Extension Tubes: Transforming Any Lens Into a Macro Powerhouse
A deep dive into the physics and mathematics of extension tubes, exploring how simple spacers can unlock macro photography capabilities in any lens
2015-07-10
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A deep dive into the physics and mathematics of extension tubes, exploring how simple spacers can unlock macro photography capabilities in any lens
Macro photography has always fascinated me: the ability to reveal intricate details invisible to the naked eye, to transform everyday objects into abstract compositions, to explore a world that exists right under our noses but often goes unnoticed. However, dedicated macro lenses can be expensive, and that's where extension tubes come in as an elegant, affordable solution.
Extension tubes are hollow spacers that fit between your camera body and lens, increasing the distance between the lens and the sensor. They contain no glass elements: they're simply empty tubes that move your lens further from the camera body. This seemingly simple modification has profound effects on your lens's optical characteristics.
To understand how extension tubes work, we need to dive into some optical physics. The fundamental principle revolves around the thin lens equation:
1/f = 1/u + 1/v
Where:
f = focal length of the lensu = object distance (distance from lens to subject)v = image distance (distance from lens to sensor/film)When you add an extension tube of length E, you're effectively increasing the image distance by that amount:
v_new = v + E
The magnification ratio M is defined as:
M = v/u
With extension tubes, this becomes:
M = (v + E)/u
For a lens focused at infinity without extension tubes, v = f (the focal length). So with extension tubes:
M = (f + E)/u
When the lens is focused at its minimum focusing distance, you achieve maximum magnification. For many lenses, this gives us:
M_max = E/f
This is a crucial formula: the magnification is directly proportional to the extension tube length and inversely proportional to the focal length.
Let's work through some real-world examples:
M_max = E/f = 25mm/50mm = 0.5x magnification
M_max = E/f = 25mm/100mm = 0.25x magnification
M_max = E/f = 25mm/25mm = 1.0x magnification (life-size)
Notice how shorter focal length lenses achieve higher magnification with the same extension tube length.
Extension tubes aren't magic: they come with several important trade-offs:
Adding extension tubes reduces the amount of light reaching the sensor. The light loss factor is:
Light Loss Factor = (M + 1)²
For 1:1 magnification (M = 1):
Light Loss = (1 + 1)² = 4
This represents a 2-stop light loss (since 2² = 4).
The new minimum focus distance becomes:
u_min = f × (M + 1)
At high magnifications, depth of field becomes extremely shallow. The depth of field formula for macro photography:
DOF = 2Ncδ(M + 1)/M²
Where:
N = f-numberc = circle of confusionδ = subject distanceM = magnificationAs magnification increases, depth of field decreases quadratically!
For this series of macro images, I used:
One of the beautiful aspects of extension tubes is that they're stackable. If you have multiple tubes, you can combine them:
Total Extension = E₁ + E₂ + E₃ + ...
For example, stacking 12mm + 20mm + 36mm tubes gives you 68mm of total extension. On a 50mm lens:
M_max = 68mm/50mm = 1.36x magnification
At high magnifications, even at f/8 or f/11, depth of field is measured in millimeters. Focus stacking becomes essential:
Extension tubes maintain your lens's optical quality because they contain no glass elements. However, some considerations:
Extension Tube Set: ~$50-150 Dedicated Macro Lens: ~$400-1200
Extension tubes offer an incredibly cost-effective entry into macro photography, allowing you to experiment and learn before investing in dedicated macro glass.
The photographs in this post were all captured using various combinations of extension tubes with standard lenses. Each image represents a different approach to macro subject matter: from natural textures to man-made materials, exploring the incredible detail that surrounds us at the microscopic level.
Extension tubes represent one of photography's most elegant solutions: using simple physics to dramatically expand your creative possibilities. The mathematics might seem complex, but the practical application is straightforward: longer tubes give more magnification, shorter focal length lenses amplify the effect, and the trade-offs are predictable and manageable.
Whether you're interested in exploring the intricate world of insects, the crystalline structure of everyday materials, or the abstract beauty of textures at high magnification, extension tubes provide an affordable gateway into macro photography.
You can view the complete collection of extension tube macro images on my Flickr album. Each image explores different aspects of macro photography technique and subject matter.
The beauty of extension tubes lies not just in their simplicity, but in how they democratize macro photography. With basic understanding of the underlying optics and some experimentation, any photographer can begin exploring the fascinating world that exists just beyond normal vision.