Tuesday, April 19, 2011

Why would you need an extension tube?

Last time on this blog we covered how and when to use an extension ring, and we wrapped up by mentioning extension tubes. An extension tube is just a really thick ring: they start at 5mm (which can be a problem when the thickest ring is 2mm, thus leaving a 3mm gap,) and they go up in size through 10mm and 20mm to 40mm, and presumably beyond.

Now the question is, why would you need these?

Well remember how we discussed using a longer focal length lens to view a smaller field of view? Well suppose that you want to look at a really small area. This is something photographers call “macro photography,” which refers to an object to image magnification of 1:1. In other words, the field of view is the same size as the sensor.

To grasp this concept, imagine that you’re using a 2/3” format sensor. This measures 6.6 x 8.8mm, so a magnification of 1:1 means your field of view will be the same size. That’s pretty small, so you should be wondering what lens to use.
The answer is, a 50mm with 30mm of extension. That’s what’s shown in the picture to the left. The upper image shows the 50mm lens with 2mm extension that brings it into focus at a working distance of 350mm. The lower image is the same 50mm, plus the extension tube, and at a working distance of 150mm.
One interesting point to note is that to get roughly the same brightness in both images, the aperture had to be opened up from f8 to f4. What this tells us is that extension tubes dramatically cut the amount of light reaching the sensor.

So there you have it. A set of extension rings and tubes really needs to be in your equipment drawer. They’ll let you get in tight on small fields of view while keeping the image in focus, and they cost far less than fancy microscope lenses.

1 comment:

Spencer Luster said...

Another good piece of advice from this blog! Glad to see it.

Nevertheless, I do have two things to add.

1) Most lenses are designed for optimum performance over some stated range of magnification. When using long extension tubes, it can be easy to move outside this range and have image quality fall off in some way. (Excess distortion, vignetting, loss of sharpness at the edges of the FOV come to mind) Our experience is that many machine vision applications suffer no excessive image quality loss. Still, beware of such possibilities for critical applications.

2) You may not have meant to write it this way, but suggesting that a 30mm extension with a 50mm fl. (presumed c-mount)lens produces 1:1 magnification is incorrect. One feature of the c-mount system is that 1:1 mag. is achieved by adding extensions that are equal to the lens focal length. Thus, a 50mm fl. lens needs 50mm extension to produce a nominal mag. of 1:1. (That is, assuming the focus ring is set to infinity. It may be that your lens had enough focus ring adjustment available so that with 30mm worth of extension you could make up for the rest with the focus ring. In general, this may not be available. Or perhaps you didn't start with a c-mount lens.)

For a detailed but readable article on using c-mount lenses for different mags, please see our article "Isaac Newton, C-mount Lenses and You". Here's a link to our page that allows you to download the PDF. http://lw4u.com/index.php/machine-vision-help/tech-tips/