Hi, all.
I have a question regarding focus distance and depth of field, shooting through a dome port. I'm using an Ikelite 6" dome, but my question is a general one.
I understand that the curved dome-water interface acts as an optical element, altering how the lens sees the scene. In particular, it makes things appear "closer" to the camera. In the case of my setup, this "virtual image" is somewhere near 12". No problems there.
My question: does the lens always think it's shooting at 12" regardless of the actual distance to the subject? Or is it some ratio? I'm trying to compute DOF and need some pointers.
Thanks,
Dan
I presume you want to do it from first principles for any lens, without getting wet. If you have the time and energy, here's how:
First you need to calculate the position of the virtual object that corresponds to the real object. You can do that with the algorithm given here:
http://www.scubageek.com/articles/wwwdome.html
[Note: If the real object is at infinity, the virtual object will be approximately at the distance 3R-4t from the front of the dome, where R=dome radius and t=dome thickness. See http://www.scubageek.com/articles/wwwprim.html ]
Next, you have to determine the near and far limits of focus of your primary lens when focused on the virtual object. Then, map these limits from virtual space back into "real" space using the same algorithm as above. The distance between those positions is the depth of field in real space.
It's tricky. Given a choice, I'd rather do in-water testing.
Les
