Center on canvas is not exact

[kathleen27066899kazr]

Here's a simple question. 

1) I have an image

2) I want to center and image that is 5" width on my 8" x 11" page-canvas - whatever you want to call the layout

3) I used the "center align tool to center the photo image on the page

4) I have a MM ruler. The image is off center by 2 mm. Left side is 68mm / Right side is 66mm

5) This is a HUGE deal when I am centering a photograph to be printed for a gallery showing. Nothing can be off. It has to be precise. So is it not possible for PShop to center exactly. EXACTLY? 

[Subject edited for clarity. Was "P: Auto Align Layers slightly misalinged]

[jane-e]

2) I want to center and image that is 5" width on my 8" x 11" page-canvas

4) I have a MM ruler.

By @kathleen27066899kazr

Photoshop works in pixels, not inches or millimeters. For testing purposes, can you change your units to pixels and tell us if that works when your center your image?

I branched your post from a primary thread about aligning multiple layers to each other.

https://helpx.adobe.com/photoshop/using/aligning-layers.html

Jane

[Bojan Živković]

Can you describe the steps to center? Are you selecting all before centering? Problems can arise if you have a width or height in pixels that is not divisible by 2, such as 1501, otherwise, everything works fine. Remember to select all and choose the layer with content before centering.

[Amiable_Wave5EC8]

To center your image precisely in Photoshop, use the following steps: select the image layer, go to Edit > Free Transform (Ctrl + T), and manually adjust the image position by entering exact values in the options bar. Alternatively, use the Align tools in the top menu while holding Shift to ensure it's perfectly centered. If you still experience discrepancies, check for any hidden guides or snap settings that may affect placement.

[Trevor.Dennis]

I suspect that the image is centred, but the layer has content that matches the background so you can't see it.  Or something like that.

Drag a guide out to the centre of the image — it should snap into place when you get it close.

Select the layer your are trying to centre, and hit Free Transform (Cmd T)

If the centre handle intersects the guide, then it is centred