How does reducing optical product size impact the design and performance of Fixed Focus lenses?
Across many industries, the trend in recent years has been all about the manufacturing of ever-smaller mechanical, optical and electronic products and devices. Reducing the size of an optical product requires innovative design and manufacturing techniques but what is the impact on performance?
There are various factors that govern the size of an optical system and for the purpose of this technical note we consider the design of a fixed focus lens. The key factors that affect the size of a fixed focus lens are the sensor it is designed for plus the aperture or f number, focal length and resolution required to meet the requirements of the target application(s). The larger the sensor area the bigger the clear aperture at the front of your lens needs to be to avoid clipping or severe vignetting.
A requirement for a large aperture (f/ number) will also affect the size of the lens. This goes hand in hand with the focal length, for example if you require a 40 mm focal length lens and an f/2 aperture the clear diameter at the front of the lens will be 20 mm diameter.
Fl = Focal length f = Aperture CL = Clear diameter
By comparison, changing optical resolution has more effect on the length of your fixed focus lens as the higher the resolution the more elements that are required for colour correction. In designing a high resolution lens you have to squeeze these elements in somewhere so inevitably the length of the lens must increase.
As an optical designer you can play around with the aperture and distortion to reduce the diameter and length of your fixed focus but this will always necessitate a compromise in performance.
As a rule of thumb, a performance optimised optical design will require the physical lens to be as long as its focal length. So, if you have a requirement for a 50 mm focal length then you should allow for a space of approximately 50 mm for the lens in your product design.
For fixed focus lenses this rule is quite loose as you can reduce the length of a design to a point, However, designing more compact zoom lenses is a much more challenging prospect.
As before a performance optimised zoom lens design will require the lens to be as long as its longest focal length. Shortening the design will always involve compromises with distortion and performance. Such compromises may be acceptable depending on the application. There is a limit as to how short a zoom lens design can go by which time the performance of the lens has dropped significantly.
In the discussion above we have only considered the space required for the lens, and not the other elements required to capture the results of your optical application.
When you have a space limited application – I would always recommend that you talk to Resolve Optics ahead of the selecting your camera or sensor if possible. This will enable us to advise how much room is likely to be required for the lens which will then give you a better idea of what camera or sensor you can fit within the space envelope.
To discuss a lens design for a space-limited application or product please contact markpontin@resolveoptics.com