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Hopefully someone can shed some simple light on a question I've have had for a while regarding image size in Photoshop CC!

When you create a document at let's say for example 91mm x 61mm

Why is it that when you've created your document, that the image dimensions in mm are never exactly what you created initially (as shown below in the screenshots.) Is it to do with the resolution not dividing exactly or something?

Thanks :)

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Photoshop is a raster image editor. It works exclusively with pixels, and only whole pixels, no half-pixels. The size and resolution you are creating your document at results in a pixel size made up of non-whole numbers, i.e. you would need fractions of pixels to create the image, which isn't possible. So Photoshop rounds your physical dimensions to the nearest size made up of whole pixels.

The calculation is a bit awkward since your dimensions are in mm and your resultion is set in inches, so convert 300ppi (pixels-per-inch) to ppcm (pixels-per-centimetre) and you get 118.11ppcm, which results in a non-whole number of pixels:

91m x 61mm at 118.11ppcm = 1,074.801 x 720.471 pixels

Photoshop rounds these values to:

1075 x 720 pixels which at 118.11ppcm is 91.02mm x 60.96mm

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I believe this is due to how Photoshop handles calculated scaling.

If you multiply your two sizes, you end up with 5551mm². I believe this is a size Photoshop can't really display in pixels. What I mean with this is, Photoshop can only assume how big that would be in pixels. If you're zooming to a picture for example, your picture might look blurred if you zoom at the factor 1.5x .

Photoshop has to guess using calculation what pixel belongs where when scaled out. I would call this the "half pixel problem", because this is what this is all about pretty much. You can't display half a pixel. You simply can't.

tl;dr What Photoshop does here, is simple. It looks for the next size that it can diplay without problems.

At the same time, it has to use a size you understand. So the trade off here is, Photoshop might round up or round down your numbers a bit, but as a reward you get a guarantee Photoshop can properly handle that size. Because this is at document creation level, Photoshop can't just assume like it does when you're zooming. It has to write something static to your disc so your OS doesn't get confused.

Different story with vectors, those are entirely calculated.

  • Your explanation seems to be a bit complicated. Have a look at the answer by Cai. – Michael Schumacher Jul 26 '16 at 15:30

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