How Your Digital Camera Works

Have you ever wondered what is going on inside that picture-taking box that you just held up to your eye, or out at arm’s length, to capture a photograph?

The Basics

The camera is, in its most simplified terms, a box that allows light to enter and strike a light-sensitive surface. This surface is either a frame of film or a digital sensor. Cameras can accomplish this task in the most simple way—a pinhole camera, for instance. Pinhole cameras may have only one moving part, or none. Or, the camera can have dozens of moving parts like the modern film or digital single-lens reflex (SLR or DSLR) camera.

In this piece, we will discuss the modern cameras popular with today’s photographers. We are going to talk about cameras in general terms, so please know that I am aware of dozens of different ways in which different cameras make images. For simplicity’s sake, we will keep it simple!

A Common Path

Modern cameras, more or less, work similarly to produce a photograph. Obviously, some are more complex than others, but, in general, light travels a similar path once it meets the camera.

• Lens
• Aperture
• Shutter
• Image Plane

How the image is viewed on the camera, through an optical or electronic viewfinder or electronic screen is one thing that differentiates different types of cameras.

The Lens

Light first enters a lens. This is an optical device made from plastic, glass, or crystal that bends the light entering the lens toward the image plane. The lens has a certain number of optical elements. These are arranged together in groups. If you look at lens specifications, you will see a mention of the number of elements and groups in a given lens. Some groups only have one element.

Some lenses have fixed focus; others have movable elements that allow the photographer to control focus. On these lenses, one or more elements can change position to focus the light precisely at the image plane.

The lens’s field of view is determined by its focal length. This is the length, in millimeters, from the rear nodal point of the lens to the image plane. Some lenses have fixed focal lengths, while others have adjustable focal lengths. Those that can change focal length are known as “zoom lenses.”

The Aperture

Aperture is created by a set of blades inside the lens.
 

Technically a part of the lens, the aperture is the size of the opening of the lens. Many designs have variable diaphragms that control how much light passes through the lens, and are not unlike that of the eye’s pupil. The diaphragm will have a certain number of blades that diminish or expand the size of the aperture as needed. Some lenses have a fixed aperture whose size cannot be adjusted.

The Shutter

Many cameras have a device that opens and closes to let light impact the image plane for a predetermined amount of time. This is the shutter and it works much like your opening and closing eyelids—if you had your eyes closed more than open!

The shutter is a complex mechanical (or electrical) system. Mechanical cameras may have leaf or focal-plane shutters. The leaf shutter opens and closes like the aperture diaphragm and the focal-plane shutter uses “curtains” that work like garage doors.

More related to the image plane than the shutter, today, some digital cameras employ
"electronic shutters" that can either turn the digital sensor on and off globally in rapid fashion,
or activate one row of pixels at a time across the frame.
 

For more information on shutters, click here.

The Image Plane

After light passes through the lens aperture and is allowed to travel through an open shutter, it strikes the image plane. At the image plane is light-sensitive chemical-based film or a digital sensor on which the projected image is recorded. This plane’s position inside the camera is often marked by this symbol: "Φ" painted or engraved somewhere on the camera body, often on the top plate.

Point-and-Shoot Cameras

Point-and-shoot (PAS) cameras are generally the most simple of modern cameras. The most basic PAS cameras have fixed focal length lenses, non-adjustable apertures, and a basic shutter design. More advanced PAS cameras may incorporate zoom lenses, variable apertures, and a combination of mechanical focal-plane shutters and electronic shutters.

The basic components of a typical point-and-shoot camera
 

Therefore, the light path through a PAS camera is very simple. To see the light that is coming through the lens, the digital PAS camera will have an electronic screen that shows the true image impacting the image plane. Or, on some digital and film PAS cameras, there is a separate optical viewfinder that, when you look through it, displays a representation of the lens’s field of view.

Today, there are several genres of PAS cameras: pocket sized, superzooms, and there are newer PAS cameras that feature “full-frame” digital sensors the same size as 35mm film frames in a compact camera. Some PAS cameras are built to be water-, freeze-, dust-, and shockproof. Smartphone and cellphone cameras are, in fact, very tiny PAS cameras.

The Mirrorless or Interchangeable-Lens Camera (ILC)

Today’s digital mirrorless cameras, also known as interchangeable-lens cameras (ILCs), have identical optical paths as PAS cameras, with the exception of having lenses that can be removed and replaced with other lenses of different focal lengths.

The term “mirrorless” comes from the fact that the cameras have similar functionality to DSLR cameras in that they can change lenses, but do not contain the reflex mirror and optical viewfinder that define the SLR.

The basic components of a mirrorless or interchangeable-lens camera
 

Mirrorless cameras can also feature electronic viewfinders (EVFs) and LCD screens, and some have optical viewfinders, as well. However, unlike SLR cameras, the optical viewfinders on the mirrorless camera does not look directly through the camera’s lens.

The Rangefinder

The film and digital rangefinder cameras have a light path that is also similar to PAS cameras. The defining characteristic of the rangefinder is how it uses an off-lens-axis optical viewfinder to compose and focus the image. The digital live view and SLR viewfinders are aligned with the optical axis of the lens, but the off-axis viewfinder introduces parallax to the view. Parallax occurs when viewing the same subject from two different angles.

To focus a rangefinder camera, a secondary image is collected through a separate window and a portion of that image is reflected, via a mirror, onto the viewfinder image. Adjusting the lens focus brings the two images together to indicate when the lens is focused properly.

The SLR and DSLR

Last, but certainly not least, is the single lens reflex camera. Despite its popularity, I chose to discuss it last here because it is the most complex of the camera systems.

One of the primary benefits of the SLR camera is the ability to look through the camera’s lens to see exactly what the film or sensor will be seeing when the shutter is opened. How does the SLR “interrupt” the light and redirect it to a viewfinder?

The light path to the image plane is similar to that of other cameras, but in between the lens and the shutter lies a mirror that blocks the light from reaching the shutter. This is the reflex mirror (the “R” in SLR). Light enters the lens and then strikes a silvered mirror inside the camera housing. It is then reflected up toward a prism at the top of the camera and then bent toward the rear of a camera through an optical viewfinder. Below the prism is a focusing screen that can superimpose information over the image.

The basic components of the DSLR
 

The photographer composes the image through the viewfinder, and when the shutter release is depressed, that mirror flips up, out of the light’s path, the shutter opens, and then the light travels to the image plane.

When it comes to manual focus, the SLR is easy. Basically, you just determine focus by looking through the viewfinder as it shows the image that is being transmitted through the lens. Autofocus is more complicated and involves a transparent part of the reflex mirror, a secondary mirror behind the reflex mirror, and autofocus sensors in the bottom of the camera. For more on how the autofocus works, click here.

End Note

So, these are the basics of how today’s digital and film cameras function. If you have questions, or would like to know more specifics, feel free to comment below or click on the embedded links.