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Lens Design

Most of the lenses we produce are lenses of our own design. We always begin the design process with an assessment of the optical requirement and look first to see if we already carry a stock lens that will work, or if a modification of an existing lens might work. If not, we then look at creating a new lens design, for which we consider a number of questions.

What type of sensor is the lens imaging onto? Is it a charge-coupled device, a complementary metal-oxide semiconductor, a linear array? What size and what field of view—the number of pixels and the pixel size will both make a difference.

Is the application a infinite conjugate or a fixed magnification? It is possible for a lens to need correcting over a range of magnifications. For instance, a lens optimized for infinity will have a much different design that that of a symmetrical design optimized for 1 : 1.

How much light-gathering ability is required of the lens? The F#, or aperture, is defined as focal length ÷ diameter, and the diameter here refers to the diameter of the entrance pupil. That in turn is defined as the image of the aperture stop. An F/1.2 lens will be very bright but the design may be complex; an F/4.0 lens, on the other hand, will not transmit as much light, but will have a simpler design. In addition, the greater the light-gathering ability of the lens, the greater the difficulty in maintaining optical performance across the image plane, and the larger the lens diameter will become. If the aperture is too small, diffraction will begin to degrade the image.

A narrower spectral range makes it easier to achieve the desired optical performance: a lens with a monochromatic requirement of 660 nanometers is much easier to design than one working from 400 to 1500 nanometers.

Are there space constraints to consider in the design? An eight-element lens design, for instance, offers great performance but requires a barrel length that may be too much for the project. We consider whether the maximum lens diameter conforms to the required F# and may also have to think about the back focal length if there are mechanical constraints in the short conjugate side. (For instance, is there a filter, mirror or mounting plate in between the lens and the image plane?) For those lenses with unusually wide fields of view, the design may call for a large diameter negative meniscus placed well out in the front but again, we need first consider whether the space constraints will allow for this.

Resolving power or modulation transfer function is the standard method for quantifying optical performance. MTF combines resolution (lp/mm) with contrast to define how sharp a lens must be. When all imaging, as in previous decades, was done onto film, contrast was not as important as resolution, as film is a medium capable of high resolution with less need to consider contrast. But with today’s sensors, matching both the MTF and the chief ray angle to the sensor results in the most efficient use of the lens design.

Design overkill means that a customer has paid for performance that isn’t necessary to the job at hand. But an under-designed lens may be failed at its final QC inspection or even worse, fail in the field. Our years of experience, our strict standards of design considerations, and the expertise of our engineers and technicians all combine to ensure that we will always match your design to your requirements, delivering to you the best possible product at the best possible price.