Macrophotography With Extension Tubes

One of the advantages of a mirrorless camera, like the EOS-M, is the very shallow backfocus requirement. The distance from the lens mount to the sensor is quite small, allowing use of just about any series of lenses on the market. All that is needed is the correct adapter, a need that several specialty manufacturers have addressed with products. The result is that the camera is useful in a wide range of photographic experiments and projects.

Gecko & Plumeria
A gold dust day gecko (Phelsuma laticauda) on a plumeria blossom
This includes older lenses from years past such as the Canon manual focus FD lenses or the Leica M lenses from decades ago. I have a number of these excellent lenses, mostly Nikon and Canon, the fast primes once treasured by photographers for their optical quality. Because these old lenses are not suited for use with modern DSLR’s they are often relegated to eBay and discount shelves in used camera stores. This does not mean they are obsolete, there are creative uses still available for these classic lenses.

I have had fun simply shooting with these old primes and the EOS-M camera. Sometimes I will grab a single lens and just go someplace to play with the camera for half an hour, sort of a self imposed creative exercise. Using one of these manual focus lenses brings back memories of my first years of film photography, before the days of auto-focus.

Combine these old lenses with an extension tube, and the rig becomes a macro-photography setup capable of fairly high magnification.

The pressure indicating gauge on a sphygmomanometer
It makes quite a stack on the front of the camera… EOS-M to FD adapter, the extension tube, and the lens. Considering the little EOS-M camera is quite small this seems even more absurd, the lens weighs twice what the camera weighs. There is also the concern of the optical accuracy in all of the couplings, a loose joint and bad things will happen at the focal plane. Fortunately the old Canon FD ring mount system, clumsy as it always was, clamps the bits and pieces together quite firmly and accurately.

Circuit Board
A circuit board with surface mount components
Manual focus with a digital camera takes time and care. Not a problem when using a tripod and a macro-photographic subject that is not moving. With a stationary target you can simply use the magnified view to focus. If this are more dynamic, handheld or a moving target, it makes more sense to use the focus peaking function on the camera. When turned on this function will highlight the sections of the image that are in focus.

Another technique I have used for handheld images is to set the camera for burst mode. I take a short series of images while slowly moving through focus, often by simply rocking back on my heels. With a little practice you can hit just the right focus, or even generate a series that can be focus stacked.

The magnification is a factor of the length of the extension tube. Calculating the magnification is not difficult, a simple equation will allow calculation of the resulting magnification.

m=\frac{\left(f\times m'+t\right)}{f}

Where m is the resulting magnification, m’ is the original magnification from the lens specifications, t is the extension tube length, and f is the focal length of the lens.

I usually go with a modest 25mm tube as any more is too much magnification for most uses, particularly handheld. Indeed, my favorite combination is a 25mm extension tube and the 100mm f/2.8 Canon FD lens. The setup has a large working distance, almost two feet away from the subject.

Plugging the numbers for the 100mm f/2.8 lens into the equation yields…

m=\frac{\left(50mm\times 0.12+25mm\right)}{50mm}=0.62

The native magnification of the 100mm lens is 0.12, now magnified to 0.62 with a 25mm extension tube. this is not quite the 1:1 magnification that some consider to be “true” macro, but it is approaching that point.

Author: Andrew

An electrical engineer, amateur astronomer, and diver, living and working on the island of Hawaiʻi.

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