The Exposure Triangle

Posted: September 21, 2010 in Photography

As I stated in my post “Understanding White Balance”, photography is all about light. Each of the three aspects of the triangle relate to light and how it enters and interacts with the camera. The 3 aspects of the triangle are;

ISO – the measure of a digital camera sensor’s sensitivity to light.

Aperture – the size of the opening in the lens when a picture is taken.

Shutter Speed – the amount of time the shutter is open.

It is at the intersection of these 3 elements that an image’s exposure is worked out. Most importantly, a change in one of the elements will impact the others. In other words, you can never really isolate one without affecting the others.

ISO

ISO is measured in numbers (100, 200, 400, 800, 1600, etc.).  My Nikon goes 200, 400, 800,1600, and HI1. HI1 is actually 3200, and no, I have no idea why they didn’t just call it 3200. The lower the number the less sensitive your camera is to light and the finer the grain. Higher ISO numbers are usually used in darker situations to get faster shutter speeds. One thing to keep in mind with higher ISO is it creates ‘digital noise’.  Instead of having a flat black, it is now more of a dark grey with specs, or pixels, of assorted other colors.

100 ISO is accepted as ‘normal’ and will give you tack sharp pictures in situations where the lighting is good.

When you do override your camera and choose a specific ISO you’ll notice that it impacts the aperture and shutter speed needed for a well exposed shot. For example – if you bumped your ISO up from 100 to 400 you’ll notice that you can shoot at higher shutter speeds and/or smaller apertures.
When selecting the proper ISO setting, ask yourself these 4 questions;
  1. Is the subject well lit?
  2. Do I want a grainy shot or one without noise?
  3. Am I using a tripod?
  4. Is the subject moving or stationary?

If there is plenty of light, I want little grain, I’m using a tripod and my subject is stationary I will generally use a pretty low ISO rating.

However if it’s dark, I purposely want grain, I don’t have a tripod and/or my subject is moving I might consider increasing the ISO as it will enable me to shoot with a faster shutter speed and still expose the shot well.

Of course the trade off of this increase in ISO will be noisier shots.

Situations where you might need to push ISO to higher settings include:

  • Indoor Sports Events – where your subject is moving fast yet you may have limited light available.
  • Concerts – also low in light and often ‘no-flash’ zones
  • Art Galleries, Churches etc- many galleries have rules against using a flash and of course being indoors are not well lit.
  • Birthday Parties – blowing out the candles in a dark room can give you a nice moody shot which would be ruined by a bright flash. Increasing the ISO can help capture the scene.
Here are some shots I took using different ISO settings. First set of 5 is from outdoors, full sunlight.
Indoor shots with no lights on.
Notice the noise in the last shot, especially on the red wall. Click images for full size.
Aperture
Aperture refers to the amount of light that falls onto the sensor.
Just like successive shutter speeds, successive apertures halve the amount of incoming light.  To do this, the diaphragm reduces the aperture diameter by a factor of 1.4 so that the aperture surface is halved each successive step.
Apertures are listed in terms of f-numbers (expresses the diameter of the entrance pupil in terms of the effective focal length of the lens; It is the quantitative measure of lens speed), which are marked on the lens.  On a camera, the f-number is usually adjusted in f-stops.
Each “stop” is marked with its corresponding f-number, and represents a halving of the light intensity from the previous stop. Modern electronically-controlled interchangeable lenses, such as those from Canon and Sigma for SLR cameras, have f-stops specified internally in 1/8-stop increments, so the cameras’ 1/3-stop settings are approximated by the nearest 1/8-stop setting in the lens. The F number can be displayed as 1:X instead of f/X
Lenses with larger apertures are faster because, for a given ISO speed, the shutter speed can be made faster for the same exposure. A smaller aperture means that objects can be in focus over a wider range of distance (depth of field).
Portrait and indoor (sports and theater also) photography often requires lenses with large maximum apertures in order to be capable of faster shutter speeds (and narrower depth of fields) in order to combat the low light problems with no camera shake.
The narrow depth of field in a portrait, as well as in macro photography, helps isolate the subject from the background.
Here are some examples of adjusting the aperture for different effects.
Large depth of field
Aperture – f22
Shutter Speed – 1/60 sec
ISO – 400
Focal length – 34mm
Focus – AUTO
Shallow depth of field (focus on subject in front)
Aperture –  f5.3
Shutter Speed – 1/60 sec
ISO – 800
Focal length – 42mm
Focus – Auto
Shallow depth of field (focus on subject in back)
Aperture – f4.0
Shutter Speed – 1/60 sec
ISO – 400
Focal length – 24mm
Focus – Manual
Shutter Speed
Shutter speed is simply the shutter time.   Normally your sensor  sits around in the dark waiting for a bit of action. It is the shutter – like the blinds or curtains which shut light out of your room – which keep the sensor in the dark. When the shutter opens, it lets in light and the sensor gets to work.Exposure time, then, is the time interval or duration during which your camera’s sensor  is collecting light to capture your image.

