What Is An f/stop?

There are two factors in creating exposure: The time light is allowed in and the amount of light that is allowed in to the camera. The math for exposure is exposure is equal to time multiplied by intensity.

E=T x I

To give a mental image, imagine that you have to fill a cup of water. There are two ways in which you can fill it at the sink: 1) open up the faucet so that a lot of water fills it up quickly or 2) let the water drip for a long time and fill up the glass that way. A camera works in a very similar way: that camera either lets in a lot of light or adjusts the time light is allowed to hit the film plane. 

How the camera measures the time of an exposure is easy. A shutter opens up for a moment and closes. This is called shutter speed. Shutter speeds can vary from the very fast (e.g. 1/8000 second) to very long (e.g. 1 minute).

What is more difficult to understand is how the camera determines the amount of light into the camera. This is determined by f/stop.

In a lens there is a device called a diaphragm (normally a series of adjustable leafs that make a smaller or larger opening in the lens). When more light is needed, the diaphragm is opened. When less light is needed the diaphragm is closed. f/stops measure the size of the opening, which photographers call the aperture.

So why are they called f/stops and how do they work?

First, f/stops aren’t fractions. It is a math equation where the f/stop number (represented as f/8, f/11, f/16 etc.) measures an amount of light. The f/stops are constant measurements. So while a small pocket camera f/8 setting may be a very small hole, and a large format camera f/8 may be a very large opening, the same amount of relative light is hitting the focal plane.

Full f/stops have specific numbers assigned to them. Each full f/stop represents a perfect measurement of light that is either double or half the amount of the next full f/stop. If you open a lens aperture one full f/stop then you have doubled the amount of light from the previous f/stop. If you close the aperture one full f/stop, you have cut the light in half.

The common full f/stops are – f/1, f/2, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/64

Many digital cameras now allow partial f/stops.

Once we know what our exposure should be (that will be covered in a different post), we can make adjustments to the shutter speed and f/stop to get the effects we want. Because shutter speeds are normally set to be in the same doubling principle as the f/stop (i.e. shutter speeds are normally 1 second, ½ sec, ¼ sec, 1/8 sec, 1/15 sec, 1/30 sec, etc.) each setting has the same effect of either adding or subtracting the amount of light to the film plan by a factor of 2.

Remember the formula for exposure (E= T x I). If you take your shutter speed and make it faster (less light), you can proportionally adjust your f/stop to allow in the equal amount of light. Adjust your shutter speed two settings from 1/125 sec to 1/500 sec (skipping by 1/250 sec), you have cut your light by 1/4th (You cut it in half from 1/125 to 1/250, and half again from 1/250 to 1/500). This means you must open up your aperture to allow four times the amount of light in, or two f/stop settings.

In another blog, I will discuss the effects you can create withf/stops.




7 thoughts on “What Is An f/stop?

  1. this was a very helpful blog. I’ve tried to understand f-stops before but with no success. Thanks so much.

  2. yes that was simple and helpful, I’m reading a book for my Canon rebel and it is referring to the “First f-stop” (brightest tones) I’m guessing they are just commenting on the aperture being the most open.. and so there for you would have the brightest tones landing on the sensor and as you closed the aperture (second f stop being half the light of the first you would commensurately get half the tonal values you would have gotten with the first on the same shot. you know I think I figured it out.., thanks again Greg the Ansel Adams want-to-be

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