calculate object size in a image

Question

I have a camera with 73º diagonal field of view and a 1/4' sensor capable of doing 1280*720

I have a fixed distance from certain object and I want to measure it on the photos taken by the camera...

I thought about trying to get the amount of centimeters by pixel giving a certain distance and the field of view angle... I don't know if this is the right way to do it....

Can someone point me to the right way and / or explain how to accomplish...

thanks a lot

Answer 1

Ok I think I have it figured out for you -

Tan 45 = 1    
// this is important


sqareroot(1280squared + 720squared) = 1468.6    
// this is the diagonal measured in pixels widths


73/2 = 36.5    
// this is half the diagonal view angle.


1468.6/2 = 734.3

Tan 36.5 = 0.73996

734.3 / 0.73996 = 992.3509  
// this is the amount of pixels you would need 
// to see 45 degrees from the center of the frame.

So to measure your object you would need to do -

((pixelDimensionOfObject / 2) / 992.3509) * distance from object = size of object.

Be aware though, that I think in order for this to be correct the object you a photographing and measuring must be in the center of the frame. Otherwise I think it becomes a lot more complicated.

Answer 2

我认为这可以反过来想知道对象的距离，我们知道对象的实际大小