The Science of Lenses – Part Two

Tim Parkin

Amateur Photographer who plays with big cameras and film when in between digital photographs.

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Those Dotted Lines (Astigmatism)

OK, we’re back to the dotted lines and these need a little more explanation as they’re quite complicated. I want to show you in a diagram what is happening as a verbal explanation is quite difficult. What I want to show is that for a single point source of light, there are two possible planes in which it can be in focus. Very odd and it took me a while to understand myself but with the aid of this diagram I hope I can help.

Here’s the first diagram showing our point of light and how we are going to separate it into two sections, one vertical and one horizontal.

We’ll take a look at the shape of the cross section of the lens for the vertical section first (looking in the direction of the red arrow). This shows the point source of light offset but the shape of the slive of the lens is normal.

Because of something called ‘Petzval Field Curvature’ [see Wikipedia], a simple lens focuses off axis points of light closer than on axis points of light.  You can see this on the right hand side of the section above. Commercial lens manufacturers try to design this field curvature out, to limited success and various side effects - more later.

If we look at the horizontal section, viewing from the green line above, we see the following..