At first glance selecting a lens for a camera system may seem straightforward, but it can be a deceptively difficult task. We often find that customers have already chosen a sensor and they want to find the most appropriate lens for their application. They will have a field of view requirement and the field of view will determine the appropriate range of effective focal lengths (EFL) for a given sensor. However, once you have an EFL where do you go from there?
Unfortunately, there is no straightforward answer. You’ll need to specify an f-number (f/#) for the lens, but keep in mind there is a direct trade-off between the camera depth of field and the image brightness. Are you willing to include a focus mechanism in your camera? Adding one will allow you to get both a large depth of field and a high brightness image.
Now you have selected an f/#, depth of field, FOV (and maybe EFL), but what about image quality? If you’re working with a lens designer you’ll need to specify a resolution metric. The value used most often is Modulation Transfer Function (MTF), and your MTF specification is going to be application dependent and require information about the sensor pixel size, the scene wavelength spectrum, the application, and what objects you’re viewing. Probably the most common mistake we see is over-specifying the MTF requirements of a lens.
And that’s not all. There are many other specifications for image quality that should be considered, such as:
- How much field curvature can the application tolerate?
- What about distortion?
- Lens shading or relative illumination?
- Have you thought about stray light and veiling glare?
- What mounting features are required?
- What are the environmental requirements?
Many of these parameters are dependent on the lens design or opto-mechanical design, and clear communication with your lens designer or vendor will help you narrow in on the appropriate lens for your application.
In future blog posts we’ll go into further detail on each of these specifications. And of course we’ll talk about testing!
Written by David Biss Ph.D., Optikos Corporation