Extension Tubes
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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.