Using PSNR to measure the quality of a degraded image against the

Hi All,

I was hoping someone would be able to explain why something I have
noticed during some image processing analysis occurs.

When I am working with extremely degraded images, and am trying
different techniques to improve them, I often measure the PSNR (Peak
Signal-to-Noise Ratio) of the degraded image against the original,
undamaged image.  Often, I find that the techniques that give the best
(highest) PSNR dB value are NOT the ones that give the best subjective
quality i.e. I chose restoration techniques that give a lower PSNR
because they LOOK better.

Can anyone explain why this happens?  Maybe it is something to do with
the fact that the image quality of the degraded image is so poor to
start with, that using PSNR is not that helpful?

Kind regards,

AJ