How to use F/Stops

To achieve an appropriate exposure, the major thing is to understand the f/stop scale.

Part of the knowledge should include the way they work, how to change them, and their meanings. This knowledge provides clues to produce excellent images.

In this article, there is detailed information about f/stop. All you need do is to try it out. 
(If you first want to learn basic knowledge about cameras click here ("Beginners Guideline"). If you wanna learn everything about the manual mode first click here ("Camera Manual Mode").)

What Is a F/Stop?

It is the measurement of an exposure. As we are aware, the settings in the triangle help us to make an exposure.  That is, shutter speed, ISO, or aperture settings.

Specifically, increasing the exposure by one stop doubles the exposure. Likewise, a decrease in the exposure by one stop reduces it.

For instance, if the aperture of a camera is f/4, shutter speed is 1/100 and ISO is 100, keeping the aperture at f/4 and the shutter speed at 1/100 but increasing the ISO to 200 increases the exposure by one stop.
If you double the ISO, the sensitivity of the exposure becomes double, thereby making the settings to jump in single stops.

It is possible to make you perplexed. However, learning it is just crucial and this is because…
Improvement in your skills will make use the manual mode frequently. This will make you to be in control as regards the camera’s exposure of the scene.
The knowledge of the one stop to use for the shutter speed, ISO and the aperture will influence the changes you make to each one.

Let us break it down:

Your shooting settings include f/2.8, 1/100 of a second, and ISO of 200. However, it is necessary that you have a shallower field depth. So you widen your aperture opening to f/2 to have a shallower field depth.

The light intensity that enters the lens is also doubled.

This jumps up one stop with the aperture and makes the exposure brighter. Hence, there is a need to adjust this by changing the shutter speed or ISO.

This requires halving the ISO to 100 or doubling the shutter speed from 1/100 to 1/200 of a second.

metering modes explained

The brief summary is that an increase in the exposure by a stop doubles the exposure and a decrease in the exposure by a stop makes it a half.

ISO Stops

We are beginning with the one that is quite easy to understand – the ISO. Moving a stop up changes ISO from100 to 200 as well as another stop up changes it from 200 to 400.

Despite the intervals not equal, the ISO doubles between stops. Let’s stop there and move forward.

But always have in mind that increasing the ISO is producing more and more grain/noise in your picture.

ISO explained

Shutter Speed Stops

When you are taking shots with a digital camera, most times, it is at a fraction of a second. Shooting using speeds of 1 second or longer still maintains the principle above.

The time is doubled from 1 second to 2 followed by 2 seconds to 4 seconds.
Shots taken at a fraction of a second like 1/200, requires halving the denominator (200) for doubling this number.

Perhaps you are a beginner; there is no need to worry. You will get used to it soon.

1/200 is two times 1/100 making it one stop and the exposure is two times the initial. 1/50 is two times the length as 1/100 and more.

shutter speed stops


Here comes some form of mathematics. I hope you will not be confused.

You are likely to hold the assumption that f/2 is two times the exposure of f/4 if you follow the logic aforementioned.
Unfortunately, it is not so because f/2 really lets in four times as much light as f/4.

Perhaps you are still wondering, however, I am assuring you that it will get clearer by sticking to it.
Things are different for the aperture scale due to the way it is measured.
The measurement of aperture is with what is known as the f/stop scale.

There is an inscription on your camera ‘f/’ or just ‘f’ with a number. It shows the aperture width and influences the exposure and depth of field. The aperture is wider with a lower number.

Perhaps this is perplexing: Why do we have the maximum aperture with a low number? You first need to be aware of the f/stop scale before proceeding to the simple answer that is mathematical.

Here is the f/stop scale: f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22.

fstops f/stops

Let us quickly look through the functions of the aperture before we move forward.

The lens houses the aperture. The aperture is a hole and it allows light to pass. It is in charge of the exposure and the depth of field. The exposure is our focus here.
You halve the exposure when you change from f/2 to f/2.8. This halves the open part of the aperture.

More importantly, for every next f/stop number, there is a reduction in the aperture size by half. This results to allowing fifty percent less light through the lens (1 f/stop). This is due to the f/stop numbers coming from an equation used in working out the aperture size from the lens’ focal length. Focal length is the ‘f’ in f/stop or f-number while the number is a fraction of the focal length and it reveals the aperture size.

Let us consider this scenario. You have a 50mm lens with the aperture of f/2. You calculate the aperture width by dividing the 50 by the 2 gives a 25mm diameter.
Find the radius by halving the 25mm diameter (12.5 mm). Then multiply the radius by itself (12.5 X 12.5 = 156.25 mm2). Multiplying the value by pi gives 490.9.

Here is the whole equation: Area = pi * r².

It is not necessary to know it but could be helpful in understanding certain things.
The following are some examples on f/stop settings:

A 50mm lens having the aperture of f/2 will have 50mm divided by 2 = 25mm width. 25mm divided by 2 gives 12.5mm; the use of the equation pi*12.5mm² gives an area of 491mm².

A 50mm lens having the aperture of f/2.8 will have 50mm divided by 2.8 = 17.9mm width. 17.9mm divided by 2 gives 8.93mm; the use of the equation pi*8.93mm² gives an area of 250mm².

It is easy to calculate the half of 491 and know that it is not up to 250. This is due to rounding off the numbers to the nearest decimal point. f/2.8 area will still be exactly half of f/2.

Here is the look of the aperture scale (not to scale):


(click to enlarge)

Here is f/stop. By now, with the new information, you should understand better the way to control your exposure.