Scale bars for photos

I was just wondering if it is possible to get an idea as to how far an object is and how big it is from a photograph (using EXIF or other camera details), without having to put a ruler of known height at the same distance and shooting again. Something like a scale bar on micrographs. eg. How high did the humpback whale jump? how long is it?

If it is possible pl send the link/name of the tool or the formula to calculate it.

thanks

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I'm sure the formula would be pretty complicated. But anyway, here's a simplified version:

ObjectRealSize = ( FocusDistance x ObjectSizeInPixels x PixelPitch ) / FocalLenght

PixelPitch is a constant and for the 350D is 0.0064 mm.

The bad news is that you wont be able to use it, because the 350D doesn't report focus distance information. Canon ceased to provide this info due to ?inaccuracy reasons? on the models following the 10D and 300D.