Sunday, February 12, 2012

Tutorial - Aperture Part 1

At its core, digital photography is one thing - the recording of light.  This recording happens by allowing light to strike a camera's sensor for a predetermined period of time - this results in an exposure.

To control the exposure (or, how bright or dark an image is), the photographer has only three tools - Shutter speed, aperture, and ISO (one could argue a 4th tool - the manipulation of light - but that's later).  Knowing how to control these variables and what happens to an image when one or more are changed is key to understanding the very basics of photography.

Aperture - In layman's terms, it's the relative size of the opening of the lens during the moment of exposure.

Usually, aperture is expressed as an f-value or f-stop, for instance f/2.8, though many times on lenses it will be represented by the number one, followed by a colon, then a number or range of numbers (e.g. 1:3.5-5.6).

The important thing to remember with aperture is that it is inversely expressed, meaning the smaller the number, the larger the opening in the lens (and therefore, the more light allowed to the camera sensor).

Lenses with large apertures (small f-numbers) are sometimes referred to as fast lenses, because the more the lens allows light to pass to the camera, the higher the shutter speed the camera can achieve (while still making a proper exposure).

Each whole "step" in the aperture is called a stop - Moving down one stop (meaning, going to a higher number) will halve the light to the sensor.  Likewise, moving up one stop (a smaller number) will double the light to the sensor.

Generally, the photographer can change the aperture using a finer step than a whole stop (typically  changing in half-stops or even third-stops), but for simplicity we'll stick to whole stops for now.

Aperture Chart

Very Fast - Fast - Somewhat Slow - Slow

An Example:

Let's say we have our camera and we've put it into Aperture Priority Mode (usually represented as 'A' or 'Av' on the mode dial).

In this mode, the photographer will choose the aperture, letting the camera calculate the shutter speed and possibly ISO.

So, we set our aperture to f/2.8, point the camera at an object, and take the image.  Afterwards, we review the image on our camera (your screen will most assuredly look a bit different than what I show below - these images are from a Canon 5D).

Sample informational photo screen (Canon 5D), showing the parameters of the image I just took.
Take note of a couple of areas, which I've outlined in boxes.

The first red box, containing 1/500, is the shutter speed of the photo that was taken.  Since I'm in Av mode on my camera, the shutter speed was automatically calculated by the camera -- I had no direct control.  

The next red box reading 2.8 is my aperture.  Now remember, in our example, we are in aperture priority, so this number can be altered by the photographer.  On the particular lens I used in this shot, f/2.8 is the fastest (smallest number) I can choose to shoot.

And while it's part of another tutorial, note the histogram outlined in blue.  In essence, this is a graphical representation of the pixel distribution of the image as it was exposed.  It's actually a very useful tool, but for now just note the location of the spikes in the histogram and where the white graph is located in the grey box.

The Next Photo

Sample photo screen #2.  I had shot this photo using an aperture of f/4
Now look how things have changed.  Before I took this photo, I adjusted my camera (which is in Av mode) to an aperture of f/4.0  I then pointed the camera to the same exact spot on my screen I had before and took the photo.

Notice what changed and what didn't?  The shutter speed, which in the first example was 1/500s, has now halved to 1/250s, meaning it took the camera twice as long to take this photo than the one above it.  Granted, 1/250s is pretty darn fast (considering the blink of an eye is a relatively slow 1/3 of a second),  but by stopping down my aperture by 1-stop, I've effectively halved the light reaching the sensor, so now the camera has to hold the shutter open twice as long in order to get the same exposure (notice the histogram (exposure) in this shot - it looks nearly identical to the one in the above photo!)

Third Photo screen example - shot with aperture of f/5.6
In the third photo, I again stop down the aperture 1 complete stop (reference the chart above and see that 5.6 is the next stop slower than 4.0) and notice my shutter speed is now 1/125s, which is only half as fast as what f/4.0 gave me and only a quarter as fast as what f/2.8 gave me.  Likewise, an aperture of f/8 (one stop slower than f/5.6) would result in an even slower shutter speed of 1/60.

But, aperture doesn't only affect how fast your shutter speed is - it also has profound effects on how your image looks.  Curious?  Read on to Part 2!

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