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

Started 2 months ago | Discussions thread
OP MrBrightSide Contributing Member • Posts: 748
Science To The Rescue Again

You are a lifesaver. I shoot in problematic light quite a bit and the hard part for me is  figuring out whether I'm making intelligent tradeoffs in shutter/aperture/light temp/ISO/gear. Being able to shoot a ColorChecker and compare the deviation between squares will go a long way to answering the questions I run into.

Then the question turns back to how many standard deviations is significant? Seems like we humans use 1 as the yardstick and that will do until experience proves otherwise.

knickerhawk wrote:

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?

There is no direct, objective/statistical measurement of noise in PS. However, there is an indirect objective measurement that provides some useful insight into noise. You can use the standard deviation measurement that appears in the histogram tool to compare uniform patches that don't have any detail. A good example is the gray patches in the GretagMacbeth chart in the DPR studio scene. Since we know that those patches are highly uniform with no intended detail, deviations in a spatial measurement of the patches is highly indicative of noise. Thus, a higher standard deviation score for the same patch under same exposure conditions is objectively measurable evidence of the difference in the amount of noise. For example:

The Std Dev for the selected region of the ISO 400 shot is 4.42 here. Note the steep relatively "thin" and lined up peaks in the histogram itself.

With the selection moved down to the ISO 3200 patch, the Std Dev score is significantly higher (12.67) and the shapes of the humps in the histograms are also spread out. Thus, we have an objective score and an objective visual representation of the difference in noise between the two patches.

While the Standard Deviation measurement in PS is useful, it should be used with caution, especially when comparing areas that are not known to be identical. Also, other image factors (such as detail and color) can be confounding, and even minor and difficult to detect differences in processing can significantly affect the usefulness of the standard deviation score as an objective noise measurement.

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.

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