This is the second post in the series "Lens Design Process with Zemax". To view the first post click here.
The lens design process starts with defining a robust set of specifications that are agreed upon by the optical designer and the customer.
Often the initial lens specification does not result in the optimum optical design. Below are a couple of reasons for this.
First, the initial spec is written to include not only the customer’s “needs” but also their “wants”. It is sometimes important to forgo some of the “wants” to design a lens that best meets the “needs” at the most affordable price.
Second, sometimes a specification can contain contradictory information. We review all lens specs we receive and can quickly identify any mistakes or contradictions that may be present.
As an example, a customer may specify a 110° wide-angle lens for a 1” image sensor with a focal length of 5.6mm and distortion < 20%. These specifications make sense when using the paraxial equation to calculate focal length from the object angle and image height (See equation and table below).
y' (mm) | θ (°) | f (mm) |
8.0 | 55 | 5.6 |
But this equation is for a paraxial lens not a real wide-angle lens with distortion. When we take into account the allowable distortion of 20% the spec no longer makes sense. We have found that when defining a lens specification it is best to only define 3 of the following: object angle, image height, focal length and distortion. Once 3 of these are defined, the 4th is also defined.
In this case the customer is most interested in making sure that the object angle of ±55° is mapped to the correct image size and that the distortion is < 20%. This means that we will allow the focal length of the lens to vary as long as the other 3 parameters are held to spec.
I have added an attachment to this post of a filled out specification sheet for this lens. This spec will be used in the following posts to illustrate the design process.
Stay tuned! In our next post on this topic we will show how to select a good starting point for this specification. Teaser:
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