CamCyclopedia
Contents
- Camera Exposure
- What is Exposure?
- Aperture, shutter speed, ISO - Auto Exposure of Galaxy Camera
- Auto Exposure
- Photometry
- Exposure chart
- Galaxy AE algorithm
- Pro/Pro Video (Manual Mode) - Reference
Camera Exposure
What is exposure?
In the mechanism of clicking a picture, light passes through the camera lens to reach the image sensor (CCD, CMOS) or the film, and exposure is the luminance that is registered by the image sensor or film. Exposure consists of three elements, aperture, shutter speed and ISO, that determine the brightness of an image.
Let us understand a few terms better to gain insight into camera exposure. Illuminance is the total luminous power incident on a surface per unit area. Luminance refers to the luminous power reflected from the subject surface to the eye of the observer and corresponds to the brightness picked up by the camera.
Why do we differentiate between illuminance and luminance? It is because light incident upon a surface gets absorbed, reflected and penetrates it in varying quantities. However, out of these, human eyes and the camera can only detect the rays reflected by the surface. That is, the illuminance may be the same, but the luminance differs depending on the reflection properties of the surface.
Aperture, shutter speed and ISO
Components responsible for adjusting exposure have other properties in addition to controlling image brightness.
Let’s understand how we can combine these three to get the optimal brightness in a picture.
The camera aperture, just like the iris in human eyes, controls the amount of light that passes through it. The aperture size is expressed in F-numbers. [1]. This number is inversely proportionate to the diameter of the aperture, i.e. the diameter is small if the f-number is big and vice versa. More light can pass through if the aperture is big, which makes the resultant image bright. Similarly, if the aperture is small, the image is bound to be dark because not a lot of light passes through it.
Also, in addition to brightness, if the aperture is big it could give an out-focusing effect making the background blurry. A ‘shallow depth of field’ only allows the camera to focus on the subjects in the frame making the rest blurry.
On the other hand, a ‘wide or large depth of field’ enables the camera to cover more area and have a sharp focus on all subjects (despite the distance between one and another) with the aperture set to a narrow diameter.
The shutter in a camera is the eyelids in a human eye. Shutter speed refers to the speed at which the shutter closes while snapping a shot. If the shutter speed is slow the sensor or the film is exposed to light for longer, making the image bright. Similarly, with a fast shutter speed, the sensor is exposed to less light making the image darker.
Shutter speed affects the amount of light coming through the filter. As a result, it is closely connected to phase changes like the movement of the subject or unsteady hands, etc. There are two kinds of shutters, one is the mechanical shutter where the physical movement of the shutter is involved and another is the electronic shutter which does not involve any mechanical parts like the former. An everyday smartphone uses electronic shutters and the shutter speed in there is indicated by exposure time (when light hits the photodiode of the image sensor).
Electronic shutters are usually a rolling or a global shutter type. A rolling shutter reads the signals on a sensor, line by line, while a global shutter reads out all sensor signals simultaneously. These two methods each have their advantages and disadvantages like image distortions as a rolling shutter effect [2] and other complications that we are not going to go into.
ISO refers to how sensitive the camera sensor can be to the incident light. Camera sensitivity uses the ISO numbering from the International Organization for Standardization. When the ISO is set to high, the gain of light in the sensor increases, causing the image to brighten and vice versa.
The ISO heightens the light that hits the sensor just like an amplifier, but this is accompanied by an increase in the noise component of the image, making it low quality and grainy.
What is AE (Auto Exposure)?
The AE feature uses the AE statistics data given by the ISP to gauge the brightness of the subject in the camera and uses control factors to automatically control the brightness to adjust it to a specific target. Here, the shift in brightness ascribed to control factors converges into a particular brightness value.
The AE statistics data above refers to the R, G, B and Y average value for each window and the assorted brightness in R, G, B and Y histogram, etc. Also, although control factors in cameras refer to aperture, shutter speed and ISO as explained earlier, things are a bit different in smartphone cameras.
A regular galaxy camera uses a fixed aperture and an electronic shutter. Given this, exposure time and ISO are the two factors that control exposure in smartphones.
The below image is a rough example to understand what AE is. If the angle of view happens to shift in an optimally exposed frame, the AE automatically acts to adjust to the corresponding lighting conditions. The AE shifts the exposure by a certain amount in every frame and they converge towards an optimal exposure. The statistical data mentioned above is used to calculate the optimal exposure.
Photometry
Photometry as the name suggests is used to measure the propagation of light and thus brightness and photometry have a close relation in photography.
