set an ISO rating.
3. Set your shutter speed to the speed nearest to that of the ISO setting.
4. Set your aperture to f16.
5. Take some pictures and, if you are using a digital camera, check the histogram and the image on the screen.
6. Be amazed at how good the exposure is.
7. Now go into some shade. It is darker than in the open sunlight so obviously you need more light. Allow more light in by opening up your aperture to f8 or alternatively by slowing the shutter speed down by two stops (eg from 1/500th to 1/125th).
8. Take the picture and check the histogram and viewing screen again. If you feel the image is too dark open up another stop or slow down the shutter speed further. Tweak the aperture and/or shutter speed settings picture the exposure is exactly the way YOU want it.
9. Pat yourself on the back -- you have just taken back control of your camera.
In the next post we will explore the reasons why we choose particular apertures and shutter speeds.
Let me know how you do.

P.S. I have no idea why these postings do not insert spaces at the beginning of the first sentence of a new paragraph. I put 'em there but this thing seems to have a mind of its own!

1 comment:

OrelleJ said...

Really interesting! I always learn something from your blog. I hope you keep writing.