A confusing number that beginning photographers face right away is the “f-stop” number. It is important because it measures one of our three primary exposure controls: aperture size. However, the concept of “f’-stops” doesn’t make sense at first glace and many are confused by it.
But understanding the figure is really pretty simple. In this article, I’m going to explain to you what this number means and how it works. But, then, more importantly I will show you how you can apply them to your own photography. I will do this in the context of measuring exposure, but also in the context of setting your depth of field.
The f/number Measures One of Our Three Exposure Controls (Aperture)
Before we get into the specifics of the aperture settings, let me show you where this subject fits in with the rest of your photography.
You may recall that the three exposure controls on your camera are: shutter speed, aperture, and ISO. The “f-stop number” we are going to talk about here governs aperture size. But the measurements for the other two exposure controls (shutter speed and ISO) are relatively straightforward, so talking about them first will actually help you understand the aperture measurement. Ultimately, it will all tie together. Anyway, here is how the exposure controls for shutter speed and aperture work:
- Shutter speed is measured in seconds, or typically as a fraction of a second. So when you see “1/250” displayed on your camera, that just means your shutter speed is one two hundred fiftieth of a second. In the same way, when you see 1/4 displayed on your camera, that is a shutter speed of one fourth of a second. For a really long shutter speed, you might see a whole number (such as 2, 8, or 15) displayed on your camera, which means the shutter speed is that number of seconds. In any event, the number you see for shutter speed will just be the time that the shutter is open.
- ISO is an international standard for measuring the sensitivity of your digital sensor. It uses nice round figures that are intuitive. Common ISO figures are 100, 200, 400, 800, and 1600. Lower ISO figures like 100 or 200 mean that your camera’s digital sensor is set to be less sensitive to light. Using a higher ISO like 1600 or even 6400 will make your digital center more sensitive to light, but will also make it more likely to introduce digital noise into your picture. But the scale is intuitive and easily understood.
With those two measurements out of the way, let’s turn our attention to aperture measurements. Here, unfortunately, the numbers are not intuitive. You will see numbers like f/2.8 and f/5.6 and f/22. Those are all common aperture settings. You may have questions, such as:
- Which one is larger and which one is smaller?
- And what does the f/ mean?
None of that is intuitive on the face of the number. But don’t worry, it is not that complicated and I will show you what these numbers mean now.
Measuring Aperture Size Using F-Stops
Now let’s talk about what the f-number actually measures.
First of all, let’s dispense with the little “f/” that goes in front of the number. This doesn’t actually mean anything. It was originally put there to denote that the aperture size was a function of dividing the focal length by a given amount, which would give you the diameter of the aperture. We needn’t worry about that. We just know that then we see this “f/,” we know the number is going to be an aperture size.
Next, I am going to tell you what the number represents. But when you don’t understand it, do not worry. It doesn’t matter. With that said, the aperture number is the number you will divide the focal length by to get the diameter of the opening that aperture.
Does that make any sense or improve your photography? Probably not.
But here’s what does matter, and what you should remember:
- the smaller the f/number, the larger the aperture size.
- and conversely, the smaller the f/number the larger the aperture size.
- the scale will depend on your particular lens, but a common range is something like f/4 – f/22.
We will get into the specifics of some of these settings in a minute.
Measuring Light in “Stops”
There is one more thing you need to understand before we get into the specifics of the numbers. And that is what a “stop” means. A stop is a doubling of light. It is called a stop because back in the days when cameras were mechanical devices there would be ruts in the aperture ring where it would “stop” as you turned it.
This concept of “stops” will be intuitive for you in the context of shutter speed and ISO, so I will explain the concept in those regards first. Then we will circle back to aperture.
Measuring Shutter Speed in “Stops”
Let’s start with shutter speed. Let’s say you have a shutter speed of 1/60 of a second. You now want to add a “stop” of light. Recalling that a stop of light is a doubling of light, you would need to let twice as much light into the camera by changing the shutter speed. The way to accomplish this may be obvious to you: you double the length of time the shutter is open. So adding one stop of light to 1/60 of a second get you a new shutter speed of 1/30th of a second.
Measuring ISO in “Stops”
It is even more simple when it comes to ISO. If you have an ISO of 100 and you want to add one stop of light, which we remember is a doubling of light, then what is the resulting ISO? Obviously it is ISO 200. And if you double the amount of light again by adding another stop of light, the result is ISO 400.
