The Exposure Triangle: Aperture, Shutter Speed, ISO

Course author Michael Freeman reminds us that “right from the start” we should familiarize ourselves with the following basic concepts.  This shouldn’t be hard to do, as all three photography classes I’ve taken spent a great deal of time talking about just these things.



What the f-stop numbers stand for  and 

How the lens aperture controls the light reaching the film

The aperture is an opening that allows light passing through the lens to reach the sensor (or film).  The smaller the opening, the less light reaches the sensor; the larger, the more.  In the camera, the opening is formed by a set of overlapping leaves and looks like what you see in the chart here.

Diagram from:  http://basicfilmfall01.blogspot.com/

The size of the diaphragm can be modified, and a series of numbers indicate relative size.  What most beginning photographers note is an inverse ratio – the larger the number, the smaller the aperture.  This is not difficult to conceptualize when you think of the diaphragm as keeping out excess light, which is why the numbers representing aperture size are called stops.  The smaller the hole, the more light is stopped from reaching the sensor.  As you can see in the chart, f4 is stopping more light than f2.8.  It is in fact allowing in half as much light.   Just to make things more confusing, stop is also a unit of measure.  Each time you move up or down a stop, you increase or decrease by half the amount of light reaching the sensor. 

How the shutter speed controls the light

When you click on your camera to take a photo, a screen momentarily flips up to allow light passing through the lens to reach the sensor and thereby make a photo.  The amount of light touching the sensor is determined by the aperture, or the size of the diaphragm.  It is also determined by how long you allow the screen to remain up.  This screen is called the shutter.  The length of time you allow it to be up, or open, is the shutter speed.   The button you push to take the photo is the shutter release.

So, why should this matter, if you’re already controlling light through the aperture?

Diagram from:  http://basicfilmfall01.blogspot.com/

Sometimes you want a large f-stop (a small aperture).  This is particularly so when shooting scenes with great depth of field.   That is, the scene you wish to photograph has objects far away and you want everything, far and near, to be as sharp as possible.  A large aperture, remember, means a small opening.  You are blocking out a lot of light.  If you use a high shutter speed, the shutter will open only for a fraction of second.  Not much light will pass through that tiny aperture.  To compensate, you can set a slower shutter speed, allowing the shutter to remain open longer and more light to reach the sensor through the small aperture. 

The diagram above shows the relationship of these two aspects of photography and how you can use them in differing combinations to produce essentially the same photo.   Notice that as you increase the f-stop (making a smaller aperture), you must also increase the amount of time the shutter stays opens, and vice versa – in order to produce a nearly identical photo.

The third variable:  ISO

The process of photographing is often referred to as the process of exposure and is presented as a triangle.  One side is aperture, the second shutter speed, and the third (which Freeman ignores for reasons yet unknown) is film sensitivity, known otherwise as ISO (which, oddly enough, comes not from the name of a standard, but from the organization that makes them, the International Standards Organisation).  Film was, and I suppose still is, manufactured with differing light sensitivities.  This has somehow been ported over to digital, and every SLR allows you to set ISO.  The higher you dial the ISO, the more sensitive becomes the sensor.  If you are shooting outdoors at night, a high ISO will allow you to capture more light.  Which sounds great and might make you wonder - why not just keep the ISO at the highest setting?  Because like most functions and features on cameras, there is a trade-off.  High sensitivity produces grainy photos, what photographers often call “noise.”  Think of your camera like a radio receiver.  The more sensitive the receiver, the greater the possibility of pulling in a signal.  But with the signal will also come hiss, pops, or squeaks.  Look at a photo shot with ISO and the noise is obvious.

Left:  6400ISO F5 160 /  Right:  100ISO F5 1.6

If you use your SLR in Auto mode, or use a point-and-click, the camera’s computer makes all these decisions for you.  Often, the computer makes very good decisions.  But there will be times when you want to achieve a particular effect, and that may be possible only by switching into one of the other modes and making the necessary adjustments to aperture, shutter speed, or ISO.

Diagram from:   http://www.s-patrick.com

I think this covers for now the fundamental ideas Freeman is after.  I hope I have summarized them well.   

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