Today I want to talk a little bit about the Aperture Priority mode on SLR camera's. Learning how to use this really opened photography up for me and started me on a much more creative and fulfilling path.
The Aperture is the hole in a lens that light travels through before it reaches the shutters in SLR camera's. Using your camera in Aperture Priority gives you the freedom to move through the lenses different aperture settings without having to worry about the shutter speed.
Understanding how to use it will enable you to do shots like this where only a precise part of the image is in focus.
This is known as having a Shallow Depth of Field. It was shot with a large aperture. F1.4 (small F numbers mean large Aperture's).
And shots like this. Expansive depth of field. Landscape photographers use a large depth of field so that they can have items in the foreground in focus as well as items in the background. This was shot at the smallest Aperture possible for the lens F22 (Large F numbers mean Small Apertures).
In Aperture Priority Mode, the aperture you select will be matched by the camera to a shutter speed that will give a good exposure.
It's important to understand that the size of the Aperture affects the amount of light potentially hitting the sensor. A large Aperture lets in more light than a small one.
With a small aperture (F22) the camera needs to compensate with a longer shutter speed. This is because the hole is small and lets in less light so it takes longer to create an image on the sensor.
Conversely with a large aperture (F1.4) the camera compensates by speeding up the shutters to stop too much light getting in and Over Exposing the image.
So remember this...
LARGE APERTURE = SMALL F NUMBER (F1.4), BLURRED BACKROUND AND FAST SHUTTER SPEEDS
SMALL APERTURE = LARGE F NUMBER (F22), SHARP BACKROUND AND SLOWER SHUTTER SPEEDS
This is why landscape photographers will tend to use tripods. Slower shutter speeds will cause camera shake and blurred images. Using a tripod allows hugely slow shutter speeds of minutes and eliminates shake.
It's also how photographers freeze action. But more on this in the next section that covers Shutter Priority, S or Tv (time value) mode.
Here is a graphical representation of depth of field.
And here is another highlighting an important point.
In this second diagram point B identifies the focus point. Notice how this is not in the centre of the focal plane (area in focus). This is why good landscape photographers focus on a point that is a third in in the image. It's also useful to know if you are ever taking group shots. Make sure that you always focus on someone in the front row, as at wide apertures may find people in the background to be slightly blurred!
A final point with Aperture priority is that it's great for creative portraits and low light shooting.
To create even more of a blurred background stand closer to the subject. The closer you are the more bokeh you'll get.
If you find yourself in low light then open the Aperture as wide as you can. Check your shutter speed. If it is above the focal length (you'll see that on your lens in millimeters) then you are lightly to be able to get a shot without camera shake. If shutter speed is below the focal length then up your ISO until it is.
I love Aperture priority particularly because I love blurred backgrounds. They make the subject stand out from the background and can often yield dreamy results. The blurred effect is often called "bokeh" (pronounced boke - eh).
Here's a nice example...
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