Extension Tubes - Calculation

Any fan of Macrophotography has used or has ever heard about extension tubes as a method to get closer to the object to be photographed, and thus be able to obtain a higher degree of enlargement.

These tubes can be used on both normal and so-called “macro” lenses, to give them the possibility of higher magnification, and are located between the lens and the camera body.

Kenko® extension tube set with contacts, consisting of three tubes of 12, 20 and 36 mm respectively

Almost all manufacturers create extension tubes that are around 12mm, 20mm and 36mm in length in the three tube set, but ... how close will using one or several tubes of different lengths allow us to get closer to the sample? and also the most common question is not only how much closer we can get with them, but how much we will increase the magnification by doing so.

Logically we will get a closer approach and therefore a greater magnification the more tubes we use assembled with each other, since even if we had enough light we could even consider assembling more tubes of a second set, that is, the more tubes we use the more we will move the diaphragm of lens from sensor of our camera and therefore less light will reach it, this being the real limitation to be able to continue assembling extension tubes indefinitely to be able to achieve large enlargements of the sample with them.

Photo by Juanjo Temiño

In order to calculate how much enlargement and what % of magnification you will get using a certain length of extension tubes, we will only need to know three lengths/distances expressed in mm (in case they are not, convert them to mm), which are the ones necessary for the calculations:

    1) THE FOCAL LENGTH of our lens (usually referred to as "F")

    2) ITS MINIMUM FOCUS DISTANCE (MDF in English = Minimum Distance Focus) measured from the sensor plane. This
        is a data provided by the manufacturers in the lens specifications and that we will call "X"

   3) And finally, THE LENGTH OF THE EXTENSION TUBES that we are going to connect between the lens and the body of
       our camera, and that we will call "T".

Thus, for example, we would have that for one of the most popular lenses among Macro photography fans, such as the Canon 100mm IS 2.8 Macro, the values would be: F = 100 mm, X= 30 cm = 300 mm (which is its minimum distance of focus measured from the plane of the sensor according to the manufacturer's catalog) and T that will be our variable depending on the tubes that we put.

Assuming that we use a complete set of extension tubes made up of three 12, 20 and 36mm tubes coupled respectively, we would have: T = 12+20+36mm = 68mm and we already know that for this lens: F= 100mm and X= 300mm.

Transferring this data to a spreadsheet with the appropriate formulas, we would obtain that the new minimum focus distance would be (rounded to two decimal places) around of 184.75 mm (instead of 300mm, which is the minimum that we could approach with this lens without extension tubes), which means that we can get 115.25 mm closer to the sample ( 300mm – 184.75mm = 115.25 mm = 11.53 cm ), which is equivalent to reducing the distance to the sample by 38.42% (magnification increase percent)

In other words, if we were using a Canon lens like the one in the example, whose magnification at the minimum focus distance is 1X (or 1:1), when using all the tubes of the kenko set that we have used as an example (12+20 +36 mmm), the new approach that we could make (only 184.75mm from the sample measured from the sensor plane) would provide us with a magnification of 1.3842X for this objective, rounding off almost 1.4X.

So that you can do your own calculations, I leave you below the link to the excel CALCULATOR, where simply by entering the three data mentioned in the fields shaded in yellow, it will automatically give you the enlargement gain in mm and the % magnificaion that we will achieve, depending on the length of the extension tubes that you install between your camera and the lens.

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