How a Camera Works

Submitted by WhiteWall Team

We use photos to capture memories and special moments. Especially in the age of digital cameras, this is easier than ever. We press the shutter button, look at the image on the display and the memory is "in the can". But what actually happens in the camera when we press the shutter button? How does light become an image? And which features play a central role in the camera? We take a look inside the camera with you.

How light becomes an image

While analog cameras expose an inserted film, digital cameras work with complex image sensors. Image sensors consist of millions of small pixels, with each pixel being assigned a light-sensitive photodiode. Photodiodes convert light into electrical charge. The more light falls on the photodiode, the stronger the electrical signal and vice versa. As not only brightness but also color information is required for a photo, the sensor is combined with a color filter consisting of the primary colors red, green and blue. Each photodiode is assigned exactly one color. This means that there are pixels or photodiodes that only record green or red light, for example. The so-called Bayer matrix is used for the arrangement of the colors in most sensors: The Bayer matrix is divided into blocks of two green, one red and one blue pixel each. The colors recorded in the blocks are later added together in the demosaicing process in order to generate exactly the colors that are required for the recorded image. The color distribution of the Bayer matrix of 50 percent green and 25 percent each blue and red results from the special sensitivity of the human eye to green tones. White balance is used by the processor to compensate for any green cast that may occur during image processing.

The autofocus

The camera and lens must work together perfectly in order to focus on a subject. In the case of mirrorless system cameras, the focus is determined via a contrast measurement, in the case of SLR cameras via a phase detector. Cameras with a so-called hybrid autofocus can do both.

Phase detection

Phase detection works in a similar way to the crop indicators on analog SLR cameras. In the viewfinder, these showed a kind of image shifted against each other, which only merged seamlessly when focusing. In this way, the camera could draw conclusions about the distance to the subject and find the required focus point very quickly. Phase detection in digital SLR cameras also works according to this principle.

In a DSLR, the light entering through the lens first hits the partially transparent primary mirror, from where most of it is directed into the optical viewfinder. The remaining light is deflected via a small sub-mirror onto a separate sensor for phase detection. Microlenses on the sensor generate a double image from the light. The camera then analyzes whether and to what extent these images are shifted against each other. Based on this information, it recognizes whether the object is in front of or behind the focal plane and passes this information on to the autofocus motor in the lens so that it in turn brings the lenses into the correct position quickly and precisely. If the double images in phase detection are exactly on top of each other, as in the sectional image display, the subject is in focus. Phase detection works very quickly, but is also prone to errors if the camera and lens do not work together optimally. The focus can then be off. If you want to be on the safe side, you should use the camera manufacturer's original lenses. When updating the camera firmware, which often also fixes problems with the autofocus, the corrections are tailored to the original lenses.

Contrast metering

As mirrorless system cameras (DSLMs) do not have the large, partially transparent mirror of DSLRs, DSLMs use contrast metering to determine sharpness. The principle is simple: the sharper the image, the higher the contrast. In order to find the optimum sharpness, the camera focuses back and forth until it has decided on the highest contrast. This can take some time, especially in low ambient light, but contrast measurement is considered to be particularly accurate as it is carried out directly by the camera sensor and the light does not have to be directed to a separate module via mirrors as is the case with phase detection on DSLRs.

Hybrid autofocus

In order to find the focus quickly and at the same time very precisely, many manufacturers rely on so-called "hybrid autofocus". Here, the approximate focus is first determined at high speed using phase detection. Contrast measurement then takes over and fine-tunes the focus. The hybrid autofocus of a mirrorless camera does not have a separate phase detection module, but works with microlenses that are placed on individual pixels of the image sensor.

Exposure metering

Have you ever had a photo that just won't turn out because it always seems too bright or too dark? Then it is helpful to familiarize yourself with how your camera's exposure metering works. Exposure metering determines whether the light is metered throughout the entire image or only in certain areas of the image. Choosing the right metering method is therefore crucial for a successful photo. SLR cameras and mirrorless system cameras offer at least three of these.

Matrix/multi-segment

Matrix or multi-segment metering is set as standard on most cameras. With matrix metering, the image section is divided into small areas and the brightness is analyzed in each of these areas. The extent to which the individual results are included in the exposure depends on the analyzed subject structure and where the autofocus point is located.

Center-weighted

This method also measures the brightness over the entire image section. In the exposure itself, the middle areas are weighted the most. With some cameras, the size of the center area can be set.

Spot

With spot metering, only the brightness of a relatively small area is measured. This "spot" is usually in the center of the image or coincides with the active autofocus point. This ensures that the camera optimally adjusts the exposure to the targeted subject regardless of the ambient light conditions. Detailed information on the ideal exposure can be found in chapter "3.4 Measuring and setting the correct exposure".

The shutter

The shutter of a camera determines how long the image sensor is exposed. Very short exposure times are used for moving subjects, for example, where the movement should be "frozen". For night shots, on the other hand, a long exposure time is required. There are various approaches to controlling the exposure time.

The focal plane shutter

The mechanical focal plane shutter dates back to the days of analog SLR cameras and is still used today in modern system cameras. It works with a first and a second shutter curtain made of narrow, lightweight metal blades. When the shutter release button is pressed, the first shutter curtain moves from top to bottom and releases the sensor for exposure. The second shutter curtain ends the exposure by also sliding its blades from top to bottom until they completely cover the sensor again. The interval at which the second curtain follows the first is determined by the shutter speed set on the camera. With today's mechanical shutters, short exposure times of up to 1/8,000 of a second are possible.

