How big would an object look at a certain distance? I've tried looking this up and found that objects only appear smaller because of the curvature of the eye. For example: 

the flowers are the same size, but since the second flower is farther, angle $b$ is smaller. 
I want to know if there is a way to calculate how big the flower would look to a human by its real size and angle. Also how do I find this angle based on the flower's distance and height?
Sorry if this is the wrong forum I didn't know which one to post this question.
 A: The curvature of the eye has nothing to do with this phenomenon. If you doubt, consider that a pinhole camera (with no lens and a flat viewing screen) also forms smaller images for more distant objects, with no curvature involved.
The reason things appear smaller at a distance is because our eyes cannot directly measure length. There isn't a way for your eye to place a yardstick next to objects you see and see how long it is. Even if there was, you'd only be able to see things that were at most as large as your pupil. What your eye can measure, though, is how large an angle is subtended by two parts of an image, which thanks to the geometry of optics, doesn't change significantly from the real world to the image in your retina. So your brain takes that information, along with a reference chart for "normal sizes" of objects and computations of parallax and perspective, and deduces the size and distance of the object.
Because the resulting apparent size is based on a number of assumptions and deductions, it's fairly easy to trick the brain into thinking something is a different size or distance (or even shape!) than it really is. Optical illusions all work by exploiting either the assumptions or processes of perception.
