Part 5 - The relationship between aperture, shutter speed, and ISO

 What is a stop?

You can alter exposure by changing aperture, shutter speed, or ISO settings.

A stop, in photography, is simply the halving or doubling of light.  It's easy to see in shutter speeds, where the change in light from 2 seconds to 1 second is a stop (2 seconds lets in twice as much light as 1 second).  It is also easy to see in ISO where you may change from a value of 200 to 100 (an ISO of 200 requires only half the amount of light on the sensor to make a proper exposure as 100 does).  Aperture is very different though.  Have a look at the exposure bar values below.

Each large bar is a 1 stop change in value

For aperture, the change is not at all intuitive.  For example, if you want to let in twice the light through the lens, which is set to f/4, you would reduce the aperture number to f/2.8.  An aperture of f/2.8 lets in twice the amount of light as an aperture of f/4.

The values may seem complicated at first, but you will soon get to understand them; it just takes a little time and reflection on photos that you take.  

A good plan when taking pictures is to have a starting point for your aperture, ISO, and shutter speed settings.  If you use aperture priority exposure mode (A), here are a few suggestions

Outdoors with a long lens:  set aperture to minimum and ISO to 200.  If the shutter speed is less than 1/200th of a second, increase the ISO to 400 or even 800.  I often like to shoot my wildlife photos at 1/400th of a second or so.

Outdoors with a wide angle zoom:  set aperture to f/16 and ISO to 100 or even lower if you like.  Shutter speeds can be fairly low, around 1/30th of a second, and lower if you have stabilization technology on camera or in the lens.  If you are doing hyperfocal images, consider increasing the aperture value to f/22 or higher.

Indoors without flash:  use a prime lens if possible (50 mm f/1.8) and set the ISO to 400.  Shutter speeds can be 1/30th or higher if stabilization technology is available.  If you are getting fast shutter speeds, consider lowering the ISO to 200 or 100.

Indoors with flash:  a kit zoom will work, but flash distances will be very short at low ISOs and high f/numbers.  Aperture priority works here, but you may want to either reduce the minimum flash shutter speed in the custom functions menu or go to shutter priority or even manual.  A low shutter speed of 1/15th of a second will allow ambient light to fill the background while the flash fills the foreground.  Keep aperture values at their minimum and ISO values from 200 to 800, depending on how your shots are turning out.

The trick to understanding the relationship between ISO, aperture, and shutter speed is to think in stops.  You can have the exact same exposure with very different settings.  Let's say that a proper exposure occurs at these settings: 

aperture:  f/5.6     shutter speed 1/250        ISO:  200

You can take exactly the same picture (a proper exposure) by increasing or decreasing those values by the same number of stops.  If you let in four times as much light through the lens (2 full stops) you would have to increase the shutter speed by 2 stops to counter the change in light.  So these would be the new values:

aperture:  f/2.8     shutter speed 1/1000     ISO:  200

The difference in the shots would be less depth of field (lower aperture) and less blur of motion (faster shutter speed).  If you were to do the reverse, these would be the new values:

 aperture:  f/11     shutter speed:  1/60     ISO:  200

In this situation, you would have a lot more depth of field (f/11) but show significant blur from subject or camera motion (1/60).  Stabilization technology can help with camera shake, but it does nothing for camera movement.

If I had a long lens on with a minimum aperture of f/5.6 and wanted a fast shutter speed to freeze the action of moving subjects, I may want a shutter speed of 1/1000th of a second.  To get this, if the light is the same, my only option will be to increase the ISO by 2 stops.  The new settings would be:

aperture:  f/5.6     shutter speed:  1/1000     ISO:  800

It is important to understand that you can alter an exposure by fractions of a stop.  The lines in the exposure bars above show divisions of thirds.  Each tick represents a third of a stop.  Three ticks means a full stop change.  Your camera uses a similar system; when you rotate your command dials to change settings, each click on the wheel represents a change by a third of a stop.

You can play with these values to get exactly what you want given the amount of available light.  Think about the factors that are associated with the three exposure parameters:

Shutter speed - affects blur from subject or camera movement.  Increase shutter speed to decrease blur or decrease speed to increase blur.

Aperture - affects depth of field.  Low aperture values (f/2) means less depth of field while high aperture values (f/16) means more depth of field.

ISO - affects noise.  Low ISO values means less digital noise in the image which means that you can do more cropping and enlarging with less loss of detail.  Higher ISO values have increased noise, but it also lets you shoot at higher shutter speeds, which is important in wildlife photography, or smaller apertures (higher f/numbers), which is important in landscape shots.

By playing with these values you can find the best balance for the desired image.  

Thanks for reading.

Eric Svendsen     www.ericspix.com