scale a photo

[TCTOOLSDESIGN]

not really anything to do with the software, but I'm hoping some good photographers could help me.
I'm running some tests on some scale bars and targets I've printed.
So I've taken a photo directly down looking at the scale bars and target. I have a circular one in the centre and length ones either side.
I've best fit a circle to the 50mm ring. It measure 29. Something. Ive scaled it up, then best fitted a circle to the 85 ring and it was perfect, within like 0.1mm.
The side ones don't work so well. I'm best fitting circles to the black dots and checking the measurements between them. Now the side ones may appear on an angle slightly, when looking at the middle target and I think this maybe doing it.
????Any pointers would be greatly received.
the length between the black dots is more like 58 59mm not 55.
Hope this makes sense.
I'm using one photo to test this.

[TCTOOLSDESIGN]

[cam3d]

Hey TCTOOLSDESIGN - What exactly are you looking to achieve? When you print your targets, there will typically be some inconsistencies as part of the print process - I'd recommend printing the scale bars then measuring/marking distances so you eliminate the print inconsistency variable.

[TCTOOLSDESIGN]

Hi Thanks for the reply.
Say you wanted to digitise a gasket. Fairly simple shape, in most cases. Quick easy way would be take a picture of it. Scale it up and "trace" it into CAD.
I checked the printing with a Microscribe Digitiser and other checks as well, so I know the printing isn't too bad. Not with the errors I'm seeing here.
I was reading about lens distortion which I'm wondering could be my issue. As the targets I'm having trouble with are near the edge of the image.

[cam3d]

I've created orthophotos in the past for manufacturing reference - This is an effective way to minimize distortion and ensure all lines are nice and straight - https://www.3dflow.net/technology/do...on-3df-zephyr/ - Could be of interest to you.