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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

Extension Tubes: Transforming Any Lens Into a Macro Powerhouse

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.

What Are Extension Tubes?

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.

Close-up macro shot of water droplets on a leaf surface
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The Physics Behind Extension Tubes

To understand how extension tubes work, we need to dive into some optical physics. The fundamental principle revolves around the thin lens equation:

The Thin Lens Equation

1/f = 1/u + 1/v

Where:

  • f = focal length of the lens
  • u = 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

Magnification Calculation

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

Maximum Magnification

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.

Extreme macro detail of flower stamens
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Macro shot of intricate flower petal texture
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Practical Examples with Math

Let's work through some real-world examples:

Example 1: 50mm lens with 25mm extension tube

M_max = E/f = 25mm/50mm = 0.5x magnification

Example 2: 100mm lens with 25mm extension tube

M_max = E/f = 25mm/100mm = 0.25x magnification

Example 3: 25mm lens with 25mm extension tube

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.

The Trade-offs

Extension tubes aren't magic—they come with several important trade-offs:

1. Light Loss

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).

2. Minimum Focus Distance Changes

The new minimum focus distance becomes:

u_min = f × (M + 1)
Macro photograph showing the intricate structure of a small insect
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3. Depth of Field Considerations

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-number
  • c = circle of confusion
  • δ = subject distance
  • M = magnification

As magnification increases, depth of field decreases quadratically!

My Extension Tube Setup

For this series of macro images, I used:

  • Primary lens: Canon 50mm f/1.8
  • Extension tubes: 12mm, 20mm, and 36mm (used individually and stacked)
  • Camera: Canon 5D Mark III
  • Technique: Manual focus, focus stacking for increased DOF
Macro shot of crystalline structure
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Close-up of fabric weave pattern
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Detailed view of feather barbs and barbules
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Stacking Extension Tubes

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

Focus Stacking: Dealing with Shallow DOF

At high magnifications, even at f/8 or f/11, depth of field is measured in millimeters. Focus stacking becomes essential:

  1. Take multiple shots at slightly different focus points
  2. Use consistent exposure across all frames
  3. Blend in post-processing using software like Helicon Focus or Photoshop
Focus-stacked macro image showing incredible detail throughout the frame
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Quality Considerations

Extension tubes maintain your lens's optical quality because they contain no glass elements. However, some considerations:

  • Electronic communication is preserved with modern tubes
  • Autofocus may struggle at high magnifications
  • Image stabilization effectiveness is reduced
  • Lens corrections in post-processing remain accurate

Cost vs. Benefit Analysis

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.

Technical Shooting Tips

  1. Use manual focus for precision control
  2. Shoot in Live View for accurate focusing
  3. Use a tripod - camera shake is magnified at macro distances
  4. Consider focus rails for precise focus adjustments
  5. Stop down to f/8-f/11 for optimal sharpness vs. DOF balance

The Images

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.

Abstract macro composition showing geometric patterns
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Conclusion

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.

Full Gallery

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.

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