Part 1
Once the specifications have been decided the optical engineer can begin the design. In this post we will be using the specifications that were provided in the previous post in this series: Defining Lens Specifications.
At this point the optical engineer must decide if the lens will be designed from scratch or if a starting point design will be used. For a simple achromatic doublet, the designer may decide to start from scratch, but when it comes to a high-resolution, wide-angle lens it will be more efficient to start from an existing design and modify it to meet the required specifications.
A helpful method for selecting the design type was first presented by Warren J. Smith in his book “Modern Lens Design”. He introduced a chart that plots each lens design type by their field angle and F/#. The chart below is adapted from Smith’s chart.
In this adaptation we have added a color code to identify camera lenses, microscopes and telescopes. Using this chart anyone with the lens specifications can decide on potential options for the lens design type that will be needed.
For our example lens, we are looking for a lens that can accept a 55° half-field of view and work at an F-number of F/4. When we use these specs in the chart we land at the middle-right of the Inverse Telephoto type or the bottom-left portion of the Fisheye type.
This is great because we know that we can now look for an inverse telephoto or fisheye lens design for a starting point. But wait, where do we find these starting point designs?
Fortunately, optical designers have cataloged lens designs in various places over the years. Below are a few resources for finding a lens design starting point:
Modern Lens Design by Warren J. Smith has many different lens designs with specifications available.
A System of Optical Design by Arthur Cox has a large collection of optical designs. Unfortunately, this book is out of print and hard to find.
US Patents
Zemax samples catalog
The optical engineer’s private catalog of lens designs.
This website has many lens designs in OSLO and Zemax formats available for download.
For this design we will use US08416512-1.zmx as our starting point from the lens-designs website listed above (I have replicated this file with all the needed catalogs as a Zemax Archive File (ZAR) for convenience as well.) Below are the specifications for the design:
Aperture: | F/1.5 |
Focal Length: | 24.67mm |
Object Distance: | Infinity-117mm |
Object Angle: | 82° FFOV |
Image Diameter: | 42mm |
This starting point is obviously quite different than the our 110° F/4 lens, but it was chosen because it is close to a wide-angle lens and it is a very fast lens (F/1.5). This should allow it to be modified to accommodate a wider FOV while working at a slower F/#.
After an initial look at the lens it doesn’t seem very good:
The max RMS spot radius gets up to 32 um and the MTF is nowhere near our desired target of 20% at 145 cyc/mm. We don't need the lens to work at F/1.5 though.
What does the lens look like at F/4?
This is better! The RMS spot radius is about 9 um and the MTF looks much better. It will take some work, but this starting point is worth moving forward with.
To be continued...
In part 2 of this post we will show how to use Zemax to scale the lens to the proper focal length and make it ready for optimization.