This page looks at two methods to increase the magnification available from the Canon 100mm macro lens: close-up filters and extension tubes.

All the images for this article were shot using a full-frame Canon camera and a cross-polarisation technique, with an iPad to act as a polarising light-table.

The test target.

The lens used was a Canon 100mm f2.8L, a high quality macro lens that reaches 1:1 magnification at its closest focus distance. Images were shot of a ruler using various combinations of tubes (Kenko and Canon) and a Raynox DCR 250 close-up filter. The lens was set to its closest focus setting and a Velbon focussing rail used to adjust the focus distance until the subject was sharp.

Focal distances were measured very roughly using a ruler, and are probably only accurate to +/- 0.5cm. Here, the Focal Distance is measured between the target and the focal plane, while the Working Distance is measured between the target and the end of the lens. Magnifications were estimated from the ruler markings.

The results are summarised in the following table – click the images for larger versions:

Magnification	Extension Tubes	Close-up Filter	Focal Distance	Working Distance	Image
1.0x	–	–	29.5cm	17.5cm
1.4x	25mm	–	30cm	15.5cm
1.8x	68mm	–	32.5cm	13.5cm
1.9x	–	DCR 250	23.5cm	11cm
2.5x	25mm	DCR 250	32.5cm	17.5cm
3.2x	68mm	DCR 250	29cm	10cm
3.8x	93mm	DCR 250	29cm	7cm

Some observations:

• for working distance, there is no great advantage to using tubes vs the filter
• using multiple tubes results in a lot of movement in the lens. Here, the camera was suspended on a tripod looking straight down – but if shooting horizontally the assembly will droop (which can not be good…)
• the clip-on attachment for the Kenko filter can too easily pop-off if not attached carefully
• with magnifications of more than 2x, the live-view image becomes very hard to focus. Partly this is because every minute vibration is magnified immensely, and partly because general softness in the image makes it tricky to decide where the sharpest focus is…
• images shot with tubes-only are noticeably sharper that images shot with the close-up filter. Obviously adding more glass is bad, and the Raynox filter is actually cheaper than the high-quality optical air that Canon sells in its 25mm extension tubes…
• the Kenko tubes work just as well as the Canon ones and cost much less…

Another curious observation was that adding a polarising filter to the end of the lens unexpectedly increased the focal distance by roughly the thickness of the filter. While I would have expected some effect, it was surprising that this was noticeable.

Not really expecting anything useful, I also tried shooting an image with absolutely everything to hand: 12+20+25+36mm of tubes, the DCR250 filter and a Kenko 1.4x teleconverter. This gives approximately 5.6x magnification – more than the Canon MPE65 lens. The stack is hopelessly unstable and impossible to use other than by pointing straight down. However, the image quality is still surprisingly good:

Lastly, if all this is a little sterile, a couple of images to show some practical application:

Close-up of a plastic ruler. This took a lot of editing in Photoshop to remove specs of dust…

Laser-cut foil with clear plastic film.