ISO Speed for Digital Cameras: Real or Imaginary
Michael Kriss
(MAK Consultants)
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Date: March 12, 2014
Description:
The concept of an ISO speed for digital cameras is somewhat of a conundrum. There is an accepted ISO 12232:2006 Standard on how to measure the ISO speed for a digital camera. In fact there are three accepted measures including the Recommended Exposure Index (REI), the Standard Output Sensitivity (SOS) and the Saturation-Based technique. These measures are all based on the final output of the digital imaging system and are often confined to a specific file format (TIFF for example) and color encoding (sRGB for example). The “traditional” negative film ISO (ASA) speed was empirically defined as the exposure that gave an excellent image when printed on either color or black and white paper. The “rule of thumb” was that on a bright sunny day the camera should be set to F/16 (or F/11 for reversal slide film) with a shutter speed of 1/ISO speed. This would ensure that there are two stops under and over exposure protection. This criterion made it possible for simple cameras to always get a good picture on a bright day. The speaker will present a way to calculate the “ISO speed” speed of the sensor rather than that of the camera. The calculation will take into consideration all the physics involved in creating photoelectrons, storing them, and the degrading sources of noise present in image sensors. The calculation will draw from the film terminology, but instead of a threshold speed calculation used in film studies, a signal-to-noise (S/N) calculation will be used for sensors. The impact on sensor speed from f-stop and shutter time manipulation will be discussed. The concept allows one to just replace the film by a sensor and use the same metering systems. Higher ISO speeds are possible by just increasing the overall gain of the imaging system (after white balance) and better image processing to hold the sharpness and lower the noise.
Further Information:
Dr. Kriss received his BA(62), MS(64) and PhD(69) in Physics from the University of California at Los Angeles. He joined the Eastman Kodak Research Laboratories, Color Photography Division, in 1969 and later the Physics Division until his retirement in 1993. In his early years at Kodak, Dr, Kriss focused on color film image structure and modled and simulated the impact of chemical development on image structure and color reproduction. When he joined the Physics Division he focused on image processing of scanned and captured digital images. Dr Kriss spent three years in Japan where helped build an advanced research facility. At Kodak he headed up the Imaging Processing Laboratory and Algorithm Developing Laboratory. He joined the University of Rochester in 1993 where he was the executive director of the Center for Electronic Imaging Systems and taught through the Computer and Electrical Engineering Department. He joined Sharp Laboratories of America in 2000 where he headed the Color Imaging Group. Dr Kriss retired in 2004 but is still active as a consultant, Adjunct Professor at Portland State University, IS&T activities, and as the Editor in Chief of the Wiley-IS&T Series on Imaging Science and Technology and the forthcoming Handbook of Digital Imaging Technologies.
Created: Wednesday, March 12th, 2014