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Lighting is Easy Blog

Free resource for learning how to use off-camera flash and studio lighting.

F-Stops Explained

Chuck Vosburgh - Thursday, February 10, 2011

If f-stops are a little confusing to you, you’re normal. Most photographers have difficulty with this and modern cameras make it more difficult. Read this and you’ll understand how. This may not be an exciting subject, but read all the way through and you’ll see why this is important to your success. 

The concept

The idea is that if you open the aperture one f-stop, it allows twice as much light through. Likewise, if you close the aperture one f-stop it allows half as much light through. F-stops are designed to work with shutter speeds, which also use the same half or double concept.

Optional scientific explanation

In case you’re interested, the f-number system is not arbitrary. The f-number is the focal length divided by the diameter of the pupil (aperture). For example, a 100mm lens with a f-stop of f-4 will have a pupil (aperture) diameter of 25mm. A 135mm lens with a setting of f-4 will have a pupil diameter of about 33.8. Both of these examples will produce the same illuminance on the focal plane.

f-number = focal length / pupil diameter

How it works

There are full stops and fractional stops. Here’s the range of Full Stops you’re likely to see:

These full-stops are standard and common to all lenses. Some lenses have a smaller range, but the numbers are always the same. The reason this is important is that they each allow either half or double the amount of light as the next full f-stop. For example, f-8 allows twice as much light as f-11, but only half as much as f-5.6. The reason this is good to know is that they correspond to shutter speeds which are also half or double the next shutter speed. For example 1/125 second is half as long as 1/500 second, and 1/60 second is twice as long as 1/125 second. When you use studio lights or flashes, everything is divisible by 2 so you can do the math in your head quickly while you’re working. I know, you’re probably thinking “yeah right”. Read on.

Why it’s confusing

One thing that makes f-stops confusing is that almost all camera manufacturers show fractional stops. In other words, f-4 and one third is shown as f-4.5 instead of f-4 1/3. There’s no indication which are full stops and which are fractional stops. Confusing. Here’s a chart  to help illustrate this:

Fractional stops are of little use in learning lighting. You need to know which ones are full stops. Why? Just about everything you need to know about lighting is based on full stops. Print the charts above and put them in your camera bag or better yet memorize the full stops.

Learning your f-stops is by far the least fun part of photography, but it will open the door to a whole new world once you do. I think it’s time well spent.

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