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Making Photographs - a tutorial by Philip Greenspun

F Stops, Shutter Speeds, Iso And How They Relate To Each Other

Do you know the secret formula of F Stops, Shutter Speeds and ISO?
When you think that the relationship between your F Stop and Shutter Speed, Focal length of your lens, and your ISO can make such a difference in the final image you create, it is surprising how few photographers actually understand the intricate relationships between them.

First we will define each factor, followed by details of how each factor affects the others.


Although you might have heard rumors to the contrary, the origin of the term F Stop is based on the initial of the inventor.

They are the diaphragm, or the opening of the lens. The F Stop determines the amount of light that will pass through the lens.


When we had film cameras, we had various forms of shutters that opened and closed and while also allowing light to "hit" the film, and could and still are used to control light, primarily, the Shutter Speed controls length of time the light will effect the film, or now "Digital Image Capture" device.


With both film and digital cameras, a standard was needed to specify how much or how little light is necessary to create an image on the film or the Digital Image Capture device. With film, the term was ASA, and recently the standard used is ISO.

With films of different speeds, we used to have to set the "film speed" on our camera. In the new digital cameras, we still set a "film speed", but in this case it makes a change in the electronic setting of the digital capture device (CCD or other type of chip).

What does matter is that we can use the ISO to control our cameras to achieve a desired result we will discuss further in this article.

Focal length of Lenses

What we will do in this article is show you how all of these elements come together to achieve desired effects depending on requirements of the assignment.

Example 1

Outdoors with bright sun.

We want to use a long, telephoto lens to capture a subject far away.

No tripod, hand hold the camera.

Even though our camera lens has a stabilizer feature, it's not 100% effective, so we will need a fast shutter speed to prevent "shake".

Using a 400 ISO, we can normally set our Shutter Speed to 500-1000, and our F Stop at F11-F15.

Next, we'll take up an example that may be more of a challenge.

Example 2

Location: outdoors, not very bright.

Again we want to use a long, telephoto lens to capture the distant subject.

We will hand hold the camera. No tripod.

We need a fast shutter speed to keep the camera from shaking. You can't totally depend on "stabilizer" lenses.

Now you have to make choices-

a. you can increase your ISO which will allow you to maintain your shutter speed.

b. reducing the F Stop (opening the lens) will also reduce the amount of area that is in focus (Depth of Field).

Let's explain relationships between all of these elements.

When you increase/decrease your F Stop one full stop- 5.6 to 8, or 2.8 to 4, reversed, these are full F Stops.

Increasing Shutter Speed, from 125th to 250th of a second, or 1000th to 2000th of a second allows half the amount of light that gets to your "film", and conversely, reducing Shutter Speed from 250th to 125th of a second, or 2000th to 1000th of a second doubles the amount of light that hits your "film".

Formula: Increasing F Stop from F8 to F11 = Losing one full stop of light.

When you increase shutter speed from 1/500th to 1/1000th, you "lose" one full stop of light.

To gain one full stop of light, double your ISO, such as from 400 to 800, or 800 to 1600.

Note: whether we are working with ISO, F Stops, or Shutter Speeds, we discuss the values in "Stops of Light".

Relationships Between Elements

Any or all of these factors can be used to control your camera settings. Use F Stop to control Depth of Field (area that is in focus). Use Shutter Speeds to control movement, or stop movement, or allow movement to create special effects. Use ISO to control amount of light.


For example, if your lens is at F8, with 1/250 second shutter speed, and you want to make your backgrounds out of focus, open your lens to F4 to reduce your Depth of Field and increase your shutter speed to 1/1000.

In this example you opened your lens 2 stops, as well as increasing your Shutter Speed to 1000. From 250 to 500 doubled the Shutter Speed, then from 500 to 1000 doubled the Shutter Speed again.

If in the example above you used an ISO of 800, it would be possible instead to reduce the ISO to 400, which reduces the light by one half, which permits you to increase Shutter Speed only once (double), from 250 to 500.

If you are using your camera on the automated "Program" setting, you are not taking full advantage of the adjustable controls of F Stop, ISO, and Shutter Speed that you can combine to unleash your creativity.

Your creativity will take a quantum leap if you learn all that these settings can do.

There are some in the photographic community who believed I was generally criticizing the "Photojournalistic Style" of photographers. However, that has not been my point. The basis of my criticism of the photojournalistic genre was that those who pursue that style, without learning the basic technical elements presented here, are shortchanging themselves and their customers. With the basics down pat, their pictures can be ever more beautiful and intriguing.

Just because it is very easy to get a new camera, set it on automatic program, point it, and shoot, the ranks of those calling them Professional Photographers is growing daily. However, many of these new photographers are lacking the basic skills that would have been required in the "old days" before all the automation in cameras. So, today it is easy to "get by" without the knowledge that was absolutely necessary in the past.

If friends and family tell someone that they take great pictures, that they have a good eye, that's not enough of a qualification to succeed as a Professional Photographer. Are they creating the best images possible by just using their natural artistic or creative ability? Probably not. If we go to the barber, or the Dentist, or Physician, we expect expert treatment. As Professional Photographers, we should strive for expert achievement, too.

How the elements discussed here work with your electronic flash will be in our next article.


About the Author
Do you aspire to be a Professional Photographer? Bob Kahn, Master Photographer, offers free articles covering basic technical elements, and skills of posing, lighting, and composition, and how to put it all together to become a successful professional photographer.