As we said before, AE calculates the brightness and adjusts the control factors to coincide with that particular brightness value. The part about calculating or identifying the brightness value lends itself to photometry.
Photometry can be done for a whole image or portions of it, like the centre or any specific area the user wants. And the current brightness value could differ based on the block it is calculated against. The illuminance varies depending on the method of photometry, and illuminance determines the exposure. The final brightness value is thus regulated.
The picture below is representative of photometry. It indicates how much weightage is given to different blocks of an image to calculate the current illuminance.
In photometry scenarios unlike the above, say in portraits, the face gets detected and either FDAE (Face Detection Auto Exposure) is applied to it or the user touches a particular block of the image and touch AE is applied with that particular area as the focus.
Exposure chart
Earlier, we read that Auto Exposure calculates the current brightness and adjusts images according to that value.
However, there is no single combination of control factors that generates this specific brightness value. (ex. ISO 50/exposure time 0.5s == ISO 100/exposure time 0.25s)
The exposure chart is a graph denoting the combination of control factors that configure a specific brightness.
This is a line chart. The x-axis of the chart represents illuminance conditions, it is dim illuminance to the left and bright illuminance as we move to the right. Shutter speed and ISO are plotted on the y-axis. The higher the values the darker the EV, and the lower the values the brighter the EV.
The bright illuminance zone is usually for outdoor venues. The exposure-time controls the same in the above diagram. The dim or dark illuminance area mainly applies to indoors or night scenes. The ISO and exposure time control the same as above.
To summarize, the exposure chart shows combinations of ISO and exposure time for all illuminance conditions from a brighter section to a darker one. The darker the illuminance conditions, the brighter the exposure value used, to tide it over and adjust to an optimal exposure.
Galaxy AE algorithm
Galaxy incorporates various AE algorithms [3], so that users can always click the kind of photos they want. AE operates in multiple situations, gleaning miscellaneous pieces of information from the histogram, color information, and FDAE and touch AE as explained under photometry.
A few of these galaxy AE algorithms are introduced in brief below.
Histogram correction algorithm
This analyses the histogram distribution of an image and makes sure the brightness of the image is not concentrated at a specific level and is balanced out well in the overall expression of the image.
Color correction algorithm
This analyses the color information of a subject and adjusts the exposure, to express the color definition in a way that is conducive to the human eye (as close as possible to how human eyes perceive colors in nature).
Backlight correction algorithm
This algorithm reduces the difference in brightness between the bright areas and the shadows in a picture to supplement under-exposure in backlight conditions.
Partitioning the skin tone on the front camera
This algorithm divides up the areas by their properties using AI learning and adjusts the brightness to make a face appealing.
Video saturation control algorithm
This algorithm analyses the brightness level of a video and restraints its saturation levels in real time.
Pro/Pro Video (Manual Mode)
Auto Exposure is provided by default in typical smartphone cameras for the sake of user convenience. Galaxy also provides the pro/pro video (manual mode) at the same time, for pro photographers who want to have control over the camera exposure.
Pro/Pro Video Mode
Pro/Pro Video can control multiple things in addition to AE, like focus, white balance, etc. But, we are going to focus on AE here.
You can enter the respective modes by clicking on more in Camera and controlling the ISO, SPEED, and EV at the bottom. You just need to adjust one out of ISO and SPEED, because the other one adjusts automatically to target brightness in response to the former. (ISO first mode or Shutter first mode)
(ex. ISO 100 manual, SPEED 1/1000 auto -> the illuminance environment darkens -> ISO 100 manual, SPEED 1/100 auto)
Of course, you can adjust both the ISO and the SPEED and the photo clicked will be in line with the configured ISO and SPEED, despite any changes in the illuminance environment. (manual mode)
Lastly, EV can be adjusted from a range of -2.0EV to +2.0EV. This tab controls the target brightness. This brightens or dims the target brightness so that the degree of change in ISO and SPEED get affected automatically, which goes on to say that this does not have much significance and cannot be controlled in a manual mode.
And just like this, the user can directly control the camera exposure to click pictures. Images clicked will bear the respective attributes of the ISO and the exposure time set. Galaxy also provides the Expert Raw app for pro users in addition to the Pro/Pro Video modes in select flagship models. You can experiment and have fun shooting different kinds of pictures using these.
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Reference
[1] https://ko.wikipedia.org/wiki/F_값
[2] https://ko.wikipedia.org/wiki/롤링_셔터
[3] The applications for the algorithms introduced could differ from model to model.
[4] Some of the images are virtual images with graphic effects for illustration purposes.
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