For many lenses, the minimum focusing distance of a lens can range anywhere from 18 inches to over 18 feet. For close-up, or macro photography, this is often much too far. For those that want to dabble in taking great close-ups, you can either purchase a set of dedicated macro lenses, which often cost upwards of £650 a piece, or you can use a set of lens spacers.
Lens spacers, more technically known as extension tubes among photographers, are relatively simple devices that connect between the lens and the camera body. While they contain no optics, extension tubes allow you to focus more closely on your subject and can magnify the subject within the frame, by adding distance between back of the lens and the film plane or sensor.
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Things you need
- SLR Camera Body
- Camera Lens
- Lens Spec Chart
- Extension Tube Set
Use extension tubes fitted for your camera's lens mount. Extension tubes exist for just about every popular lens mount from Canon's EOS to Nikon's F mount.
Determine how close you need to focus. Using your lens of choice, frame up your subject, then measure the distance between the subject and the film plane of your camera. This is your needed minimal focal distance.
Calculate the built-in lens extension. The built-in lens extension is the distance between the film plane and the lens by itself. To calculate, use the formula: d = f * m.
Where f is the focal length of the lens, m is the maximum magnification factor determined by the lens spec chart on your particular lens, and d would be the built-in extension of the lens in question. To see the formula in action, for a Canon 28mm f/2.8, with a magnification factor of 0.13, the built-in extension would be the focal length multiplied by the maximum magnification factor, or 28 * 0.13 to equal 3.64mm.
Calculate the length of extension tube needed to focus on your subject. Use the formula: 1/(d+e) = 1/f - 1/s, with the variables representing the needed minimum focal distance, f represents the focal length of the lens, and d is the built-in extension of the lens, and e is the length of the extension tube, all in millimetres.
By substituting the variables for minimum focal distance, built-in extension, and focal length, you can derive the length of the needed extension tube. For example, if using the same 28mm lens, and a minimum focal distance needed of 150mm, you could determine that you would need an extension tube roughly 31mm in length to achieve focus on a subject that is 150mm from the lens.
Stack your extension tubes if necessary. Most extension tubes are sold in sets of various length. You can stack multiple extension tubes together to get your desired extension tube length. For example, if you needed to add 32mm of extension, you could use a 20mm and 12mm extension tube together.
Attach your extension tube or tubes to the camera body before attaching the lens. Extension tubes attach to your camera just like a camera lens. You then mount your lens directly onto the extension tube.
Focus normally and take your shot.
Tips and warnings
- While the formula is presented to understand specifically how extension tubes impact focus, for less precision, you can simply experiment with stacking the tubes until you are able to focus to your desired distance.
- While dedicated macro lenses exist for close-up photography, extension tubes are much cheaper to purchase.
- While using extension tubes, it is possible to lose up to 3 stops of light.
- For digital SLRs, extension tubes may not be able to communicate exposure or autofocus back to the camera.
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