When I bought my first single lens reflex (SLR) camera some years ago, it didn't take me long before I started experimenting with different techniques and settings. One of those techniques I was keen on was taking photos with shallow depth of field.
Here's an example of what a "shallow depth of field" means:
Only the doll on the right side is in focus, and even, if you pay attention, her right eye is in better focus than her own left eye. Almost everything else in the picture is blurry and out of focus. As a photographer I'm really making sure that my audience pays attention to the doll on the right.
My objective today is to explain how depth of field works so you can take more creative control over your photos. If you're new to photography, or you're not quite sure how to achieve this kind of look, read on.
At first, when I was learning my way around photography, it was all mostly by luck and chance. But being the techie that I am, that did not work for me. I needed to know how to consistently replicate what I wanted to achieve. That's when I started reading about how to use the different settings on the camera. Technically this topic could go on for a while as full courses are build around it. But I'll try to keep it to the basics, still with the big picture (pun intended) always in mind.
There are many factors that determine the look of a photo or an exposure. From that plethora of factors there are 2 main ones that determine the amount of light entering the camera (after all, photos are all about light):
For a shallow depth of field we'll be working with the aperture, but before we do that, let me briefly talk about the shutter speed.
See, when you take a photo, the lens will allow light to hit the sensor on the digital camera. That's why photos are also called "exposures"; because the film or sensor is exposed to light while the photo is being taken. So, the shutter speed will determine for how long light will hit the sensor. A fast shutter speed will open the shutters, let an amount of light hit the sensor, and quickly close. A slow shutter speed will stay open for longer, allowing an exposure that takes more time to record.
On the other hand, the aperture determines how big the opening of the shutters will be. In simple terms, how big the hole that will allow light to hit the sensor will be. A larger opening will let more light hit the sensor than a smaller opening at the same shutter speed. And this is where physics get really interesting. If we have a large aperture (a large opening), we will allow the light that hits the sensor to be more dispersed. In a smaller aperture, the light rays are more paralel and less dispersed.
That's it! I just spilled the beans for you! To get the shallower depth of field you want to get as large an aperture as your lens will allow.
Take a look at the following two photos. On the first one the aperture is as wide as that lens can do, letting lots of scattered light enter the camera and hit the sensor. This big aperture creates a fuzzier out-of-focus area or a shallow depth of field. On the second photo, you can clearly see a small aperture created by the shutter blades on the lens.
Ok, enough theory, let's put this to practice.
First, what we'll do is switch our camera to "Aperture Priority". In the case of the Canon cameras, you'll find an "Av" setting in the dial or in the menu. If you have a Nikon camera, look for the capital "A" in the dial. If you have a different kind of camera, search online for "aperture priority" and the brand and/or camera model.
This is the dial of one of my Canon cameras:
In Aperture Priority we'll control the aperture and the camera will calculate the shutter speed based on the lighting situation. I strongly recommend you use a tripod to avoid any camera shake.
Next, let's crank the aperture to the maximum.
Note the "Av" for "Aperture Priority" and "F1.8" indicating the aperture setting.
Here's where many people get confused. Really confused, because camera manufacturers refer to aperture as "f stops" which mean focal _ratios_. The kicker is that it's a ratio between the focal length of the lens and the diameter of actual aperture. So, when you see an aperture of f/1.8 it means that the aperture is roughly half (1/1.8 = 55%) of the focal length. Compare that to an aperture of f/22 which is only about one twentieth of the focal length (1/22 = 4.5%). By the way, I use this numbers because the lens I used has a maximum aperture of f/1.8 or F1.8 as it appears on the display on my camera, and a minimum aperture of f/22 or F22. Depending on the lens the aperture ranges will be different.
Ok, ok, ok… I got a bit too technical here. If you're really interested in the physicality of it all, do a search for "focal length" or "f-number" or "f-stop". But if you're not that curious and still confused simply inverse the numbers. Meaning 1.8 will give you a much _larger_ aperture than 22. So a smaller f stop digit on your camera settings means a larger aperture, which automatically translates to a shallower depth of field.
I recognize that this concept may still be confusing but I hope that the following comparison helps you relate in physical terms to what's going on.
The top row shows two very similar photos. They were taken with the same lens, with the same focus, both set with "Aperture Priority". The one on the left I set the aperture value to be "F1.8" as it reads on the bottom row of the display on my camera. F1.8 is the maximum aperture that lens can do. The photo on the right was then taken with the smallest aperture that the same lens can achieve: "F22" as shown in the camera display. The photo on the left has a very shallow depth of field. Only a few centimeters around the burgundy flower in the middle of the shot are in focus while stuff closer to the camera and further back from the burgundy flower are out of focus. My idea behind that photo is for you to pay attention to that small flower amongst the other pinkish flowers.
On the other hand, the photo on the right has way more flowers in focus. So, even when I was focusing on the burgundy flower, most other pinkish flowers are fairly crisp.
Now, pay attention to the second row of photos. Here I show you the exact same lens I used, with the same aperture used for each of the photos. The one on the left has a very large opening (aperture) and the one on the right shows a tiny hole (aperture).
Something that I must mention is that the camera calculated the shutter speed automatically based on the lighting environment. The photo on the left had the shutter open just for 1/40 of a second while the one on the right had to stay open for a full 4 seconds to capture the same amount of light. Why such a difference? Well, take a look at those apertures. The f/1.8 on the left lets lots of light in, so the shutter didn't have to stay open too long, while the f/22, with its tiny aperture, required a much, much longer exposure.
Just to wrap up, different lenses allow for larger or smaller apertures depending on the lens itself. So, unless you're using a similar lens to the one I used, you most likely will get different maximun and minimum aperture values on your camera display.
For reference, I used an "inexpensive" EF 50mm f/1.8 II fixed or prime lens as pictured below.