What is Aperture?

This post is a part of the series of posts on Photography Basics.

I’ve been reading about it ever since I decided on nurturing my interest in photography. And, the more I read about it, the more I feel the need to read about it. Of course, experimentation follows reading – in fact, experimentation trumps reading – but it is always good to know things before you try them. And, hence this post.

In photography, what is aperture?

In simple terms, it is the hole/space through which the light (and hence the image) travels through the lens onto the imaging sensor and, if available, through the viewfinder. The aperture is one of the most important points to consider while capturing images. The other important points are the focal length, the ISO, and the shutter speed.

Basically, it is the right setting of the aperture that helps you bring either everything into the focus or only the subject by blurring the background. But, how does that happen? That happens because the aperture controls the amount of light entering into the lens and onto the imaging sensor. And – unfortunately – here’s the catch! A large aperture doesn’t mean more light onto the sensor. In fact, it is the opposite; A small aperture means that the lens is open wide enough to pass abundant light onto the imaging sensor.

How does the aperture affect the depth of field?

You can control the light entering the lens using a diaphragm. A diaphragm is a device (if that is the right word) that functions much like the pupil in a human eye – Diaphragm controls the diameter of the lens opening. The structures within this diaphragm are called stops. Each stop represents are definitive number that defines the opening of the lens.

The lens aperture is typically written as an f-number, which is also called f-stop. This f-stop represents the ratio of focal length to the opened aperture diameter. The following picture, which is taken from the Wikipedia page for aperture – which I think best describes the concept – represents the range of aperture. Remember that lower the aperture number, larger the opening of the lens.

Image Credit: Wikipedia
Lens exposure per f-stop: Wikipedia (Image Credits) best describes the concept of aperture. Look how the aperture settings change for each f-stop


Wikipedia mentions, “Reducing the aperture size increases the depth of field”. This means that you will choose a smaller aperture number when you want to cover everything between the subject and the photograph’s actual plane of focus, with the subject and the plane of focus included. For a shallower depth of field, and to isolate the subject from its plane of focus (Yes, for that creamy bokeh!), you will use a large aperture – typically something like f/4.

Let me make it simpler for you:

  • If you want the entire picture in focus, use the following formula:

Entire Area in Focus = Greater DOF = Smaller Opening in Lens = Higher f-stop

(f/8 through f/16)

  • If you want only the subject in focus, use the following formula:

Only Subject in Focus = Shallower DOF = Larger Opening in Lens = Smaller f-stop

(f/5.6 through f/1.4)

You can have f-stops that are smaller than f/1.4 and larger than f/16, but I am only using those figuratively here – to give you the idea.

How does the understanding of aperture affect my photography?

Switch from the automatic mode on your camera. If you are using a DSLR, switch to the “A” mode from the “PASM” modes available. For those who are new to this setting, the DSLR allows modes other than the two at the extreme ends of the spectrum – the Automatic and the Manual modes. The PASM here stands for Program, Aperture Priority, Shutter Priority, and Manual Mode. For those of you who are using a point and shoot (P&S) camera, use the manual mode and set the aperture. You will have to dig into the menus, because every P&S has its own style of menus.

Select Aperture Priority as the mode for shooting. Aperture Priority is like a semi-automatic mode of shooting – but with a difference. Based on the aperture you set, the camera calculates and chooses the best possible ISO and shutter speed. You can use this to practice and master your understanding on aperture. Once you are habitual with the change in the shutter speed and ISO based on the aperture you choose, you can experiment with the Manual Mode, while changing any of these elements – Experimentation, as I said, trumps theoretical knowledge.

Do “fast” and “slow” lenses have anything to do with aperture?

A little background before we come to that question… Lenses come with either fixed aperture or variable aperture. For example, the 70-200mm f/2.8 denotes that across the focal length of the lens (70mm and 200mm, respectively), the lens will maintain fixed aperture of f/2.8; and the 70-300mm f/4-5.6 means that at 70mm, the lens will get you f/4.0 aperture and at 300mm, the lens will get you f/5.6 aperture.

Let us go back to the basics now. The lens aperture, as we discussed, denotes the lens opening diameter (managed by the diaphragm). A larger opening – which means a smaller f-number – will help you capture a lot of light, and a lot of details in your image. So, for a smaller f-number, opt for faster shutter speed. If this is too technical, read how simply Wikipedia defines this for you, “The aperture is proportional to the square root of the light admitted, and thus inversely proportional to the square root of required exposure time, such that an aperture of f/2 allows for exposure times one quarter that of f/4.”

Note that for both the fixed and the variable aperture lenses, we consider the maximum aperture opening as the most useful. This value of the maximum aperture opening is also called the lens speed. Lenses with aperture openings equal to or wider than f2.8 are called faster lenses. These lenses will give you faster focusing (usually, in combination with sharper picture, better detail retention, and faster shutter speeds). And, because you can focus faster with these lenses, they are called “fast” lenses. The lenses, though, are typically expensive.

I hope I have understood the concept of aperture, and that I was able to reproduce my knowledge correctly. I’d like you to let me know if there are any improvements.

Happy clicking!