Canon Extension Tube EF-12 II
Canon Extension Tube EF-25 II

Extension tubes are designed to enable a lens to focus closer than its normal set minimum focusing distance. Getting closer has the effect of magnifying your subject (making it appear larger in the viewfinder and in your pictures). They are exceptionally useful for macro photography, enabling you to convert almost any lens into a macro lens at a fraction of the cost while maintaining its original optical quality.

Extension Tubes are designed with all the circuitry and mechanical coupling to maintain auto focus and TTL auto exposure with most Canon lenses given there is enough light to activate the cameras AF system properly.

Kenko Extension Tubes can be used individually or in any combination to obtain the desired magnification.


Specification

Model Canon EF-12II Canon EF-25II Kenko 12 DG Kenko 20 DG Kenko 36 DG
Dimensions (DxL), mm 66.5x12.3 66.5x27.3 62x12 62x20 62x36
Weight, gr. 66 95 60 85 125

Image Magnification Ratios and Exposure Factors @ f50mm

Extension Tube, mm Distance reading of master lens, m Image magnification ratio Field coverage, cm Distance from
lens front to subject, cm
Exposure factor
12 inf 0.24 10x15 21 1.5
0.5 0.35 6.9x10.3 16 1.8
20 inf 0.4 6x9 13 2
0.5 0.51 4.7x7 14 2.3
36 inf 0.72 3.3x5 7.7 3
0.5 0.83 2.9x4.4 7 3.4
12+20 inf 0.64 3.8x5.6 8.5 2.7
0.5 0.75 3.2x4.8 7.5 3.1
12+36 inf 0.96 2.5x3.7 6 3.9
0.5 1.07 2.3x3.4 5.5 4.3
20+36 inf 1.12 2.2x3.3 5.3 4.5
0.5 1.23 2x3 4.9 5
12+20+36 inf 1.34 1.8x2.7 4.5 5.6
0.5 1.47 1.6x2.4 4.2 6.1

Formulas for calculation

The law of focal lengths is 1/a + 1/b = 1/F, where a is the object distance to the lens, b the image distance, and F the focal length.

The magnification M = b/a = b * 1/a = b * (1/F - 1/b) = b/F - 1.

The image distance b = F+L, where L is the length of extension tube used, so the magnification is M = (F+L)/F - 1 = L/F.

Suppose the f/ratio of the lens is n, thus the actual lens aperture is F/n. (n could be 2.8, 4 or whatever for f/2.8, f/4 and such.) The effective focal length of the lens with extension tube is F+L = F + F*L/F = F * (1+M). Thus, F/n = F*(1+M) / (n*(1+M)), so the effective f/ratio is n*(1+M). In other words, n is multiplied by a factor of 1+M.


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