Exposure

In order to move beyond the P mode of your camera you must understand the basics of exposure. In terms of photography exposure refers to letting light get to your cameras film or sensor, in the case of digital cameras.

A big part of making a good photograph is getting the proper or desired amount of light onto the film or sensor. When you click the shutter release on your camera the shutter opens for a small amount of time allowing the light reflected from the scene to pass thru the aperture opening in the lens and fall onto the sensor. The camera uses the f-stop and shutter speed to vary the size of the hole in the aperture and length of time the shutter is open. The camera can do this totally automatically (Auto mode) with help from you (P, A and S) and it can give you total control (M). The other special modes not mentioned leave the camera in full control.

Aperture and Shutter Speed
The size of the hole in the aperture is referred to as the f-stop and the length of time that the shutter is opened for is referred to as the shutter speed. Now think about that a little, we have 2 ways to control the total amount of light hitting the sensor. A big hole in the aperture will let a lot of light in while a small hole only lets in a little light. A longer shutter open time will also let more light in while a shorter shutter time opening will let less light in.

The proper exposure of light to the sensor can be had with various combinations of the aperture and shutter. A big hole in the aperture with a short shutter open time will allow the same amount of overall light in as a smaller aperture opening and a longer shutter speed. So now its starting to get a little confusing whats the difference between big hole (aperature) and short time (shutter speed) or little hole and a long time.

How Do You Choose?
Understanding how the various shutter speed and aperature opening sizes (f-stop) affect the looks of the photo is the key to picking a faster shutter or smaller aperature. A larger aperture while letting more light in also reduces the depth of field in the photo. The shutter speed will effect things that are in motion in the photo. If you have a longer shutter opening and a quickly moving subject, the subject may vary well be blurred in the photo.

Another motion issue to deal with in relation to shutter speed is camera shake. The longer the shutter is open the more prone you are to shake the camera while the shutter is open. The longest shutter speed that you can use without moving the camera will vary based on the zoom level of the lens and your ability to hold the camera still.

Sometimes You Can't
Now that we know how the 2 exposure control methods effect the looks of the photo we can start to decide when to use a larger versus smaller aperture or faster versus slower shutter speed. Sometime you really can't choose and are forced to go one way or the other based on the current lighting situation. For instance if in a low light situation, you don't have a tripod and your subjects are moving you know you must steer towards a larger aperature to get a faster shutter speed. i.e. a bigger hole (aperature) lets more light in so the shutter can stay open for a shorter period to reduce possible blur.

I have purposely left out any specific measurements and settings so as not to confuse you any more at this point. I have also not mentioned the effects of the ISO setting either. Right now it is just most important to remember that bigger hole (aperture) lots of light means shorter shutter open time while the same exposure can be had with a smaller hole (aperture) and longer shutter open time.

Next lesson will be on ISO, actual f-stop numbers and shutter speeds.

Please post comments, questions or mistakes in my writings to this in the D40 Plus groups thread on this article.