Aperture 102

Now that one has a Rookie's understanding of aperture, what does it mean for the picture? Two main things occur when changing the aperture. First is the total amount of light coming through and second is how much focused light is coming through.

I put my camera dial on "A - Mode" and took these 3 shots below as I adjusted the apertures from,     f2.8  /  f10.0  /  f32.0. In "A-Mode" I was able to adjust the aperture and exposure and my camera decided my shutter speed.

The picture below was taken at ISO 100, f2.8, shutter 1/125 and exposure ±0. My aperture was set at its widest ratio 1:2.8 (f2.8). The amount of light coming though the lens was at its maximum capacity.  It was like opening a faucet completely open, allowing the water to rush in without any restrictions. Because the light comes in so freely my camera decided at exposure ±0 my shutter speed would be at (1/125sec). Also notice how only 1 pole is in focus. The wider your aperture curtain becomes the shallower your Depth of Field (DoF) becomes.

The picture below was taken at ISO 100, f10, shutter 1/8 and exposure ±0. See how more poles are now in focus at f10, compared to f2.8. At f10 almost all the poles are in focus. This is because the light is being restricted by the reduced aperture's curtain diameter which causes only focused light to come through. Also note, the shutter speed. Did you notice that the camera decided to to open the shutter for longer (1/8sec) to obtain the same exposure levels as the picture on top.

This last picture was taken at ISO 100, f32, shutter 1/1 second and exposure ±0. Now look at how everything is in focus even the background. Reducing the aperture's curtain width to a pin hole size restricts the light coming through the lens. Similar to a squirt gun, a pin hole size opening straightens out the stream of water going through it. Same goes with your lens, as the amount of light is being reduced by the smaller aperture curtain width, your lens only allows focused light though it and onto the sensor. This is why the entire image is in focus. Did you notice the shutter speed for f32? The camera decided to keep the shutter open for longer to gather enough light to have the same exposure level as the other pictures.

The Rookie Explanation. 

If you want a blurry background effect then you set your aperture to its widest curtain settings (example, f2.8). If you want everything in focus set your aperture to is smallest curtain setting (example, f32). The smaller your aperture curtain the longer you need your shutter to be open to have a properly exposed image.