Ultra-high-contrast digital sensing technique could put an end to “blown out” skies

The image sensor of a digital camera does not directly capture digital data. Instead, it captures light signal as analog data and then uses an analog-to-digital converter (ADC) to convert the fluctuating voltages of the light analog signals into digital strings of ones and zeroes. If an incoming signal exceeds the voltage limit of the ADC, the ADC will cut it off (i.e., flatlines at the maximum voltage). This phenomenon is familiar as “saturation” in digital images — when, for instance, a sky that looks blue to the naked eye is captured by your camera as white; we then say that the sky has overexposed or “blown out.”

There are a number of tricks you can use to avoid blown-out skies — and thus capture back that blue sky:

  • Expose for the sky. Point your camera toward the sky, press the AE-L (Auto Exposure Lock) button on your camera (if there is one) to lock exposure on the sky, then recompose to take your shot. The sky will turn out blue — though the rest of the image might now appear too dark. You could also dial in a negative exposure compensation to darken the sky and bring out the real blue color.
  • Shoot using the HDR feature (if your camera has it). With one press of the shutter button, the camera takes a number of shots at different exposure (e.g., one shot exposing for the bright sky and a second shot exposing for the darker foreground), then merges the best exposed parts of the images together to produce one photo with the sky (top) and foreground (bottom) parts perfectly exposed. You can also opt to do that merging yourself manually in Photoshop or another image editing software.
  • Use a Graduated Neutral Density (ND) Filter on your lens. That will darken the sky part of the image while leaving the bottom part unaffected. If you expose for the bottom part, the sky will be underexposed (due to the Graduated ND Filter’s effect, and which will prevent the sky from overexposing), thus giving you a picture with both top and bottom parts of the image properly exposed.

In the future, we may not need to do any of these adjustments anymore. Researchers from MIT and the Technical University of Munich have put forward a technique that they call “Unlimited Sampling” which, theoretically, can accurately digitize signals whose voltage peaks are far beyond an ADC’s voltage limit. The consequence could be cameras that capture all the gradations of color visible to the human eye (as well as audio that doesn’t skip, and medical and environmental sensors that can handle both long periods of low activity and the sudden signal spikes that are often the events of interest). Maybe, one day we will really realize the promise Kodak made to us a long time ago (with a slight twist): “You Press the Button, [the Camera Does] the Rest.

If you want to know the details about this promising technique, read more about Ultra-high-contrast digital sensing.

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