Tuesday, May 4, 2010

Aperture

I'd like to discuss aperture and speed this week. These two things are what the photographer is primarily controlling with the manual modes of a typical SLR camera. I talked about how light is everything. Aperture and Speed control how much light is allowed into the camera. This is where "stops" come in. A "full stop up" allows exactly twice as much light into the camera as the stop below. With modern digital slr cameras, our settings aren't set to full stops anymore. Often there are multiple fractions of stops, so the old understanding of twice as much light doesn't apply with all the numbers the camera shows.

Aperture controls the size of the opening when the shutter opens to allow light in. If you squint your eyes, you generally think you can see farther. It is this way with the camera. If the opening is large, the plane of focus is narrower. If the opening is small, the plane of focus becomes a much larger area. Here is an example of wide aperture:

Notice how thin the area is that is actually sharp. You can see the buttons on her dress, her face, her hand, and the plant are all fairly sharp. Her feet and everything in front and behind her start to get fuzzy. Here is another example:
Notice how blurry the railing behind her is. If you look closely, the front of her face is sharper than the top of her head or her ears.
In both of these I used an aperture of f/1.2. Notice that I placed the 1.2 as the denominator of the fraction. This is helpful. In your camera, aperture is just shown as the number. But you need to know that the SMALLER the number - the WIDER the aperture (because it is in actuality the denominator, and not a whole number). A lot of standard lenses only "stop down" to about f/4 or f/5.6. Notice we write it as f/number while the camera only shows us the number. Therefore an aperture (as seen in the camera) of 8 is actually a smaller opening than 4.

Now lets talk about Speed. This is referring to how fast the shutter opens and shuts to allow light in when you push the button. If the aperture remained the same, every time you changed the speed a full stop (note that on the old SLR cameras, the only options were full stops - this is no longer the case and when you dial the speed settings up or down, we actually see a lot more numbers, or fractions of stops) you let in twice as much light as the stop before it. Once again we will notice that the camera shows us only the denominator. For example: if it shows us "60" it means 1/60 of a second.

So, we use a combination of speed and aperture to define how much light we let in, and in the process we set the camera to take a faster or slower picture and also set it to have more or less of the picture in focus.

In the manual section on your dial, Av is where you set just the aperture and let the camera guess at the appropriate speed to let the right amount light in. Tv is where you set just the time (speed) and allow the camera to guess at the right aperture. M is where you set both yourself. I often work in Av mode, because I like to adjust the aperture as I like and then let the camera set the speed. I do have to watch the speed to make sure it isn't so slow that it shows my unsteadiness in holding the camera (blurriness) or movement of what I'm taking a picture of (unless that is what I want). There are times I set the speed to make really sure that the shutter can move fast enough to freeze the action. In the following example I still used Av mode, but I was watching to see what shutter speed it was using.

Here the aperture was f/2.8 (shown as 2.8 in the camera) and the speed was 1/2500th of a second. This speed froze the action of my son jumping through the air. We do not see any blurring of him. The camera opened and shut so fast that it caught a fraction of a moment and froze it in picture form.

If you look back to my blog post in mid March at the picture of my little girl in pink running, you'll see a lot of motion blur. The sharpest thing in the photo is her face. Her feet are a blur, her arms, and everything around her. In that photo I used an aperture of f/16 (Av mode) in order to get the speed very slow (1/25th of a second). I panned with her, keeping my focus on her face and slowly pushed the shutter. I was intentionally trying to capture a sense of the motion by keeping the shutter speed so slow that she was moving the whole time the shutter was open.

I hope that clarifies and explains a few things. Feel free to post questions and I'll do my best to help!

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