Extension tubes are an easy way to get extra magnification in an image. An extension tube is a fixed length spacer that goes between the lens and the camera body. This means that they will only work on interchangeable lens cameras such as SLR’s. There are no optics/lenses within an extension tube, just air space. When you add an extension tube onto a lens it moves the lens farther away from the camera body.
- No lenses inside
- Easy to use
- Can use more than one at a time
- Relatively limited amount of extension you can add by stacking tubes – becomes unstable as more are added
- May lose the electrical or mechanical connection from the lens to the camera body
- Will lose working distance
Since extension tubes go between the camera and the lens; they may need to provide two functions within the connection – mechanical and electrical coupling depending upon the lens that is being used. The mechanical connection allows the camera body to control the aperture on the lens and the electrical connection allows the lens and camera to communicate properly. A few extension tubes also allow normal auto focus function. The electrical connection is an important connection because in newer cameras all of the lens functions, including focus and aperture, are controlled by the camera. If your extension tube does not have the proper mechanical or electrical connections, you may have a hard time using it with some more recent lenses – especially those without a manual aperture ring. When using older lenses with manual apertures and manual focusing, the electrical and mechanical connections are less important. Not all cameras require both machanical and electrical connection. Canon bodies only need an electrical connection as they control the aperture electronically.
Effects of Extension Tubes
Extension tubes work to increase the magnification of the image by increasing the distance from the lens to the detector. This provides closer focusing and increased magnification.
Adding an extension tube to a lens has a similar effect to moving a movie projector farther from the screen. The image projected by the lens will be larger. Since your camera detector has not changed in size, it will see a smaller portion of the enlarged image and will give a more magnified image.
More technically, an extension tube increases the image distance – the distance from the lens to the detector. This will create more magnification but will also cause an obligatory decrease in the object distance. Less object distance means that you will lose working distance.
Extension tubes will also limit the far focus capabilities of the lens. If the focus range on your lens was 6 inches to infinity without an extension tube, it will be something like 3 inches to 4 feet with an extension tube. You will lose the ability to focus at a distance, but you will now be able to focus closer at a higher magnification.
There are no optics inside of an extension tube but one should not assume that this lack of glass will not affect image quality. The image does degrade in a relatively slow and predictable way. This degradation is related to increasing the distance from the aperture to the detector resulting in an increase in the effective aperture. An increase in the effective aperture results in more diffraction and less resolution.
Thankfully the loss of resolution is slower than the gain in magnification, so you will tend to get finer detail in your images as the extension is increased. The added magnification and resolution gained with extension does have limits and at a certain point the amount of magnification gain will be roughly offset by the loss in resolution and no further significant gains will be available. This point is termed “empty magnification.”
Empty magnification is a rather vague and subjective term as there is no objective definition as to when it is occurring. I think that the take-home point for me is that you can increase the magnification by twice but you get significantly less than that in gained resolution. You don't likely have enough extension tubes to get into the realm of empty magnification, although if you are using a bellows (a large variable extension tube) it becomes much more feasible.
The amount of extension needed to add magnification varies by the focal length of the lens. To get the same increment of magnification with a 100 mm lens will take twice as much extension compared to a 50mm lens. This means that a shorter focal length lens will get more added magnification for the same amount of added extension.
For a 50 mm lens focused at infinity, the detector needs to be at the focal length of the lens - 50 mm behind the rear principal plane of the lens - to form an image. In order to focus at 1:1 you will need to extend that same lens by an additional 50 mm. You will need an extra 50 mm of extension for every unit of magnification beyond 1:1 (2:1 will require an additional 50 mm from 1:1).
The problem with adding an extension tube to an existing lens is that you will lose working distance quickly, similar to add-on macro lenses. This is not a significant problem with longer focal length lenses. These lenses already have relatively long working distances and can afford to lose some. The problem lies with short focal length lenses; the lenses where adding extension is also the most effective. These shorter focal length lenses already have limited working distance related to their short focal length and adding extension tubes will quickly reduce the working distance to virtually nothing. If you have limited working distance you will have a difficult time getting decent lighting.
That being said, adding an extension tube to almost any lens can add magnification with good quality as long as the amount of extension is not overdone. Below is a table showing the increase in magnification you can expect from adding extension tubes to various lenses (Of note: Using the Nikon 18-70 zoom lens with 25 or 50 mm of extension turns it into a usable macro lens.)
|Lens||No extension||+ 25 mm||+ 50 mm|
|28 mm (non-macro)||0.25 : 1||1.1 : 1||Zero WD|
|50 mm (1:1 macro)||1.0 : 1||1.6 : 1||2.2 : 1|
|55 mm (1:2 macro)||0.5 : 1||1.0 : 1||1.4 : 1|
|18-70 zoom (@70 mm)||0.2 : 1||0.6 : 1||0.8 : 1|
|105 mm (1:2 macro)||0.5 : 1||0.75 : 1||1.0 : 1|
|200 mm (1:1 macro)||1.0 : 1||1.3 : 1||1.5 : 1|
You may have noticed that a couple of lenses don't seem to follow the math for extension vs. added magnification. The two lenses that stick out for me are the 1:1 macro lenses in the list (200 mm, 50 mm). The 200 mm lens adds 0.5x magnification for only 50 mm of extension. You would expect 50 mm to only give you about 0.25x for this lens. The reason in this case is that the 200 mm lens isn't a 200 mm lens at 1:1. Most modern lenses that can focus to 1:1 achieve this by a combination of extension and focal length shortening. The 200 mm lens in question is actually about 100 mm at 1:1 focus. With that knowledge, the number make more sense.
More than one extension tube can be stacked to increase the total amount of extension. They are commonly sold in sets of three which can be configured in a variety of ways. If you want to use more than three at a time it might be a better idea to acquire a bellows which will give a more secure setup and more flexibility than a set of extension tubes. Bellows will be discussed separately in a later article.