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Do You Know The Secret Of F Stops, Shutter Speeds, And Iso?

Feb 22, 2008
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.

Before we discuss how the parts fit together, we will first describe each separate factor.


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

How much light comes through the lens is determined by the F stop setting, which refers to the degree of opening of the lens diaphragm.

Shutter speed

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.


When film was used, a standard was needed to determine how much or how little light is necessary to create an image on the film. Various "speeds" of films were produced which differed in their ability to capture light. The term for that standard was ASA, from American Standards Association. A new term, ISO (International Standards Association), became popular a few years ago.

Even though we don't use film anymore, we do have the ability to set a "film speed" in our new digital cameras. My understanding of the particulars is that it is actually an electronic change to the DCD (my own term, there are CCD's and other chips, but for this article their names are not important)

Later in this article we will discuss the use of the ISO to attain a particular result.

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 One

Location: outside, bright sun

We are shooting a distant subject with a long, telephoto lens.

We will hand hold the camera. No tripod.

Since we cannot depend totally on stabilizer lenses, we will need to use a fast shutter speed to keep the camera shake from affecting our picture.

Setting ISO to 400 will usually allow Shutter Speed of 500-1000 with F Stop at F11 to F15.

In this next example, you might have problems taking pictures of your subject.

Example Two

We are outside, cloudy, not very bright.

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

No tripod. Hand held camera.

Since we can't depend on the stabilizer lens 100%, we need a fast shutter speed to prevent the camera shake effect in our image.

Now you have to make choices-

a. if you want to maintain your shutter speed, you could increase the ISO.

b. you can reduce your F Stop (open your lens) You sacrifice the amount of area in focus (Depth of Field)

Now we will sort out the relationships among these factors.

Increasing or decreasing F Stop one full stop would be 5.6 to 8, or 2.8 to 4, or vice versa.

If you increase shutter speed from 1/125 of a second, to 1/250th of a second, or from 1/1000th of a second to 1/2000th of a second, you reduce the amount of light to your film or chip by one half, and if you decrease shutter speed from 1/250 second to 1/125 second, or 1/2000 second to 1/1000 second, you double the amount of light getting to the film or chip.

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

If you increase your Shutter Speed from 500 to 1000th of a second, you "Lose" one full stop of light.

Doubling ISO from 400 to 800, or 800 to 1600, gains one full Stop of light.

Important note: "Stops of Light" is the terminology we use whether we are discussing the effects of working with ISO, F Stops, or Shutter Speeds.


You can use any and all of these elements to control your camera settings. F Stops control Depth of Field (amount of are in focus). Shutter Speeds control movement, or stop movement of action, or allow movement for effect. ISO controls amounts 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.

Though some "Photojournalistic" style photographers have at times thought I was criticizing their entire body of work, they misunderstand my viewpoint. I want them to realize that by incorporating the basic technical skills in their work, their work will jump to new heights of creativity.

So many eager individuals, with a love of photography and some small successes, decide to jump into the business of Professional Photographer, before they know the basics of what it takes to create the beautiful pictures that become treasured records of their clients' life events.

Simply being told by friend and relatives that they "have a good eye", is simply not enough. There is not question, they achieve images, but are they achieving the best image they can achieve? When we get our haircut, or our visit our Dentist to remedy a painful toothache, we expect these people to do the best they are capable of, not just what they can get away with.

How the elements discussed here work with your electronic flash will be in our next article.
About the Author
Do you want to be a Professional Photographer? Bob Kahn, Master Photographer, offers free lessons covering basic technical elements, and techniques of posing, lighting, and composition, and how to put it all together to become a successful professional photographer.
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