At very fast shutter speeds, the second shutter curtain moves downwards before the first has even reached the bottom. You can imagine that not the entire sensor is exposed, but only a narrow slit that moves from top to bottom. As such a slit can lead to problems with flash photography, the technical specifications of all cameras include a so-called flash sync time. This is often specified as 1/250 second. The flash sync speed is the shortest time in which the shutter is still fully open. At 1/320 of a second, the second shutter curtain would already slide from top to bottom before the first shutter curtain has reached the lower end of the sensor, which would result in part of the upper image area being darkened despite the flash.

The electronic shutter

The electronic shutter has a significant advantage. In contrast to the mechanical version, it has no moving parts and therefore works both silently and without vibrations, which can lead to blurring in the image with long exposure times. In simple terms, the electronic shutter in the CMOS sensors used in almost all consumer digital cameras today works as follows: As long as light falls on the sensor, it is continuously converted into electrical charge at the photodiodes. To start recording, the charge at each photodiode is briefly reset to zero. This "restart" corresponds to the first shutter curtain. At the end of the exposure time, the sensor is read out line by line from top to bottom with the conventional electronic shutter. This moment can be compared with the second shutter curtain.

Electronic exposure is very fast and therefore enables very short shutter speeds that would not be possible with a mechanical shutter. The Canon EOS R3, for example, allows shutter speeds of up to 1/64,000 of a second. However, a disadvantage of the conventional electronic shutter occurs when either the subject or the photographer moves the camera. As the sensor is read out line by line, it can happen that the subject is already in a different position in the lines read out further down than at the beginning of the readout in the first lines. This can lead to distortions, also known as "rolling shutter". This effect can be recognized, for example, by the fact that the rotor blades of a fan appear slightly distorted in the image.

A combination of both methods

With modern digital cameras, both shutter methods can be combined. The first shutter is triggered electronically. The advantage: there is no noise or vibration. For the second shutter, the blades of the mechanical shutter move downwards.

Global shutter

With the Sony Alpha 9 III, the Japanese manufacturer has introduced the first consumer digital camera with a so-called global shutter. This is an electronic shutter in which the sensor's photodiodes are not read out line by line, but all at the same time. The global shutter thus avoids the rolling shutter effect that occurs with conventional electronic shutters. Sony's global shutter sensor in 35 mm format enables very fast exposure times of up to 1/80,000 of a second and is therefore particularly recommended for distortion-free sports and action shots with fast shutter speeds. In addition, even flash photography at 1/80,000 second is possible. This was previously only possible with analog or extremely expensive digital medium format cameras with an electronic shutter built into the lens.

The image stabilizer (IBIS)

Camera shake mainly occurs in photos when the ambient light is very weak - in other words, when the camera shutter has to remain open for a relatively long time in order to obtain a sufficiently bright image. The most effective way to prevent camera shake is to use a tripod to stabilize the camera during the exposure. However, few photographers want to carry this bulky tool around with them all the time. This is where image stabilization comes into play.

In SLR cameras, image stabilization is used exclusively in the lens. Individual lenses are moved vertically and horizontally to counteract camera shake during exposure. With many modern mirrorless system cameras, manufacturers go one step further and also equip the camera sensors with image stabilization. Sony, Panasonic and Olympus were the first manufacturers to offer what is known as an IBIS system ("In Body Camera Stabilization"). Other manufacturers have since followed suit.

The principle is always the same: With sensor-based stabilization, the camera's sensor is held in position by electric magnets. If vibrations or movements are measured, the camera compensates for them by shifting the sensor accordingly. Modern IBIS systems stabilize the image in five axes. They compensate for both vertical and horizontal wobbling, tilting, panning and rotation around the optical axis. Some providers also offer hybrid autofocus. The image stabilization units in the camera sensor and in the lens work together and enable freehand shots with comparatively long exposure times. With the OM System OM-1 Mark II, for example, up to 8.5 f-stops longer exposure times can be photographed hand-held than would be possible without stabilization.

Image processing

After the camera is triggered, the light on the camera sensor is converted into electrical signals which are passed on to the camera's image processor. The image is processed there. Depending on which image format is set in the camera, there are various options. When shooting in RAW mode, the camera saves all available data in a file and does not process it internally. It is therefore also referred to as a raw data format. RAW images can then be edited as required using special RAW converter software.

However, if you want to edit your images directly in the camera in order to share them with others as finished and space-saving JPEGs, you must first make some settings on the camera for development. Important: When taking photos in JPEG format, the post-processing takes place before the image is written to the memory card and cannot be undone. If you do not want to leave the software-based, internal processing in the camera to the automatic system, you can intervene manually. White balance, noise suppression or lens corrections can usually be accessed via a separate entry in the menu. There are usually separate presets for correction functions such as color style, contrast, sharpness or saturation, which can be changed individually if required. In addition, many manufacturers also offer artistic effect filters. With Fujifilm, the film simulation can also be used to simulate the look of old analog films.

Conclusion

In the fascinating world of photography, the camera is the tool that captures our memories and preserves moments for eternity. The technology behind the camera is complex, but it allows us to capture our creative visions and share them with others. By understanding the interplay of light, sensor and mechanics, we can develop our photographic skills and create even more impressive images. We are here to help and advise you to perfect your images.