Pixiq

Once upon a time, buying a set of extension tubes was the generally recommended route to close-up and macro photography. You might be forgiven for thinking that they are redundant now that so many lenses offer a ‘macro’ setting and most macro lenses offer a reproduction ratio of 1:1 (life-size) where subject details appear at the same size on the sensor.

However, extension tubes can really extend the creative possibilities with various lenses you might already have in your camera bag. One thing, though I tend to use them singly since they are not convenient to remove and change at speed.

Here are some possibilities with links to earlier posts where I have discussed the basic techniques in detail.

• Wide Angle and ultra-wide angles – the thinnest of tubes (typically 12mm) can help you get dramatic wide-angle close-ups when used with 24mm, 28mm or even 20mm wide angles and some wide angled zooms. With shorter focal lengths even the thinnest tube causes considerable enlargement and can brings the subject impracticably close to the front lens element.

• Macro and other lenses: extension tubes boost magnification with no recognisable deterioration in image quality. There is a loss in light reaching the sensor but TTL exposure systems compensate and make awkward calculations for correction a thing of the past. You can do the same with a teleconverter but slight image deterioration and a bit of internal flare can be the result thanks to the addition of more glass elements.
• Telephotos and tele-zooms – adding a tube means you can no longer focus on infinity but you get a dramatic change in close-focus capability, especially with something like a 300mm lens that has internal focusing. Many tele and macro lenses shift elements inside to get closer without changing the tube length but focal length is reduced in the process. It gives a ‘macro’ capability with much greater subject to camera distance useful for skittish lizards and shy butterflies that insist on staying out of reach in trees.

• Reversing lenses: reverse a lens on to an extension tubes and you will produce a combination that gives you much greater than life-size.

About a year ago I surprised myself and bought a set of Kenko fully coupled tubes for Nikon…they were not cheapbut I have not regretted the purchase. Whereas earlier tubes were, at best, capable of allowing open aperture metering – these tubes permit complete coupling between camera and lens (even full AF) and are very well-made. Canon users can buy a couple of tubes (12mm and 25mm) that indvidually that do the same. They look identical to the Kenko tubes but for the logo …especially since Kenko also make a single tube: Uniplus tube DG 25 that raises suspicions even more. When it comes to modern ‘macro’ work, I feel that Nikon does not serve well the army of afficionados who use their equipment. Their own-brand tubes are the same designs introduced a long time ago…excellent accessories, but. Their 8mm tube is one of the thinnest made and particularly good with wide-angles which is some consolation, although Canon once made a 4mm tube that I still have for its FD mount accesories.

How Extension tubes work

When the lens is moved away from the centre light rays spread out. This produces a larger image on the sensor and the lens focuses closer to the subject. However, because the rays spread the light intensity (brightness) is reduced but when using TTL metering the camera corrects for the under exposure.

Stripped to basics, an extension tube is a metal tube with a lens mount at one end and body mount at the other which serves to increase the space between lens and camera sensor. There are no lenses within them. An earlier generation of macro lenses would only go close enough to reproduce an object at half life-size and the lens relied on an extension tube to produce life-size reproduction

You can calculate the magnification you get  with a formula that works for a lens set on infinity 

Magnification (M) = tube length (d) in mm  ÷ focal length (f) of lens (mm)

NB. The maximum magification is slightly greater if the lens is set at its closest focus. If you think of that as due to a bit of internal extension you can, from the manufacturer's quoted reproduction ratio work backwards, to find what extension caused this…. then add it to the tube length to get the ‘effective’ extension. The problem is that modern macro lenses focus by shifting several elements internally and thus changing focal length so this just gives an idea. Some examples – 'ball park' because of assumptions just to give ideas:

• A 24mm wide angle used 12mm tube gives ½ life-size on the sensor and even greater scale of reproduction at its closest focus but the front element is just a few cm from the subject.

