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11. Reverted Lens
In paragraphs 3, 4, and 6 of this essay I talked about "distortion", "resolution" and "color correction".
I already gave to understand and I stress it right here again: These are "precious" parameters in so far as they can cost lots and lots of money for designing, for manufacturing, and hence, for purchasing an objective lens system.

So, if we really need a lens of high quality, we'd better use it in such a way that these quality parameters are not impaired. That means especially: Always use a lens at the magnification it is intended for.

(If someone offers you a lens and he cannot state the optimum magnification, be careful. Either he doesn't know his product or the product isn't worth while.)

Of course, there is a range of magnifications within which the lens will work sufficiently. You know these photo lenses performing at object distances from 0.6m up to infinity ... and performing well.
But there is a limit. If you get closer than 0.6m to your object, then indeed you might regain optimum definition in your photograph by inserting and adaptor ring between lens and camera (increasing image distance). Taking these things too far, you're going to notice that "optimum" definition is no longer good definition; that distortions occur; that lack of color correction becomes visible.

The experienced photographer now surprises us: he unscrews the lens and screws it revertedly with the filter thread to the camera.
Why that?   --   Take a look at fig. 11!

fig.11: Reverting Rays (13kByte)

A fundamental principle of (conventional) optics is:
"If a ray propagates               from A (via B, C, D) to E,
then a ray can as well propagate   from E (via D, C, B) to A."

Making use of this principle, you step from the top of fig. 11 to the bottom. Gaining and saving all the lens corrections (for distortion, resolution, and color) that you have payed for.

This way, a lens that has been designed for   m = 20   can as well be used for   m = 1/20 = 0,05   and so on.

Remember: you may not just revert the lens. But you are always allowed to
interchange object and image
as long as you
keep the small item at the "small" side of the lens
and the big item at the "big" side.

I hope you enjoyed it and found it useful.

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Last modified Nov.28th, 2002 18:30