Selecting a lens

If you followed the instructions in my last post (Image processing at the speed of light ) diligently and studied the “Lens elements” tutorial on the web site of Axis Communications you’ll now understand how to specify a lens. You’ll understand for example, why a fixed focal length 25mm C-mount with manual iris control is a good general purpose lens to keep in your toolbox.

So here’s my question: why spend $300 or more when a lens of the same spec can be had for $90 or less?

Well the simple answer is that the more expensive lens will provide a better image. In this context, “better” means one with higher resolution, or in other words, better able to “see” fine detail.

There are two ways to judge how good a lens is: refer to the MTF graphs, or look at the price.

The MTF (modulation transfer function) graphs show how much detail a lens can transmit, at a given aperture setting, for a required level of contrast. Okay, that’s my simplistic explanation. To get a much better explanation, I suggest you read, “Do Sensors “Outresolve” Lenses?” by Ruben Osuna and Efrain Garcia and published on “The Luminous Landscape.” This paper does a terrific job of explaining how to interpret an MTF graph, and will also take you on a journey of discovery relating to resolution, diffraction, and related optical topics.

Unfortunately though, being able to read an MTF graph doesn’t do you a lot of good, for the simple reason that few lens vendors put the info on their web sites. A notable exception is Linos who provide detailed information (in German) about every lens. (The image at the top of this post was lifted from the Linos web site, www.linos.com .)

So the other option is price. When it comes to lenses, you do get what you pay for. A better lens needs superior components, more of them, assembled to higher tolerances, and that costs money. The manufacturers know that most folks don’t know the difference been a lens of top quality and one that’s more average, so they are forced to be as competitive as they can. So spend more money and get a better lens. (Of course, it’s not a linear relationship, so if you really need the sharpest image possible be prepared to spend a lot of money.)