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

Started 2 months ago | Discussions thread
57even Forum Pro • Posts: 14,116
Re: How Do I Measure Noise Using Photoshop? And What Is A Significant Change?

tbcass wrote:

57even wrote:

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.

Simply knowing the amount of noise is only half the story. The nature of the noise makes a difference in how effective noise reduction will be on that noise. The more fine grained the noise the easier it is to reduce without smearing detail. I think chroma noise is also more difficult to reduce.

It depends which raw processor you use. This is not a product of the sensor, only the software you use to demosaik it. Adobe, C1 and Silkypix have different default noise levels, and all add sharpening and NR to the data.

But the JND is the same, whatever. So, if the noise level is 0.8%, increasing it by 1dB to 0.9% should be noticeable.

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