• A 105mm AF macro with IF becomes closer to 80mm focal length used at life-size. To give 1:1 with an 80mm lens requires an ‘effective’ extension of 80mm. Adding a 36mm tube gives an extension of 116mm which with the formula provides M = 1.45. If you have this on a DX camera with crop factor 1.5 this would mean using 1.5 x 1.45 = 2.1 magnification to fill the frame to the same extent…

• A 300mm f/4 ED-IF AF-S has a quoted repro ratio of 1 : 3.7  (x 0.27) on its ‘macro setting. Using the formula you’d need an extension of  300 x 0.27 = 81mm. I have no data about the change in focal length of this lens due to internal focusing so, assuming it stays at 300mm, a single a 36mm extension tube would give effective extension of 116mm and a magnification of 116 ÷ 300 = 0.39 or 1: 2.6. The inevitable  focal length change with an IF lens means it would be closer to half life-size I would guess…

Sets of extension tubes often  come in threes and can be assembled or used singly to produce a range of different extensions and thus magnifications:

Kenko Quality.

When buying independent lenses and accessories it is as well to be careful. I have no hesitation, as a Nikon user, with buying Sigma lenses - my eulogies on both their 150mm f/2.8 macro and 15mm f/2.8 diagonal fisheye (my most frequently used lenses) have already been written on Pixiq and I have long used Novoflex accessories. Mechanical quality with both is excellent whatever internet know-alls (who have never used them) might say.

In fact, you have to be careful when using for bargain-priced tubes for the rear end of a lens is complex with contacts and small levers and a badly made extension tube can prove a disaster when its machining has not been precise. Repairs will prove costly and there is no sympathy for those who have dared to buy out of the system.

With that caveat, I am impressed with the build quality of the Japaneses-made Kenko extension tubes: they fit smoothly together and to lens and camera body with no noticeable playand the AF works a treat with the tubes fitted. Naturally, older auto-tubes made by Nikon and Canon were made from thicker gauge tube and had a more ‘solid’ feel as, of course did lenses in the good ol’ days.

The Kenko tubes come in a set 12mm, 20mm and 36mm and when used together provide 68 mm of extension: possible extensions in mm are 12, 20, 32, 36, 48, 56 and 68. Magnification is thus in steps but you can crop later in post production.

They are available in fittings for  Nikon AF-D, Minolta AF/ Sony Alpha  and Canon EF & EF-S at an on-line price of around $179.00. Beware: if buying used, the older tubes did not work with Canon EF-S mounts. The 12mm Kenko tube is sometimes advertised as a separate item ($99.00) but is seldom available unless in a set: the 25mm tube UNIPLUS DG tube is available separately in the following fittings: Nikon AF-D, Minolta AF/ Sony Alpha and Canon EF (not EF-S).

Practicality of Extension tubes in the field

In the past, the fact AF was not transmitted meant little for it was best to switch it off to stop a lens hunting when doing macro work. There has been huge improvement in focus speed and on all but the breeziest of days many macro lenses can follow subject movement…the infuriating way is that they then shoot off to infinity and it’s best to use a focus limiter if you have one.

I tend to use them singly as mentioned above citing that they are not convenient to remove and change at speed. Maybe it was just Sod's law but I always found the wrong tube or combination was on the camera when once-in-a lifetime images were on the horizon or closer. I have never quite got over the trauma of missing a shot of a golden eagle that went just over my head in a wild part of France when I was photographing orchids close-up and I had only one camera body with me. Fumbling with a mix of panic and excitement I could not get the tubes off and, maybe it was just my paranoid imagination, but as the bird did a ‘victory’ roll it parted two feathers at its wingtips and flicked that time honoured “V" for victory – or words to that effect.

 On another occasion, way back, I was using a medium format Bronica S2A outfit high in the Welsh mountains on a bitterly cold January day to photograph mosses for a museum assignment. I managed to drop a tube plus detachable lens mount in an acid bog… I learned then that having a full set of jewellers screwdrivers was not the advantage it seemed when the tube had to be disassembled,  reassembled and work. I learned that day about something called a multi-start helical thread.

Those who know about the things will ‘feel my pain’, to use that touchy-feely terminology.

© Paul Harcourt Davies 2011