Points in Focus Photography

Extension Tubes and Magnification

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Last time I posted, I noted that the easiest way to figure out magnification with extensions tubes is just to measure it. This time I’m going to show you how I measure and the results of as many configurations I expect that I’ll use.

Note, if you’re a Canon user and you stick to only their 12 mm and 25 mm extension tubes, and you don’t stack them, you can get the magnification and working distances from the manuals for their lenses.

I’ve found that the most efficient way to measure this is to use a movable ruler that’s on parallel to the film plane and on axis with the lens. How you move the ruler can be as simple as sticking to a glass with some gaffer tape and moving along the top of a table. For my tests, I pulled out a section of track for some large scale trains and used one of the cars as my movable carriage.

The whole point here is to be able move quickly from setup to setup without having to readjust a tripod. I did that for one lens when I first got the extension tubes and found it to be a pain in the rear.

As for the rule, you want it as parallel to the film plane as you can get it, but it does’t have to be absolutely perfect. We’re not interested in measuring this to the micrometer, we just need a close enough approximation.

Keep in mind, if you were to calculate the magnifications, you have to assume that the published focal lengths and magnifications for the lens in question are accurate. This further presents a problem, as the published focal length is almost never the actual focal length of the lens. Moreover, for virtually all modern inner and rear focusing lenses, the published focal length is the focal length at infinity focus; as you focus closer to the camera, the focal length decreases.

In any event, I suggest making two rough measurements for your tests. One will be the width of the field of view at the close focusing distance — as given by the ruler.

The second measurement should be the distance from the front of the lens to the ruler. This isn’t critically important to get to the mm, but it is useful as a guide if you don’t want to get to close to something.

Below is a table of magnifications and working distances I’ve measured so far with various Canon lenses and 12, 20, and 36 mm Kenko extension tubes. I’ll be adding to these tables, as I complete more measurements in the future. I’ll also be updating the linked reviews with information regarding the image quality of these lenses at various magnifications.

Magnification and working distance for lens and extension tube combinations
Lens Bare 12 20 36 12 + 20 12 + 36 20 + 36 12 + 20 + 36
Canon EF 70–200mm f/4L IS USM (200 mm) 0.22x / 38″ 0.29x / 30″ 0.33x / 27.25″ 0.41x / 22.25″ 0.40x / 23.38″ 0.49x / 18.88″ 0.53x / 18.88″ 0.60x / 17.38″
Canon EF 100–400mm f/4L IS USM (400 mm) 0.33x / 23.75″ 0.39x / 20″ 0.42x / 15.38″ 0.53x / 15.38″ 0.44x / 16.38″ 0.60x / 14.13″ 0.64x / 13.25″ 0.71x / 12.13″

Also, with a 12 mm tube, all of these lenses can mount a Canon 1.4x or 2x teleconverter. I don’t have a 2x TC, but I also ran the tests with my 1.4x TC.

Magnification and working distance for lens and extension tube combinations with a 1.4x II teleconverter
Lens Bare 12 20 36 12 + 20 12 + 36 20 + 36 12 + 20 + 36
Canon EF 70–200mm f/4L IS USM (200 mm) 0.30x / 38″ 0.36x / 33.38″ 0.4x / 31.75″ 0.49x / 28.25″ 0.47x / 29″ 0.54x / 26″ 0.58x / 25.13″ 0.65x / 23.88″
Canon EF 100–400mm f/4L IS USM (400 mm) 0.44x / 23.75″ 0.50x / 21.75″ 0.55x / 20.88″ 0.65x / 19.13″ 0.61x / 19.38″ 0.73x / 18″ 0.64x / 17.5″ 0.84x / 17.88″

Remember, if you stick to Canon’s 12 mm and 25 mm extension tubes, the magnification and working distances are published in the manual for your lens.

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