If you missed the intro to this series you can find it here!
So the first thing that you need to learn on the long road of learning to become a great photographer is the "Magic Triangle" of exposure. This can be controlled via 3 seperate modes. Tv (shutter speed priority) Av (aperature priority) and M (manual mode)
The three elements of the triangle are "Aperature" "Shutter Speed" & "ISO". Centered among all of this is your camera's light meter. This will be your guide to help determine what to change in the triangle.
Quoting from an unknown source, Aperature could be related to a set of Drapes on your window. When you change the aperature you either Increase or Decrease the amount of light that will enter the lens. This is measured in F/Stops and most consumer lenses will range from f/1.8(being the largest opening) down to f/22 (being the smallest). That's Right, no typo here! The higher the number the smaller the opening... I will not get into the science here, that is what Wikipedia is for, but for the purposes of learning, the smaller the number referred to a specific lens means it is a "faster lens". All that needs to mean to you at this point, is that you can capture more ambient light through the lens while shooting in darker settings.
Aperature also has another affect on what your camera sensor records. You probably have seen the acronym DOF before. This stands for depth of field a.k.a depth of focus. When you use a larger aperature - smaller number = "Shallow DOF - the image will be sharp on whatever subject you have focused on (within the depth range) and blurry in all the areas that are not in the focal zone (front and behind the range of focus). This is a great technique when shooting portraits because you can bring your subject into crisp focus and blur out the distracting background. Ideally using a zoom lens and stepping further back from your subject will give you the shallowest DOF. Vice versa to create a deep DOF, bump up the aperatue to at least f/8 and for the sharpest of Landscapes you might even notch it up to f/22.
However here's the tricky part... Because your decreasing the amount of light that is entering the lens, you will need to compensate by increasing the Shutter Speed. When shooting in the range of f/22 you will more than likely want to have a tripod handy!
Shutter Speed, again quoting from an unkown source, could be related to the shutters on the window. The longer you open them, the more time that light can enter the window into the room (or camera Sensor/Film in this case). Typicall shutter speed will vary from 30 seconds all the way down to 1/4000 of a second. Unlike the aperature being directly related to the lens you are using, the range of shutter speed will depend on the camera body you are using. Also most DSLR cameras are equipped with "Bulb Mode" which will allow the shutter to stay open as long as you hold the button down.
As you can probably guess by this point, shutter speed also effects the amount of movement in the image. The shorter the shutter opens the less movement you will see. Hence the shutter speed correlating with "motion blur". The longer it stays open, the more motion you will see. In regards to sharpness, if using an Image Stabilized lens, it would not be recommended to shoot "hand held" at anything slower than 1/60th of a second with a short lens to achieve a nice crisp image. Even then, unless you have robot arms, any sort of enlargement will not benefit from maximum sharpness. Also keep in mind that the further you are zooming the more shaky the image will be. If you have one lying around, grab a laser pointer and stand at one end of your hallway. Point it at a single point on the furthest wall and see how steady you can keep it centered on that spot... Your probably starting to get the picture!
On the contrary, if you have a tripod handy you could open the shutter for say 15 to 30 seconds for shooting various scenes. Im sure everyone has seen those beautiful light streaking photos of cars passing by. This effect is achieved by using long shutter speeds. Keep in mind that when your using shutter speeds in this range, the camera will become very sensitive to movement. Steps should be taken to minimize movement, such as, using a self-timer mode or remote to trigger the camera & even resting a soft weight on top of the camera to minimize movement from wind. If movement/motion blur in your image is not acceptable for the particular scene that you're shooting, such as shooting people (depending upon your artistic vision for the photo), consider increasing the ISO and decreasing the shutter speed.
ISO is the camera sensors Sensitivity to light. Typically the lowest ISO setting will be ISO 100 or even ISO 50 on newer point and shoot cameras. With conventional film, the more sensitive the film is to the light, the grainier the image would be. Same goes for Digital, the more sensitive the sensor(think of this as your film for digital cameras)the more noise you will see in the photo. Camera makers are finding new ways to control this noise and some high end cameras will effectively reduce this noise all the way up to ISO 1600 but if your shooting with a Point and shoot you will probably notice undesirable noise at ISO 400. DSLR's handle the noise a little better because the sensor is substantially larger, therefore the pixels are larger and the more light they can capture. But that's a whole other blog series.
What you need to know... Firstly, turn off the automatic mode for ISO which will boost the ISO whenever it feels it is needed (low light situations) and start controlling it yourself. Start with the lowest setting (100) and adjust by reading your light meter. Take a few shots to determine the minimum ISO you can use to achieve the proper exposure for your scene. Then leave it for the period you spend shooting in that environment.
Of course this all ties in together and you may not need to increase the ISO.
Think of these three elements as a balancing act. When you change one setting you inherently affect 2 others. You need to ask yourself some important questions when deciding to make your changes to these settings.
Do I want an extremley sharp image? Do I want to show motion in photo? Can I use the flash to compensate for a lack of light instead of bumping the ISO? Do I want the background to be in focus or just the subject?
When you give to one of these settings you may have to take from the other. If you want to shoot a beautiful portrait with a nice blurred background than your probably going to opt for a Large aperature. Say, 2.8 and remember that your DOF will be quite shallow so keep in mind {Manual Focus on the eyes} to get them pin sharp and let the rest of the image fall where it may. If the rest of the face is too soft, then bump up the Aperature.
If you use a setting of f/2.8 chances are that you will need to decrease your shutter speed. If there is a lot of ambient light (let's say a sunny day) you will want to bring it down significantly. You may even find that you can't even expose your shot properly at your fastest shutter speed.
This would be typical if you were using flash for fill light and your "maximum sync speed is 1/200th of a second. All this means is that flash could not fire fast enough to be effective at any faster speeds. So then in turn you would need to decrease your aperature (make it a higher number). If your shooting in a house or shade you will find yourself needing to slow your shutter down or open your aperature wider.
Assuming your shooting in Manual mode so that you have total control over all of these settings simultaneously, you can use your exposure meter to decide which way you need bring the settings. If your not familiar with using it, the simplest way would be using "matrix or evaulative metering mode". Look through the viewfinder and point your camera at the subject your photographing. Notice a little bar with a +/- and some vertical lines? Adjust your shutter speed or aperature until there are no bars to the left or right of the center. Take a shot and if you see shadows that are too underexposed than increase your settings incrementally. On the other hand, you can also shoot in "Tv" or "Av" mode. "Tv" (shutter priority) will allow you to control the ISO and Shutter speed and the camera will take a light reading on the scene and adjust your aperature. Same idea for "Av" aperature priority. In these modes you can also change your Exposure Compensation. All your telling the camera here is that it did not expose your photo the way you wanted, so either make it lighter/darker in typically steps of 1/3 of a stop. It will change the aperature or shutter speed accordingly, depending upon what mode your shooting in.
Well this was quite the longwinded post and I still have not even scratched the surface of exposure. If you would like to learn more, there is an excellent book available by Bryan Peterson called Understanding Exposure"
Part 2 of 3 "Image Composition" now available here
If you know of anyone who would enjoy or benefit from this series, please pass it along!
Here a few more diagrams to help understand Exposure
Diagrams courtesy of Silverstrand Photography
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