Canvas Calibration - Invalid Sizing?

[marshall_x]

Wondering if there is some sort of guideline (or trick) one needs to adhere to when Photographing objects?
Distance object should be away from camera, perhaps? Preferred resolution of the photo? or anything really to ensure results are correct.

While trying to use the calibration function for a canvas, I'm getting invalid results.

I took a photo of an object on a cutting mat of my wife's which it has a grid on it. According to the calipers the distance between those lines is 25.4mm (1in.) When I use the calibration function, I place a dot each line 25.4mm apart and set the distance in the text box to 25.4mm .. However the object in the photo ends up being larger than what it is in-hand. 

One of the measurement of the object in-hand is 61.7mm. When I use the measuring tool in Fusion to measure the same distance between the same points, I get 74mm..

I'm not sure what might have happened to return such invalid results..

[Drewpan]

Hi,

My first thought was a parallax error but that is a bit big of an error to be so. When I photograph and import, I have

a couple of engineering rulers I put onto the surface and I photograph directly over the top of the view I am trying

to achieve. The ones I use are the smaller 6"/150mm metal types that I flip to get the measurement I want. The only

hard and fast rule I use is to get the ruler measurements as big as possible in the photo so I can get my callibration

as accurate as possible. I also don't peg to a small measurement, I will calibrate the greatest distance I can so that

any errors are spread over a distance and minimised. Most resolution settings are fine but as usual, the bigger you

can make an un-distorted picture then the more accurate the calibration.

While it is possible to get some very accurate calibrated measurements, using the calibration tool on a canvas is only

really to get a canvas to work off. It is not an exact point by point guide, although it can be if you are careful enough.

Cheers

Andrew