Idea of extension tubes is to get some distance between lens and camera's sensor. By doing this the minimum focusing distance will get smaller and magnification increases. Basically picture projected to sensor is enlarged and therefore also distortions are enlarged. Due to enlargement also some light is lost.
I use extension tubes to use my non-macro lenses to take close-up / macro photos. For real macro photos I prefer my macro lenses, but some times it's not possible to carry all lenses or the ugly bokeh of Canon EF 100 f/2.8 USM Macro is not suitable for the photo. For example Canon EF 200 f/2.8 L USM Mk II and Canon EF 85 f/1.2 L USM Mk II are good lenses for flower photography with entension tubes - both have excellent image quality (the magnifigated image is still usable) and have excellent bokeh.
Formulas to calculate the effect of extension tubes
Using formulas below take into account that
- internal focusing lenses usually don't have their nominal focal length when focused close
- lens may not have the focal length manufacturer has claimed it to be, e.g. 70-200mm zoom might be 72-192mm zoom in real life
Due to these reasons formulas may not give exact figures but they can be used to estimate what kind of effect is going to happen when extension tube is used.
Added magnification = Extension / Focal Length
By using this formula and lens manufacturer's information what is the magnification of the lens itself.
These magnification figures can be turned to more usable dimensions of the area which is in focus by using formula:
Width = Sensor Width / Magnification
Height = Sensor Height / Magnification
Effective F-stop = (Lens F-stop * (Focal Length + Total Extension) ) / Focal Length
This light loss is happening in addition to light loss happening when lens is focused. You can calculate your lens' own extension while focusing with formula: "lens' own extension" = Focal Length * Magnification.
Example of formulas in use
I used in this example Canon EF 85 f/1.2 L USM Mk II which has minimum focusing distance of 95cm and maximum magnification 0.11x. Below calculations show the formulas when this lens is used with 12mm Extension Tube and aperture f/1.2 on full frame camera (sensor 36mm x 24mm).
Lens' own extension = Focal Length * Magnification = 85mm * 0.11 = 9.35mm
Added magnification = Extension / Focal Length = 12mm / 85mm = 0.14
Therefore the total magnification at minimum focusing distance is 0.11 + 0.14 = 0.25 and 0.14 at infinity.
Which can be then calculated to dimensions at maximum magnification...
Width = Sensor Width / Magnification = 36mm / 0.25 = 144mm
Height = Sensor Height / Magnification = 24mm / 0.25 = 96mm
And in minimum magnification (lens focused to infinity):
Width = Sensor Width / Magnification = 36mm / 0.14 = 257mm
Height = Sensor Height / Magnification = 24mm / 0.14 = 171mm
Effective F-stop = (Lens F-stop * (Focal Length + Total Extension) ) / Focal Length = (1.2 * (85 + 21.35) ) / 85 = 1.50
Kenko Extension Tube Set: 12mm, 20mm and 36mm
Canon makes two extension tubes: EF 12mm Extension Tube II and EF 25mm Extension Tube II. These are
quite expensive considering that there are no optical elements, just air. Due to this I ended up purchasing Kenko
Extension Tubes, which cheaper alternative and more versatile since the set contains 3 tubes.
Kenko Extension Tubes are providing full electrical connection between the lens and camera. I have never
had any problems (e.g. Err-99) with the Kenko tubes. Tubes can be used one at a time or then they can be combined.
Therefore following extensions can be achieved with the tube set: 12mm, 20mm, 32mm, 36mm, 48mm,
56mm and 68mm.
Generally I don't have any complaints about the Kenko extension tubes. Some users have mentioned that
they have dropped their lens since it's too easy to press the lens release button. The release button haven't been
problem for me.
Photo of the Kenko extension tubes