Lens focal lengths – an illustration

If you’re new to machine vision, unless you’re a keen photographer all the talk of focal length will be a little baffling. “Experts” will tell you that the focal length relates to the field of view, but if depending on how you like to learn, that might not help. Speaking for myself, I’m a kind of “let me do it and I’ll understand” learner, so I had to work with a range of lenses to figure out what it all meant.

I was fortunate in that I had lots of hardware to work with, and some free time. You might not be in that position, so I thought I could at least show you what changing focal length does to an image.

These pictures were all acquired under fluorescent light and at a working distance of about 350mm. They start with a 12mm lens and work up through 16, 25 and 35 to 50mm. The camera was a regular VGA resolution monochrome item with a 1/3” sensor.

You’ll notice that the last image is slightly out of focus. That’s not because I was in too much of a hurry to do it properly but because the 50mm wouldn’t focus at that working distance. In a later post we’ll talk about using extension rings and tubes to bring this image into focus.