Problem with increasing "Canvas Size"

[Irish-Valk]

I'm having an issue when it comes to resizing the canvas for my images. I have been using this function in my work flow to add borders to my images as follows. 
Using Image>Canvas Size with the 'Relative' option selected I usually set the first border to .5cm (white) then I set a second border to .01cm (black) and then set a third border to 5 or 6cm (white). This has always worked perfectly for me up to the latest update.
Since the latest update Photoshop does not apply the increase evenly on each side of the image. It applies more of the increase to the Bottom and RHS than it does to the Top and LHS.  In my example above the very thin black border I tried to apply doesn't appear on the Top or LHS at all, it has all been applied to the Bottom and RHS. On closer exemination all of the borders (canvas size increases) seem to be applied more in favour of the Bottom and RHS.
On experimentation I have found that by changing my 'Rulers" and "Grid Lines" to measure Pixels instead of metric centimeters/millimeters, when I apply my borders using the same work flow with "Image>Canvas Size" as before but using Pixels instead of centimeters/millimeters in the "Canvas Size" window it seems to work perfectly.
I am using a high spec Windows 11 PC and currently running Photoshop Ver: 25.5.1 since 2024/03/02

[Semaphoric]

What is the resolution (in PPI) of your document?

[Irish-Valk]

2680 x 3424px (270ppi)

[Semaphoric · Accepted solution]

I think the trouble is Photoshop having to convert from physical units (Inches, metric, points) to pixels. In the Canvas Size dialog, if you enter your size in centimeters, you can change the measurement to pixels, and it will show how many pixels that would be. Since you want the same size on all sides, it has to be an even number, because there are no fractional pixels.

If the value is odd, you would have to change it, and decide to go up or down. If you do this often, it could be scripted.

[Irish-Valk]

Semaphoric, you are spot on. The basic unit that Photoshop works with is pixels and pixels do not translate evenly into imperial or metric measurements. If a user defines his/her measurements in imperial/metric then photoshop has to translate that measurement into pixels. Since pixels are a fixes size and cannot be split then the translation from imperial/metric has to be rounded off to the nearest pixel, hence the inaccuracy. 
Since figuring this out I now always set my rulers and gridlines to Pixels and increase my canvas size by even numbers to insure eveness of canvas size on all sides. 

[Irish-Valk]

Thanks for the reply. Yes I had considered your reasoning and it makes abolute sense. As I said, when i use pixels it does work correctly. 

My problem is it has always worked using centimeters as well and I've checked back on my images and the borderd appear to be accurate. This is not the case since the last update..

[Lumigraphics]

Photoshop works in pixels, not centimeters. So this isn't surprising.

[Irish-Valk]

Yes I understand that.

However the Canvas Size dialogue offers several options as in pixels, centimeters, inches, points, among others. The 'centimeter' option has been working pretty accurately up to the recent update. If they cant get it to work accurately why have the option there in the first place... I appreciate the most accurate option will always be pixels and pixels cannot be broken into parts and they dont fit neatly within centimeters or inches.. 

[Semaphoric]

I don't know what your previous settings were, so I doin't know why it was working the way you want. As far as I know, it has always worked like this.

Using physical units certainly has its place, especially for print, but the rounding errors are much more apparent when using small values.

[Lumigraphics]

If your image resolution changes, sizing will change. 1cm @ 72ppi is not 1cm @ 300ppi.

[ChrisPonthier]

I am having the same issue with canvas size.  If I do not crop the photo canvas size works correctly prividing a white border.  If I crop the photo canvas size produces a large canvas that is clear, and the image size gets altered.  Have not had this problem before the new release.  BTW in most instances the option to convert inches to pixels is greyed out.