Object area::relative Relationship:: how to FIND OR solve the problem

Hi member:

I try to find area and circularity of different objects by use webcam to make a classification by use neural network

I face a problem and its important which is

when I take an image for object and camera near object by Meter then area will ex.. 100 pixel

but when camera near same object by 0.5 meter then the area will ex 150 pixel

 how can i solve this problem!!!!

how can i determine the exact Relationship in this case if the image size =640*480

best regards

best regards

m.s