Lens Tutorial



This note gives a tutorial on lenses and gives some common lens formulas.

I attempted to make it between an FAQ (just simple facts) and a textbook.

I generally give the starting point of an idea, and then skip to the results, leaving out all the algebra.

If any part of it is too detailed, just skip ahead to the result and go on.

The theory is simplified to that for lenses with the same medium (e.g air) front and rear: the theory for underwater or oil immersion lenses is a bit more complicated.



Part a - Object distance, image distance, and magnification

Throughout this article we use the word "object" to mean the thing of which an image is being made.

It is loosely equivalent to the word "subject" as used by photographers.



In lens formulas it is convenient to measure distances from a set of points called "principal points".

There are two of them, one for the front of the lens and one for the rear, more properly called the primary principal point and the secondary principal point.

While most lens formulas expect the object distance to be measured from the front principal point, most focusing scales are calibrated to read the distance from the object to the film plane.

So you can't use the distance on your focusing scale in most calculations, unless you only need an approximate distance.





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