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Extension tubes are elements that are mounted between the camera and the lens. They don’t contain optical elements and their only function is to move the lens away from the camera. This way, they allow focusing at closer distances (the working distance decreases), which leads to greater magnification.

The formula used to calculate the maximum magnification achieved with extension tubes is:

where:
ML – maximum magnification of the lens
Еxt – added extension [mm]
F – focal length of the lens [mm]

* When the lens is focused at infinity, ML=0

It is obvious that the more extension we add and the shorter the focal length of the lens is, the greater the magnification will be.

We should keep in mind that a lot of lenses focus on closer distances by insignificantly decreasing the focal length. In some macro lenses, this is strongly pronounced. On the one hand, this could be assumed an advantage, if we’re seeking more magnification, but this could also be considered a disadvantage, because of the larger viewing angle that we get. With Sigma 105/2.8 EX Macro, for example, the focal length at maximum magnification becomes approx. 65 mm. That’s the reason why 1.85x magnification can be achieved with only 56 mm extension.

There are two types of extension tubes – with or without electric contacts transmitting the communication between the camera and the lens. The lack of this communication deprives you of auto focus, EXIF data related to the lens, and aperture control. In this case, you have to either shoot using the widest aperture, or set it before shooting and use the system this way. If your lens has an aperture ring, this is quite easy, but if it doesn’t, you’ll have to mount the lens on the camera, set the desired aperture, and take off the lens holding the DOF Preview button. When the aperture is set previously, the viewfinder will be much dimmer, making focusing even harder. In any case, I recommend choosing extension tubes with electric contacts. Their price is higher, but they’re much more convenient and pleasant to use.

The main advantage of extension tubes is their relatively low price. Using them in combination with some of your available lenses, you can easily start shooting macro. On the other hand, if you already have a macro lens, they would be very useful for getting even more magnification. It is believed that extension tubes don’t degrade image quality, because they don’t contain optical elements. This, of course, is not completely true, because using them, we look through a smaller part of lens’ viewing angle. With larger extension this would certainly affect image quality.

Loss of light when using extension tubes is their biggest disadvantage. With a 50 mm lens and 12 mm extension, we lose approx 1.5 f-stop, with 20 mm – 2 f-stops and with 36 mm – 3 f-stops of light. This shouldn’t bother you, but the more unpleasant thing is loss of infinity focus and that the working range becomes very small. If, for example, we use a 50 mm lens and 20 mm extension, we will be able to shoot within the 0.4x to 0.55x range. If we want to increase or decrease magnification, we will have to change the extension tube or add another one. This not only takes time, but can also be a reason to miss a good shot, and can cause dust to get inside the camera.

We have to keep in mind, that using extension tubes moves the weight of the lens further away from the camera, which causes additional stress on the camera mount. It is advisable not to let the camera mount take the weight of the whole optical system.

Camera manufacturers also offer extension tubes for their systems, but they’re usually too expensive. Since tubes don’t include optical elements, the main requirement is for them to have a sturdy construction. Kenko and Soligor offer sets of 3 tubes (12, 20 and 36 mm) that fully satisfy this requirement.

To me, this is the first thing I would use in order to achieve greater magnification with one of the lenses that I already have.

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