Eye dropper not reading grey scale correctly

[nmoran]

I have an image of a series of stepped blocks of grey tones going from fully black (100%) to white (0%). This is a .psd file that I have opened in Affinity. I decided I would measure the tones for accuracy before printing . Using the eye dropper tool and the greyscale colour model in the info window I started with the 0% tone (white). It reads as 0% in the info window so it is white.I measuered the 5% grey tone it reads as 5% ,so it is correct.  I measure the 10% block of grey tone it measures as 8% ,20% measures as 17% ,30% grey is 25% ,40% block as 33% and so on and so forth up to the 100% grey tone (black).I have attached the image/file I am measuring.I would love to know what the issue is here. I have incidentally opened this image some years ago in photoshop and all grey tones measured correctly.Any help would be appreciated.Nick

half letter 3-step tab test.psd

Edited January 21, 2023 by nmoran

[R C-R]

If I add grey quick shapes like rectangles or ellipses to the document set to 10% increments, they do not match the greys in your file. I assume this has something to do with opening the PDF document in Affinity, but I am not sure how.

Anyway, the greys shapes I added do show the correct 10% increment values in the info panel.

[lacerto]

It is related to the original file probably being viewed as a grayscale image using Dot Gain 20% profile (which might have been the default grayscale profile in your Adobe environment). Affinity apps do not come with equivalent grayscale profiles but can use them, though. I attached below a conversion of the original RGB file to grayscale/8 TIFF using the Dot Gain 20% target but saved without a profile. When you open this in Affinity Photo, it will by default assign it with the D50 profile and view it as a neutral Gamma 2.2 image, and you should get readings that in K scale show the expected values (give or take the rounding errors).

half letter 3-step tab test_dotgain20.tiff

Note though that if you open the original 16-bit RGB file in Photoshop and use the K-scale to read the values, you will get similar values as in Photo (ones you mention in your post) also in Photoshop. The file may have been inadvertently converted at some point, explaining the differences.

[nmoran]

Thank you very much both for the imput. I have opened lacerto's converted version.Most of the measurments match but as you pointed out there does seem to be some kind of rounding discrepancy here and there. The eyedropper measures 50% in some areas of the midtone greytone but 51% in other areas.This won't prevent me from proceeding as it negligable. Colour managment etc is something I would like to know more about though. Thanks again

Edited January 22, 2023 by nmoran

[lacerto]

8 hours ago, nmoran said:

The eyedropper measures 50% in some areas of the midtone greytone but 51% in other areas.

I think these discrepancies are basically results of Affinity apps handling colors (including grayscale) based on HSL, so if you change the color model of Color panel to HSL and use the Color picker tool (even with "Point 1x1" ratio) to sample intensity values, you would get exact readings (100, 95, 90, 80, etc., but  "reversed" since they are light values rather than pigment values), while the K value of the Info panel would still show something like 51% or 91% (that is, inaccurate conversion values). NOTE: I am referring to the converted TIFF file that I attached in my previous post.