Understanding a Modern Digital Camera

A computer vision engineer’s perspective

Photo by Damir Babacic on Unsplash

The camera captures images, ez. pz. End of story, right? RIGHT? NO!
A camera measures light, it captures the number and direction of photons that make an image and there is a lot to it one must learn to be a better computer vision engineer.

The camera also transforms a 3D scene into a 2D image, so it reduces the actual dimensionality (depth is lost!)

In order to have an in-depth understanding of how a camera works, it is important to understand the different elements and components of a camera.

Elements of a digital camera

A digital camera consists of various components as described in Figure-1

Figure 1: Overview of a digital camera (Figure by Author)

Sensor

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A camera sensor is an array of light-sensitive cells and it converts the incoming light to intensity values.
A bigger sensor can collect more light per time interval giving a higher quality image.

So, a sensor measures incoming light and converts it into intensity values, but what about the color?
To obtain color, there are 2 common approaches

1. Three-Chip Design
   Use separate filters filtering out for red, green, and blue lights that are in turn measured by 3 different chips.
   This results in a very high-quality image but makes the camera bigger and more expensive
2. Single-Chip Design
   Use a single chip that has pixel-dependent color filters, i.e. different pixels of a single chip is sensitive to one of the RGB color, and during the final image creation, colors are interpolated.
   This results in a lower quality image but helps in reducing the size and cost of the camera

Shutter

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The shutter is basically a curtain over the camera sensor.
It controls the time duration for which the camera sensor is exposed to light.

A longer exposure time means more light for the sensor to capture which results in brighter images.
Longer exposure time also means a moving object would result in motion blur.

There are different types of shutters and how they operate, the most common being rolling shutter and global shutter.

1. Rolling Shutter
   The shutter rolls (moves) across the exposable sensor area.
   This results in pixels at the same line being exposed to the light at the same time.
   Rolling shutter produces distortions in the case of fast-moving objects or cameras because the position of the object might have changed when the top-most row of the sensor was exposed to the light by the shutter compared to the bottom-most row.
2. Global Shutter
   Global shutter exposes the full camera sensor to the light at the same time resulting in lower distortions of the object being captured.

Aperture

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In order to completely understand what is the use of lens and aperture, one must know how the image is formed in the camera.
You can go through this blog to understand camera image formation.

Aperture is the ‘pinhole size’. In other words, aperture defines the size of the opening in the barrier (that is in front of the sensor) which governs the amount of light that is being passed to the sensor.

Figure 2: Different camera apertures (Source)

Aperture governs the portion of the lens exposed to light.
If a very large portion of the lens is exposed to light, then due to lens errors, some of the parts of the image may become blurry.
If the aperture is small, only a small portion of the lens is exposed to the light reducing any lens error and resulting in sharper captured images.

Depth-of-field refers to the range of distance that appears acceptably sharp and is directly governed by the aperture. (for eg. in the image below, the aperture is large, causing the middle of the image to be sharp and the rest to be blurry)

Photo by Stanisław Gregor on Unsplash

Lens

Photo by Ahmad Mohammed on Unsplash

The goal of a lens is to obtain images that are

1. Not distorted
2. Sharp
3. Contrast intensive

There are various types of lenses available for a modern camera. The choice of the lens depends on

1. Field of view
2. Distance to the object
3. Amount of available light
4. Price

Summary and Conclusions

Now we know what are the various components of a camera and what role do they play in image formation.

As a follow-up to this read, I would recommend going through this post to understand how the image is captured by a camera.

References

1. Photogrammetry Course Lecture by Cyrill Stachniss
2. Photogrammetry Course Lecture Notes