isolate part of a photo and combine it with other photos, so that each photo piece is the true size?

[cybilc52659059]

How do you isolate part of a photo and combine it with other photos, so that each photo piece is the true size?

I have photos of mandibles with a ruler in the photo for scale. I would like to isolate a specific tooth from each photo and combine all the teeth into a single photo; so I can visually compare them.  Each tooth needs to be the correct size (the size it was in real life). The photos were taken at different distances with different cameras (resolution).

I have creative cloud. I know there are size issues if you copy and paste photos of different resolutions into new files. This seems like a problem that the technology exists to solve, but I do not know where to start looking for the correct method.

Can anyone please offer guidance?

[Trevor.Dennis · Accepted solution]

Open the master document, and make a selection of the ruler.  Copy to a new layer (Ctrl j or Cmd j)

Open the other images, and drag the copied ruler layer to each one in turn.   There several ways to do this, but the easiest is drag the layer to the other images tab which will open that doument, and then drag into the middle of the other document.

Position the ruler from the master to align with the ruler in the other document, and free Transform the background layer so the two rulers match.  You'll need to click on the padlock icon on the background layer to make it an ordinary layer before you can Free Transform it.

The other document now matches the scale of the master document, and you can select and copy the relevant object back to the master document