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.