f-number equivalent between m43 and FF

Started Mar 25, 2014 | Discussions thread
crames Regular Member • Posts: 192
Re: Noise is good?

Jack Hogan wrote:

crames wrote:

Jack Hogan wrote: ...Snipping for clarity...

If we wanted instead to do the opposite and upsample the image of the ROI with the smaller number of pixels to the same number as that of with the larger number we would have to add noiseto the upsampled image in order to provide the dithering necessary to allow us to have comparable images. Simply magnifying it makes it blocky as shown and doesn't do it. If noise is added in order to dither out the blockiness the image with the fewer collected photons will appear noisier, theoretically in the same SNR proportion as in the paragraph above

Jack, I upsampled the image with the ideal sinc interpolation filter. No blockiness has been introduced by the upsample, at all.

Upsampled noise looks noisier, not due to any change in the standard deviation but due to the way magnification shifts the visible frequencies in the noise to more sensitive regions of the eye's contrast sensitivity function.

Your suggestion to add more noise to dither out blockiness (caused by, what, nearest neighbor upsampling?) would be part of an very poor workflow and is a complete red herring. If you don't purposely add more noise to an upsampled image what do you get?

Well, if you upsample it all the way to one single pixel (independently of upsampling algorithm chosen) you ideally get a pixel with a value equal to the average of all the others. If you then display this single pixel at the same size as your longer fl image above (the larger one), the SNR you will perceive will be entirely due to the setup and physical characteristics of your display medium, having severed any ties to the SNR of The Signal. No?

Downsample it all the way to one single pixel, yes.

I am not suggesting that this be part of a workflow (most images are purposedly not enlarged to the point that a displayed pixel is larger than human CoC when projected on the retina, otherwise posterization becomes visible and objectionable). I am simply saying that to compare a visibly posterized image to one that is not is unfair to one and not apples-to-apples. In that case adding noise is one way to level the playing field.

GB's argument has to do with photon counts and photon and read noise. These should all be measurable. Visibility of noise is a completely different can of worms that IMO is best kept out of the discussion or made into a separate one.

Post (hide subjects) Posted by
(unknown member)
(unknown member)
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark MMy threads
Color scheme? Blue / Yellow