What are CCD and CMOS Sensors in Digital Cameras?

In Uncategorized by Rohit KumarLeave a Comment

Today, my dear friends, we will deal with a theme that is fundamental to the entire photographic world, we will talk about the symbolic element of the transition between analog and digital photography: the photographic sensor.

We will analyze together important topics such as:

  • The main features of the CMOS sensor and the CCD sensor.
  • The importance of the size of the photo sensor.
  • What are megapixels?
    and much more.

Whether you own a compact or an SLR, I would say that you can not miss this article because:

The sensor represents the heart of your camera, the nerve center of the system, the magical place where light is transformed and comes to life.

The photo sensor

The photographic sensor is an electronic device that once hit by the light rays coming from the lens, collects and processes all the information related to the incident light.
It is then inside the sensor, that the information of intensity and color range is processed and then transferred to the memory card.

We can, therefore, imagine the sensor as a large deposit, where the light is stored and packaged and then transferred to the sd card.
The quantity and quality of light stored in the warehouse depend on the efficiency of the many suppliers it uses:

  • The diaphragm opens and closes and regulates the amount of light that will hit the sensor.
  • The shutter speed determines the time in which this information is collected.
  • ISO control determines how much light information needs to be amplified before it reaches the sensor.

CCD Sensor vs. CMOS Sensor

These two types of sensors, although both were invented in the late ’60s and ’70s, have followed a clearly different path of growth.

Immediately after its birth, the CCD sensor practically obtained the monopoly of the cameras on the market until the ’90s, while the CMOS sensor seemed to disappear into thin air.
The main cause of this phenomenon was to be found in the great complexity of construction of the CMOS sensor, which for economic factors strongly limit the expansion.

Image credit Quec.li
CCD vs CMOS Infographic

Which is better today: CMOS sensor or the CCD sensor?

Recently there has been a trend reversal since the two types of sensors are practically the same.
This change, of course, has taken place, as often happens, for purely economic reasons.

The CMOS sensor is now present on almost all cameras on the market mainly because of:

  • Lower production costs
  • Best performance in critical lighting conditions

Ccd sensors are now found in medium format cameras since the color rendering is still better than that of CMOS sensors and in some compact and bridge.

What does a photo sensor look like?

One of the most common mistakes you make when talking about the photo sensor is to think of it as a single element that works on its own.
This small component, in addition to being formed by millions of photosites or pixels, is a very complex system, characterized by multiple elements that interact with each other. We can have, for example:

  • A color filter like Bayer or Foveon filters
  • Sensor matrix
  • Amplifier
  • Digital Controller
  • A/D Converter
    ..

How does the photo sensor work? What are pixels?

Now let’s try to understand how the sensor works and especially how it can perceive every color of the scene framed.

When the photons, which carry the information of light, collide and interact with the siliceous elements of the sensor, electrons are released and a small electrical discharge forms. This phenomenon is known as the Electric Photo Effect.
The electrons that are released are collected from the individual pixels, each pixel has a maximum capacity of electrons that can collect. This maximum is known as the full capacity of the pixel.
A pixel can only display a single color, the color of each pixel is determined by the amount and type of information it collects from the light.

The more electrons accumulate on a pixel, the lighter the corresponding tonal value in the image is.

So for example in a photograph:

  • A white pixel contains the maximum amount of electrons.
  • Whereas a black pixel does not contain electrons.
  • All values in the first two cases listed produce tonal values in grayscale.

Color in Photo Sensors

This process takes place because the sensor cannot determine the color by electron counting alone and needs a color filter.
Inside the filter, combining the total value of the received gray and the intrinsic color information of the filter, the final color is determined for each pixel.
The filter that is usually used is a Bayer filter that has a pixel pattern: red – green – blue-green. Green is used twice in an attempt to imitate more closely the vision of human sight, which sees this color better than any other.

The repetition pattern of a Bayer RGB filter. Wiki

Photo sensor dimensions

One of the most important classifications of image sensors is based on their physical size, which is usually expressed in mm and in terms of height and width.
The size universally recognized as standard is the equivalent to the format provided in film cameras, which is 36 mm x 24 mm.
In modern cameras, sensors of this size are only mounted in professional Full Frame cameras, because with the advent of digital photography, for purely economic reasons, several sensor models have been built smaller than the standard format.
This differentiation of the photographic sensors led to the birth of the crop factor and the end of equivalent focal length.

The different dimensions of the image sensor. Image Source Wiki

As you can see there is a huge difference between the medium format (the only sensor with dimensions greater than those of full frame) and the other formats, the differences going down in the scale are thinner but remain evident as the effects caused by this diversity will be evident.

Sensor size and image quality

The different sizes of the format also have an obvious impact on the quality of the image produced.
As we have seen, the sensor is composed of many small units of light storage, called photosites or pixels.

These units, like the sensors, can also vary in size between different camera models.
This means that:

For the same number of pixels, a smaller sensor must have smaller pixels than a larger sensor.

But as we have seen, each pixel has a maximum electron capacity.

So this means that the larger pixels will contain more light information than the smaller one.
More information means more detail in the image quality and less noise in the final product.

Difference between Cmos sensor and Ccd sensor

Now let’s try to understand how the different types of CCD and CMOS sensors work.
Both types of sensors work on the basis of the electric photo effect we mentioned earlier.
The difference in acquisition in the two sensors lies in the system of reading the electrons.

When the exposure time ends:

In the CCD sensor, the electrons excited by the photons migrate towards the base of the sensor. The transfer takes place by varying the voltages between the various pixels, the information is amplified and converted into an analog signal.
In CMOS sensors, the information generated by electrons is encoded directly on the pixel. Subsequently, the signal of the individual photo-sites is collected to transform the data into an image, using a digital signal.

What are megapixels?

One of the most sponsored features on the market is the number of total pixels contained in the sensors, this size is generally expressed in megapixels (1 Megapixel = 1 million pixels).
For example, a 30 Megapixel camera sensor will contain 30 million pixels inside.
Now I hope the world doesn’t collapse on you but I tell you:

Megapixels are important, but not as the physical size of the photo sensor.

The closer the pixels are, the more the electrons it contains interact with each other creating that annoying problem known as background noise.
That’s why a larger sensor with fewer pixels has better image quality than a small sensor filled with pixels, remember that when you buy your camera.
If you think about it it is for this reason that compact cameras, despite having many megapixels have image quality much lower than an SLR, because the sensor is very small in comparison.

Conclusions

If you’ve come this far, you should have understood a lot about the photosensor and its potential by now.
Keep in mind everything you’ve learned if and when you want to buy a camera and never forget your photographic needs:

If, for example, you need a very high definition of your photographs, maybe for big prints or you think to make big cut-outs in post-production, it might make sense to buy a camera with a standard sensor.
But if your maximum use of the camera is to take simple photos to share with friends, it doesn’t make sense to spend a lot of money on a professional camera body. Instead, buy a pair of optical lenses and you’ll surely have more fun when you shoot.

Anyway, when you think about the photo sensor you imagine that it is a sail on a beautiful white boat and that the light is the wind that pushes it and remembers that:

The bigger the sail, the greater the surface that the wind can exploit to improve your navigation.

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