Perceived contrast of electronically magnified video

It has been observed that electronic magnification of imagery results in a decrease in the apparent contrast of the magnified image relative to the original. The decrease in perceived contrast might be due to a combination of image blur and of sub-sampling the larger range of contrasts in the original image. In a series of experiments, we measured the effect on apparent contrast of magnification in two contexts: either the entire image was enlarged to fill a larger display area, or a portion of an image was enlarged to fill the same display area, both as a function of magnification power and of viewing distance (visibility of blur induced by magnification). We found a significant difference in the apparent contrast of magnified versus unmagnified video sequences. The effect on apparent contrast was found to increase with increasing magnification, and to decrease with increasing viewing distance (or with decreasing angular size). Across observers and conditions the reduction in perceived contrast was reliably in the range of 0.05 to 0.2 log units (89% to 63% of nominal contrast). These effects are generally consistent with expectations based on both the contrast statistics of natural images and the contrast sensitivity of the human visual system. It can be demonstrated that 1) local areas within larger images or videos will usually have lower physical contrast than the whole; and 2) visibility of ‘missing content’ (e.g. blur) in an image is interpreted as a decrease in contrast, and this visibility declines with viewing distance.