Telecentric Lens Size Control
This is Section 5.3 of the Imaging Resource Guide
The field of view of any Telecentric Lens is limited by the front optic diameter; the larger the required field of view, the larger the front optic diameter. Telecentric Lenses can grow quite large and heavy with small magnifications, as such magnifications require large front optics; this can be prohibitive in some setups where weight or size is a concern. The f/# of a lens also plays into the size of the lens, particularly at large magnifications where the front optics also grow with the speed of the lens. Figure 1 shows two different 4X telecentric lenses of different f/#s. The lens at the top of Figure 1 shows a system that runs at f/6, and the lens at the bottom runs at a much slower f/16.
Figure 1: Two 4X Telecentric Lenses, operating at f/6 (top) and f/16 (bottom), showing how f/# can partially dictate Element Size for Telecentric Lenses
If a particularly large field of view is required, Fixed Focal Length Lenses can be augmented with additional optics in order to produce a pseudo-telecentric lens for large fields of view. Figure 2 shows a 25mm Fixed Focal Length Lens that has been augmented with a Fresnel Lens to make a telecentric lens that has an 8” field of view. This type of augmentation always has lower image quality than a specifically designed Telecentric Lens, but can be useful where cost-sensitive, large field of view applications require telecentricity. This approach allows for minimal flexibility with regards to working distance, as well as substantial chromatic aberrations due to the use of a Fresnel Lens.