Measuring Aperture in “Stops”
But you can take all of this intuitive stuff and throw it out the window when it comes to aperture size. The numbers simply do not make sense. I could explain it all to you, but it would not matter and it would not improve your photography at all.
The best way to deal with this is simply to look at this chart of aperture sizes measured in full stop increments:
These are aperture sizes in 1-stop increments. Just get familiar with these numbers. You might even memorize them.
Also, note that your camera is probably set up to change settings by smaller increments than a full stop. For example, my camera is set up to change in 1/3 stop increments. This means that every time I click a wheel or dial on my camera to change aperture, shutter speed, or ISO, the change is by 1/3 of a stop. If I want to change by a full stop, I have to move 3 clicks. Your camera is probably set up to change in 1/3 stop increments as well, although some cameras default of changing in 1/2 stop increments. Whatever your present setting, this can usually be changed in the camera’s menu, so just use whatever increment you are most comfortable with.
How Aperture Changes Affect the Exposure of Your Pictures
Now let’s talk about changes in the aperture settings. First let’s talk about these changes in the context of exposure. Then we will come back and talk about depth of field.
When you make your aperture smaller, you let less light into your camera. Your picture will be underexposed unless you make an offsetting change. Such an offsetting change is either going to be a corresponding increase in the length of time the shutter is open, or in the ISO setting. But how much should you change those? For example, if you reduce your aperture by 1-stop (say from f/5.6 to f/8) and you want to keep the same overall exposure, how much should you change your shutter speed?
Here it is important to understand that a stop is a stop is a stop is a stop. By that I mean that no matter whether you were talking about a stop of light as a doubling of shutter speed, aperture, or ISO, it is still a doubling of light. It is all the same thing. So if you reduce your aperture by one stop, you increase one of the other settings by one stop. In our example above, if you shutter speed was previously 1/30, you would make it 1/15 to offset the 1-stop reduction in your aperture.
Therefore, if your camera is underexposed buy one stop according to your cameras meter, you can either: double the shutter speed, increase the aperture buy one stop, or double the ISO.
Let’s make it more concrete now. Let’s say your camera is set at a shutter speed of 1/200 of a second, ISO 200, with the aperture at f/5.6. You now want to add a stop of light because your cameras meter says you are underexposed by one stop. You can increase your aperture buy one stop from f/5 .6 to f/4.0. Again, refer to the chart above.
The point is that you can increase or decrease exposure by making the aperture larger or smaller. It is done according to a defined scale. Once you understand that scale, you will see how it fits in with shutter speed and ISO to control exposure. You can make changes to all three controls and make them work together to get the proper exposure.
How Aperture Changes Affect the Depth of Field of Your Pictures
The final step of our journey today – an perhaps the most important one – involves changes in aperture and its effect on depth of field.
If this is new territory for you, you may want to review the prior article on aperture and depth of field. The key takeaway is that a large aperture means a shallow depth of field, and conversely, a small aperture means a deep depth of field. The following chart should make the relationships clear to you:
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The point here is that increasing the aperture to allow more light into the camera has consequences to your picture. The consequences are a decreased depth of field. Therefore, if you open up the aperture to allow more light into your camera and get a proper exposure, the background of your picture may be blurry.
The shallow depth of field might be a problem. For example, if you are shooting a landscape you will typically want the picture to be tack sharp from front to back. The shallow depth of field would create blur and potentially ruin the picture.
On the other hand, the shallow depth of field might actually be desirable. For example, in portraits, you might want the background to be blurred. The shallow depth of field might make your subject stand out better and make the picture better.
Some will be quick to point out to you that aperture is not the only thing that affects depth of field. And that is true. It is also affected by the focal length and the distance between objects. But let’s be clear: aperture is most important variable. It is also the only variable you are likely to control at the moment of capture. When you are considering the proper exposure, you usually already have your shot lined up. The subject matter is defined. The relationships are set. You are down to just changing the camera controls. At that point aperture is the only one that matters with respect to depth of field.
There is no set measurement of blur and your background or other criteria for setting your aperture. You will just have to work with it and gain experience. But to help you in the meantime, here are some pictures shot at different apertures with different depth of field to help you get a better sense of things:
This may seem like a roundabout discussion, but there is a reason for that. The strict definition of aperture and the way it is measured will not help your photography one bit. But understanding how aperture size fits in with other concepts of exposure and depth of field will improve your photography significantly.