Our surroundings are subject to a significant variance of luminous intensity (dynamic range), from the bright sunshine to dim light of stars.
Image sensors have only limited dynamic ranges and are unable to capture images with the capacity comparable to the human eye. Bright photographic subjects will result in the stronger shadows in surrounding areas, and conversely, dark objects in focus will result in the highlight blowout. Image sensors for smartphone cameras have even smaller dynamic ranges compared to SLR cameras due to their small sizes, which reduces the light receiving capacity. This is why photos taken against a light source with a smartphone come out very differently from how we see the same scene with our eyes.
HDR is a feature that enables to capture images with a large variance in brightness (dynamic range). The HDR mode alters the sensor operation to enable a wider range of luminosity to be captured, and automatic brightness adjustment reduces the blackout or blowout. The photographic image becomes close to the human vision.
Sony‘s image sensors come with an exposure control technology and signal processing function built in, realizing the HDR function in the sensor itself. In particular, the sensors compatible with three exposure levels can not only produce images that are close to human vision, but also realize the same effect in video images.
A landscape in the sunlight seen from a dark indoor environment―a scene with a wide dynamic range such as this can be captured with details in the light and dark areas intact, avoiding blackout and blowout.
In fact, the dynamic range is also wide in nightscapes. The HDR function reduces the blowout effect of illumination, making it possible to capture an image just like we see it.
Firstly, shutter speed is adjusted so that images are captured in differing exposures. In the QBC sensor, four pixels of the same color are configured to three different exposure settings: short, long, and intermediate. Then, three versions of an image are captured at the same time.
The resulting three images are synthetized in the image sensor, creating a signal set with a wide dynamic range. Gradation correction is applied to adjust the brightness based on the luminance distribution of the data, reducing blowout and blackout. The result is an image that is close to how it appears to the human eye.
The HDR function can complete these processes of multi-exposure capture, synthesizing, and gradation correction within the sensor‘s logic chip in real time. This makes it possible to the application of HDR to video shooting.