Shutter speed is measured in seconds – or in most cases fractions of seconds. The bigger the denominator the faster the speed (ie 1/1000 is much faster than 1/30).
In most cases you’ll probably be using shutter speeds of 1/60th of a second or faster. This is because anything slower than this is very difficult to use without getting camera shake. Camera shake is when your camera is moving while the shutter is open and results in blur in your photos.
If you’re using a slow shutter speed (anything slower than 1/60) you will need to either use a tripod or some some type of image stabilization (more and more cameras are coming with this built in).

Shutter speeds available to you on your camera will usually double (approximately) with each setting. As a result you’ll usually have the options for the following shutter speeds – 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8 etc. This ‘doubling’ is handy to keep in mind as aperture settings also double the amount of light that is let in – as a result increasing shutter speed by one stop and decreasing aperture by one stop should give you similar exposure levels (but we’ll talk more about this in a future post).

Some cameras also give you the option for very slow shutter speeds that are not fractions of seconds but are measured in seconds (for example 1 second, 10 seconds, 30 seconds etc). These are used in very low light situations, when you’re going after special effects and/or when you’re trying to capture a lot of movement in a shot). Some cameras also give you the option to shoot in ‘B’ (or ‘Bulb’) mode. Bulb mode lets you keep the shutter open for as long as you hold it down.

When considering what shutter speed to use in an image you should always ask yourself whether anything in your scene is moving and how you’d like to capture that movement. If there is movement in your scene you have the choice of either freezing the movement (so it looks still) or letting the moving object intentionally blur (giving it a sense of movement).

To freeze movement in an image you’ll want to choose a faster shutter speed and to let the movement blur you’ll want to choose a slower shutter speed. The actual speeds you should choose will vary depending upon the speed of the subject in your shot and how much you want it to be blurred.
Focal Length and Shutter Speed – another thing to consider when choosing shutter speed is the focal length of the lens you’re using. Longer focal lengths will accentuate the amount of camera shake you have and so you’ll need to choose a faster shutter speed (unless you have image stabilization in your lens or camera). The ‘rule’ of thumb to use with focal length in non image stabilized situations) is to choose a shutter speed with a denominator that is larger than the focal length of the lens. For example if you have a lens that is 50mm 1/60th is probably ok but if you have a 200mm lens you’ll probably want to shoot at around 1/250.

Here are some shots I took of a ceiling fan using different shutter speeds.

Aperture – f4.5  (yes, the fan is on!)
Shutter Speed – 1/10 sec
ISO – 400
Focal length – 32mm
Focus – Auto
No tripod
Aperture – f2.5
Shutter Speed – 30 sec
ISO – 400
Focal length – 28mm
Focus – Auto
Tripod
Here is the same shot with the same settings, but without using a tripod.
So, there you have it. Pretty much the basics of how to take a great shot. Use this information and practice, practice, practice. Get out your camera manual and see how to change these settings, then start shooting.  If you understand these concepts, you should be shooting ‘gallery ready’ shots in no time.  It is not the camera that takes the great shots, it is the photographer.
Leave a comment, or send me an e-mail if you have questions. Good luck, and enjoy!


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