determining orginal dimensions

[Shinnen]

Hi All,
This may be asking the impossible, but here goes.
As some of you probably know I have a friend who takes a lot of pictures of artworks, which she sends to me, mainly for straightening. (Usually the perspective has to be corrected.) Recently, after 'fixing' one, I noticed that the moon was elliptical. To me, this meant that I didn't have the height to width ratio right...... so I corrected it by increasing the height until the moon was once again round. This got me to wondering.... how often am I getting the H/W ratio wrong. Now here's the impossible question. I know what the original dimensions of the picture are; but is there a way of knowing what the dimensions of an artwork (usually rectangular) are, in the picture. Often I can make a good guess, by making the subject 'look right', but not always. Any ideas would be appreciated.
Thanks,
...... john

[Antonio]

it is difficult, perhaps you can use the plugin Measurements.
With this plugin you can get pixel distance between 2 points. If you calque the distance of a square or round object (width/height) perhaps it is possible to calc the ratio for correction...

[Shinnen]

Thanks Antonio,
I'll give that a try.
..... john