Inside the camera: a complete guide to structure and function

Unlock the secrets of digital cameras: Dive deep into the essential components & discover their unique roles in photography.

Author: Marco Crupi

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Components of a Digital Single-Lens Reflex Camera (DSLR).

This article is a segment of the Digital Photography Course. Click here to return to the main course overview.

In a previous photography course article, I discussed the various types of digital cameras. Here, I will explain the operation and basic components of a camera.

The operation of a digital SLR camera is incredibly intricate. Still, it can be simply described if we avoid diving into technicalities not pertinent for this course. The light (represented by the yellow arrow in the top image) passes through the lens and is reflected by the mirror onto the pentaprism (or pentamirror in more affordable reflex cameras). This allows us to view the scene through our camera’s viewfinder. When the shutter button is pressed, the mirror and the shutter curtain lift, allowing light to hit the sensor, capturing our image. This process is elaborated further in the subsequent sections of this article.

Components of a Mirrorless Camera (MILC).

In mirrorless and compact cameras, where there’s an absence of the mirror and pentaprism, the operation varies slightly as light directly strikes the sensor​.

In the following section, I will delve into the details of individual components of a digital camera.


A lens could be permanently attached to a camera, as seen in compact and bridge cameras, or it might be interchangeable with other lenses boasting various focal lengths, apertures, and different features.

The lens's aperture at various openings.

The light passing through the lens is controlled by a circular or polygonal mechanism made of thin metal blades known as the diaphragm​​.

I have discussed the diaphragm and types of photographic optics in detail in the following article: Comprehensive guide to choosing your photographic lens.


The reflex or SLR (Single Lens Reflex) cameras are named after their viewing system comprised of a mirror set at a 45° angle relative to the lens and a pentaprism or pentamirror.

The mirror reflects the light that passes through the lens, allowing the image to be viewed from the viewfinder. The role of the pentaprism is to correct the image, which would otherwise have its left and right sides reversed. During the shooting phase, the mirror lifts so that the image can be captured by the sensor; when in the raised position, the mirror covers the focusing screen window, preventing stray light from entering through the viewfinder. This mechanism allows the photographer to see the exact image captured by the lens through the camera’s viewfinder.

As previously mentioned, regarding compact, bridge, and mirrorless cameras, this optical viewing system is absent. The light passes through the camera lens and strikes the sensor directly. The scene is therefore viewed directly from the rear LCD screen and, if available, from the electronic viewfinder.

The absence of the mirror and pentaprism/pentamirror offers numerous advantages, including the opportunity for manufacturers to create smaller and lighter cameras, faster focusing speed, higher shooting speed, and the elimination of micro-movements caused by the mirror lifting. These micro-movements can induce slight blurring during long exposures.

The only evident drawback of mirrorless cameras compared to SLRs is the higher energy consumption, leading to shorter battery life.


The Shutter.

The shutter is a device responsible for controlling how long the sensor is exposed to light, a duration that is adjusted using the shutter speed function of our camera. It consists of two fabric or metal surfaces arranged parallelly along the focal plane. When they move vertically, they form a slit that allows light to pass through.

The slow-motion movement of the shutter curtains.

I will discuss shutter speeds in a dedicated article.

Technology advances and in the not too distant future this component will disappear from the camera, an example is the Sony Alpha 9 III which has replaced the shutter with a technology called “Global Shutter“.


The viewfinder is the device that allows users to preview the framed scene.

In digital cameras, two types of viewfinders are available: optical and digital.

Viewfinder in an SLR camera.

Optical viewfinders are used in SLR cameras (both digital and analog). For cost reasons, only high-end professional models offer a full view of the framed scene. In entry-level and semi-pro models, only about 90-95% of the framed field is visible.

The digital viewfinder (also called an electronic viewfinder or EVF) is found in non-SLR digital cameras. It consists of a display that shows a real-time preview of the captured image.

An SLR camera in Live View mode.

Using the display is referred to as activating Live View mode. In this mode, light directly hits the sensor, the data is processed by the camera’s CPU, and then displayed on the camera’s rear monitor. On SLRs, when Live View mode is active, light doesn’t reach the viewfinder because the mirror is raised.

Articulated rear LCD display.

It’s always a good idea to choose a camera with an articulated rear LCD display. It allows for shooting in scenarios that would be challenging with a fixed display. For instance, at concerts, it’s not uncommon to need to take photos with the camera held above one’s head. With an adjustable display tilted downward, framing the scene becomes easier. Conversely, there might be a need to shoot with the camera resting on the ground or just a few centimeters above; in such situations, being able to tilt the display upward has always been beneficial.

LCD screens often have poor visibility in bright lighting conditions. That’s why mirrorless cameras might come with a small Galilean-style optical viewfinder (OVF) or an electronic viewfinder (EVF).

With SLRs, we see the scene as it truly is through our lens. Only after taking the shot can we see the effect of our adjustments on the photo. In contrast, mirrorless cameras offer a constant preview of how the photo will turn out based on our settings.


CCD Sensor – Photo by Daniel on Flickr.

Before the advent of digital technology, images were captured on film. Today, in modern digital cameras, it’s all about the sensor.

The most commonly used sensors in cameras are CCD (Charge-Coupled Device) and CMOS (Complementary Metal-Oxide-Semiconductor). There’s also the FOVEON sensor, currently used only in cameras produced by Sigma.

CCD and CMOS have distinct characteristics and applications:

  • The CCD offers superior image quality and generates less noise than the CMOS.
  • The CMOS consumes much less energy than the CCD.
  • The CMOS is more cost-effective than the CCD.
  • The CMOS allows for faster continuous shooting.

Given these features, CMOS sensors have been implemented in compact cameras and smartphones, while the CCD has been dedicated to cameras targeting high performance in terms of image quality. Although there used to be a clear distinction between CCD and CMOS, as technology has advanced, the differences between the two have become less pronounced. It’s now common to see CMOS sensors in mid to high-range cameras.

What are Megapixels?

A sensor’s resolution is measured in megapixels. In digital photography, a megapixel equals one million pixels. The term is used to denote both the number of image pixels and the total pixels on the camera sensor. To calculate megapixels, multiply the two dimensions together. For example, a sensor producing images of 4288×2848 pixels has 12.2 megapixels (4288 × 2848 = 12,212,224).

Due to misleading marketing, there’s a misconception that a higher megapixel count automatically means better image quality. While a higher pixel count can, in theory, offer greater resolution impacting print size, image quality also depends on:

  • Dynamic Range: The sensor’s ability to capture detail in high light areas and shadow zones.
  • ISO Sensitivity: A sensor’s light sensitivity defined by the ISO (International Standard Organization) standard. Advanced sensor technology reduces image noise at high ISO levels.
  • Sharpness: Refers to the overall clarity of an image in terms of both focus and contrast. A sharp image appears clear and realistic with high detail, contrast, and texture. Images lacking sharpness or described as “soft” can appear blurry and lack detail.

Dynamic range and ISO sensitivity are parameters exclusively dependent on the sensor, influenced by its size and technology. In contrast, sharpness depends not only on the sensor but also on the lens’s quality.

These concepts are quite complex, and I’ve tried to express them in this article as simply as possible. I’ll dedicate separate articles to them in this photography course.

Sensor Sizes in Digital Cameras.

To put it simply, given the same technology and megapixel count, the larger the sensor, the more dynamic range it will have, and the less noise it will produce at high ISOs, compared to its smaller counterpart. That’s why having 18 megapixels on a 36x24mm (full-frame) sensor is a different ball game compared to having them on a 6.16×4.62mm sensor.

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