I understand that chromatic aberrations occur because the different wavelengths of light are separated, but I don't exactly understand what conditions control it. I understand the photography aspect better than the physics of this subject now. I know that CAs are most common at the edges of photos, especially when the lens has been stopped down all the way (the aperture is completely open). Can any explain/point me to the reasoning behind this? Why should aperture affect this? Are there any equations which exist to explain it? And why at the edges only? Thank you.