How Do I Measure Noise Using Photoshop? And What Is A Significant Change?

Started 2 months ago | Discussions thread
57even Forum Pro • Posts: 14,106
Re: How Do I Measure Noise Using Photoshop? And What Is A Significant Change?

MrBrightSide wrote:

Question 1: Long ago I remember reading that you could use the statistics tools in Photoshop to measure the noise in a photograph. There must be something in the program because it has noise reduction so it must be able to identify noise. How does it work?

Failing that, what other simple methods are there to measure noise in digital photos.

Question 2: Once the noise is measure what constitutes a significant difference in the noise level? Does it fall at 1 percent noisier? 10 percent? 100 percent?

Question 3: There is no question 3. This is a reminder that no one has the patience for your thoughts about subjectivity and how everyone has a different pain threshold when it comes to noise.

The human JND for differences in luminance at the level we might typically associate with the mid-tones on a display is about 0.8%.

As we zoom out, the noise level drops because we are no longer seeing single pixels but combinations of pixels with lower average deviation.

Try this - create your own grey patch in PS, then progressively add gaussian noise until you can just see it. Then downsize the image by 50% which will reduce it to half.

A noise level of 0.8% in terms of SNR is about 42dB. Reducing it by half would be 45dB.

A third stop increase in noise, or -1dB, would be noticeable, but 'significant' will depend on your tolerance of course. One stop, or -3dB would be roughly equivalent to doubling the ISO level.

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"A designer knows he has achieved perfection not when there is nothing left to add, but when there is nothing left to take away." Antoine de Saint-Exupery

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