Basic Operation #
The word camera originates from the Latin word meaning “chamber.” And in a way, a camera truly is a chamber: a light-tight box with an opening that allows light in.
The objective of a camera, any camera is to allow the correct amount of light to hit the image plane or film*. We call this “Exposure.”
The primary camera used in this course is called a Digital Single Lens Reflex camera. This is the descendent of the Single Lens Reflex camera from the days of film.
1971 Film Camera. Modern Day Digital Camera
When you press the shutter button on your DSLR camera, three things happen in sequence:
- The Mirror in the camera moves up to clear a path for the light to strike the sensor.
- The Aperture opening closes down to the pre-set F-stop.
- The Shutter opens for the pre-set amount of time to allow light to strike the sensor.
|The workings of a DSLR camera
There are 3 key camera components that control exposure: #
ISO Setting #
Shutter Speed #
You can see the relationship here in a graphic commonly known as the Exposure Triangle: (Links to an external site.)
The ISO setting represents the camera’s sensitivity to light. The higher the ISO, the more sensitive the camera is. The lower, the less sensitive it is.
The Shutter of the camera is a set of small metal curtains in front of the image sensor that open for a specific amount of time.
Shutter speed examples. Notice that the higher the shutter, the faster the curtain travels
Aperture: The Meaning of the Numbers. #
The Aperture, describes the size of the diaphragm opening in the lens.
Of the three camera controls regulating exposure, the aperture is often the most confusing initially.
The number used for the aperture settings is called an F Stop. At first glance, the F Stop numbers do not seem to make much sense. But they do once you realize the F Stop number is the ratio of the lens’s focal length to the diameter of the aperture opening.
F-Stop = Focal Length / Aperture Diameter #
As you can see from the above Figure, the aperture is a numerical value for how big of a hole the diaphragm is making. A larger hole means that more light is let in at any given time. The reason for this complicated equation is to ensure that F Stops are the same, no matter the camera, format, whether it is film, digital, or even video. All F-Stop settings are universal.
So, with that, you can use this little guide to remember which f stops will allow more or less light:
A lower number = a larger hole.
A higher number = a smaller hole. #
Camera Shooting Modes #
As you can probably tell, there are a lot of shooting modes available on your camera. Just a quick glance at some of the control wheels on cameras shows a dizzying array of options. These options control things such as what you choose to prioritize: shutter, aperture, or completely automatic functions. Consult your camera manual for a full description of the camera’s functions.
A sample of control dials from various DSLR and Mirrorless cameras:
Most cameras have what is called a light meter inside of them that work to determine proper exposure. And todays modern cameras have very intelligent light meters with different modes that will get you in the ballpark most of the time. The ultimate goal, is to get a good exposure.
A good exposure analogy is a pail and a sponge. The pail corresponds to the lens, a hole in the pail works the same as the aperture, and the amount of time water is allowed to flow through the hole could be considered the shutter speed. The size of the sponge to be filled would be the equivalent of the ISO. If you poke a small hole in the pail, it will take a certain amount of time for the sponge to become saturated. If you poke a larger hole in the pail, it will require a shorter amount of time for the sponge to fill. This picture is pretty easy to visualize and the concept is the same for a camera exposing film. The more light to pass through the lens at any given time, the shorter the shutter has to remain open for the film to become correctly exposed.
It can start to get confusing when you consider that there are three factors to getting a correct exposure:
ISO, Aperture, and Shutter speed.
Changing any one of the above variables requires changing at least one, possibly both, of the others. The following formula is not a formula to determine exposure, but to show the relationship between the variables:
Aperture + shutter speed = amount of light needed to get the correct exposure for a particular ISO
Smaller ISO numbers need more light. Larger ISO numbers need less light. That is because for any given aperture, the less light needed, the faster the shutter speed is. It is important to realize that the above formula is for the amount of light needed, not the numerical ISO assigned to the film. Now at this point in time, it is not necessary to know all of the formulas needed to get the correct exposure. It is however very important to understand fully the relationship between all of the variables that you will be dealing with.
- For any given ISO if the aperture is changed to let in less light then the shutter must remain open longer to let more light in to compensate.
