Friday, February 5, 2010

Aperture and Depth of Field (Exposure part 1)

This is the third post in my ongoing “Lessons from Photography Class” series. You can find post one (Basic Definitions) here and post two (Camera Controls and White Balance) here. In this post I want to go over specifically what the aperture is and how it works.

Since a camera is basically a light tight box, the aperture and the shutter are how light enters the camera. Like I said in my first post, exposure is basically the end result of light acting upon the photosensitive material (i.e. the film or sensor). The intensity of that light (controlled by the aperture) and the time interval it does so (controlled by the shutter) are what make up the exposure (ISO also has a role to play but I don’t want to over-complicate things so we’ll just ignore it for now).

Just think of your eye. If the aperture is your pupil, then your eyelid is the shutter. When your eyelids are closed, light can’t enter your eye. But if you keep your eyes open for a long time then sometimes too much light can enter your eye (I know I can’t be the only one who gets headaches when it is too bright outside). The shutter works the same way. Does your eye only depend on the eyelid to control the amount of light though? No it doesn’t. It uses the pupil (in the sense that is dilates and contracts) to control the flow of light in conjunction with the eyelid. Your camera uses the aperture the same way.

part of my "Depth of Field" assignment

I don’t know if that analogy helped make things clearer or just complicated things more, but my point is that the aperture and the shutter work in conjunction to control the amount of light that enters your camera. If too much light enters the camera then you have an overexposed picture that will be very white. If not enough light enters the camera then you will end up with an underexposed picture that will be really dark/black.

Overexposed = too much light.
Underexposed = not enough light.

Easy enough to remember right?

Now onto aperture specifically; once again the aperture is the opening through which light passes into the camera. It is usually housed in the lens (though not always) and it is expressed by an “f” over a number (example: f/22).

In my class, people seemed to have had the most trouble remembering that a large aperture opening is represented by a small number. So f22 is a very small aperture opening while f/2.8 is a very large aperture opening.

a. Example of a large (f/2.8) and small (f/16) aperture setting
Body Jewelry Gauge Chart

For me, I just think of ear gauges or wire gauges but whatever method you use to remember doesn't matter as long as you do remember. That's the most important part. In the end it doesn't really matter if you can remember the aperture scale if you don't know whether or not f/5.6 is a large or small opening.

In fact, after my test on Tuesday, I wouldn’t be surprised if I completely forget the aperture scale. It’s just not needed in order to take pictures as long as you understand the way aperture works. The most important thing to know about the aperture scale is that closing down one f/stop halves the amount of light and opening up one f/stop (or going to the next lower setting) doubles the amount of light entering the camera.

Here is a diagram that was in my text book that I think explains it visually really well:


Now some of you may be wondering why the hell any of this matters; well beyond how the aperture affects exposure (which I’ll get into at some time in the future), the aperture setting also controls the “depth of field.” The Depth of Field, or DOF, is basically how much of the picture is in focus.

If you want a large depth of field (which means everything in the picture is in focus) then you use a small aperture opening. If you want a shallow depth of field (which means the background will be blurry) you use a large aperture opening. An easy way to remember is by thinking of your aperture setting as distance, miles, feet, yards, etc. (Like if you’re taking a picture of a person standing in front of some buildings and you want the buildings to be in focus along with the person you would choose f/22 since you need 22 feet in focus. It’s not a perfect example but I hope you catch my drift.)



I really hope I didn’t make this too complicated. I was originally going to post on aperture alone without bringing exposure into it, but I don’t think that’s a good thing. Aperture and shutter speeds are the way you expose your film/sensor to light and it’s important not to forget that. The way I got a grip of the different aperture settings is by taking the same photo over and over using different aperture stops (as you can see by my pics). I’d really recommend this as it gives you a hands on approach to everything I talked about in this post.

Next time I’ll talk about shutter speeds and then I plan on doing a post about how aperture, shutter speed, and ISO all relate in regards to exposure.

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