Depth of Field, Shutter Speed and ISO
A Relationship Revealed!

The relationship between Aperture, Shutter Speed and ISO or Film Speed is one of the most misunderstood concepts in photography. Here we will try to unlock the mystery! The Aperture or f-stop controls the amount of light that falls on the film or sensor. The Shutter Speed controls the length of time the film or sensor is exposed to the light. And the Film Speed or ISO is how sensitive the film or sensor is to the light. But what do they have to do with each other?

Aperture

We need to take a closer look at how the Aperture controls the amount of light reaching the film or sensor. Let's start with your Aperture at f/22. This admits X amount of light. Open your Aperture one full stop to f/16, this will admit twice the original light. If you open your Aperture two full stops to f-11, you admit 4 times the original light. Open up one more full stop to f-8 and you admit 8 times the original light. Do you see a pattern developing (photographic joke) here? For each one full stop increase in Aperture, you admit 2 tmies the amount of light.

Shutter Speed

Shutter Speed sounds pretty straight forward, and it is! A Shutter Speed of 1/125th-second admits twice as much light as 1/250th-second or one full stop of light. Four times as much light as 1/500th-second or two full stops of light. Eight times as much light as 1/1000th-second or three full stops of light. Have you seen this pattern before?

The Relationship

Take a good look at the list of settings below. Can you see the relationship?

The Advantage

Let's say you wanted to re-create one of those famous "Calander Shots" of a waterfall where the water looked smooth and milky and everything else was in sharp focus. You would use a slow Shutter Speed, 1/30th-second to blur the water and an Aperture of f/5.6 to create the desired effect.

But what if you wanted to freeze the water it time so you could see every ripple or splash. You would use a fast Shutter Speed, 1/500th-second to freeze the action of the water and an Aperture of f/22. You have just created two images with two dufferent effects with the same exposure value!

The ISO Factor

Film Speed or ISO is the sensitivity of film or digital sensor to light. ISO 200 is twice as sensitive to light, or one full f-stop as ISO 100. ISO 400 is four times as sensitive to light, or two full f-stops as ISO 100. The pattern is re-appearing!

This is really handy in low light conditions. Let's say you want to take a picture indoors using a 50mm lens and for some reason you can't use a flash. You want a good Depth of Field so you set your Aperture at f/5.6. You meter the scene and to achieve the correct exposure, the proper Shutter Speed would be 1/15-second. Too slow for hand holding and you don't have or can't use a tripod.

You can now use the relationship between Aperture, Shutter Speed and ISO to your advantage! Increase your ISO from 100 to 400 or even 800. An ISO of 400 would give you two more full stops of light and you could increase your Shutter Speed by that amount. from 1/15th-second to 1/60th-second. You can now "get the shot".

There Is a Downside

So why not keep the ISO at 400 or 800 all of the time? The answer is, the lower the ISO the better the quality of the image. To get more light sensitivity from the sensor, an additonal charge, or gain is applied. As the ISO gets higher the additional charge appears as noise in the image. Those seemingly random red, blue and green pixels that appear in the darker portions of the image.

If your camera is equipped with Noise Reduction, be sure you have it activated and most good Image Editing Programs like Photoshop have filters to remove noise from images.

The Last Word

Understanding the relationship between Aperture, Shutter Speed and ISO can help you be more creative, "get more shots" and become a much better photographer technically. So get out your camera and start experimenting with Aperture, Shutter Speed and ISO.

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