- If the numerical value of the ISO goes up then less light is needed and the aperture must be closed down to let less light through, or the shutter speed must be increased or both to lesser degrees.
These are the three variables for getting good exposure. The correct way to look at it is that for any given ISO there is only a certain amount of light that will give the correct exposure. Then any combination of aperture and shutter speed that gives that amount of light will be a correct exposure. Just as in adding two numbers that equal 10 there are a number of correct solutions:
1 + 9
2 + 8
3 + 7
4 + 6
5 + 5
All of these combinations are correct. And just as when you increase a 1 to a 3 the other number must be decreased by the same amount from a 9 to a 7, if you increase the light from either the aperture or shutter speed you must decrease the other by the same amount.
Since your light meter will give you the correct combination of aperture and shutter speed assuming the ISO is set correctly, all you need to know is how much of an increase in one is equal to a decrease in the other. We already know that from the one stop increments listed for both the aperture and shutter speed in the previous lesson. Just remember, decreasing the f-stop number lets in more light and decreasing the shutter speed lets in more light. Therefore, if you move down in the f-stop you must move up in the shutter speed.
Remember the word f-stop refers to the representative number of the aperture while the word aperture refers to the actual size of the opening. Also the shutter speed is actually a fraction of a second when in the normal range. Refer back to the aperture and shutter speed diagrams of the previous lesson.
E.g. Assuming that f-8 @ 1/250 is the correct exposure: if the f-stop is changed from an f-8 to an f-16, 2 stops, the shutter speed must change in the opposite directions 2 stops from a 1/250 to a 1/60. The f-8 opening is larger than the f-16 aperture opening; therefore light is decreased. The shutter speeds are actually 1/250th sec and 1/60th sec. 1/60th of a sec stays open longer than 1/250th , letting in more light.
For any given ISO film, the smaller the f-stop, the faster the shutter speed needs to be. And the vice versa holds true, the larger the f-stop the slower the shutter speed needs to be. That is because the size of the aperture gets smaller the larger the f-stop number gets. Note the diagram above.
Basic Exposure Formula (or The Sunny 16 Rule)
BEF states that for a given ISO rating the correct exposure at midday f16 at a shutter speed of 1/ISO, or whichever shutter speed is closest to this value. Since most cameras list the shutter speeds without the fraction, if you pick the closest number on the shutter speed dial to the actual ISO of your film you should get a correct exposure on a sunny day between the hours of about 11am-3pm. This is not the correct exposure for morning, evening or cloudy days. There are corrections that you can make for the above rule that should get you close to the correct exposure. But without artificial light the exposure should only increase, i.e. smaller f-stop number or longer shutter speed. During a partly cloudy day increase by 1-1.5 stops. A completely overcast day might require 2-3 stops more exposure. Light shade as in shadow but with a good amount of fill light from the surroundings might also be 2-3, but deep shade might require 4-5 extra stops from BEF.
There are two ways that you can train your eye to see these differences. The first is to take a lot of pictures, keep good notes and check your results. Once you become adept at using a light meter, use it to check your guesses. It is instantaneous and quite accurate. Once you feel comfortable with your guesses, go out and shoot a few frames without the assistance of your meter to see how good you really are. This may seem to be a waste of time and if your meter never breaks down then it may be. But is very good to know just in case. Because there are certain lighting situations that are very difficult to meter, such as a night scene with various light sources, where an adjustment to BEF as discussed above will get you a close exposure. These adjustments will be discussed in more detail later.
The aperture, together with the shutter, combine to determine the total amount of light that strikes the image plane. The ISO (or speed) you use will determine the correct amount of light needed. Each roll of film, or digital sensor chip, has an ISO rating. The easiest way to think of the ISO is to realize that it is just a standard. At this point it is enough to know that a film rated ISO 100 needs the same amount of light as a digital camera set to ISO 100. Once your camera’s light meter is set for the correct ISO, a light reading will give you the two numbers, aperture and shutter speed.
*(You will often see the word “film” used when explaining different concepts in exposure. Film and digital terminology is interchangeable when discussing exposure. 100 ISO film is the exact same sensitivity as a digital camera set at 100 ISO. Film is still used by many photographers, and the word sounds much better than “digital imaging chip.”)