WHAT IS AN EXTENSION TUBE?
An extension tube is a spacer that sits between your lens and the camera body to alter the MFD (minimum focus distance). The MFD of a lens is a measurement of the closest point a subject can be from the camera’s sensor, whilst still being able to focus. If a subject is closer to the sensor than a lens’ MFD, you cannot focus on that subject. The thicker the extension tube used on a lens, the higher the lens magnification becomes as you’re able to move the lens much closer and still achieve focus. What all this means is that you can turn a normal lens into something with much a much higher magnification factor to achieve macro-like images, without needing a dedicated macro lens.
Later in the article I will discuss the knock-on image quality issues caused by extension tubes, but the other important thing to realise is that when extension tubes are in place, you can no longer focus all the way to infinity. This has few practical implications because most people are using extension tubes to try to focus on something at a very close distance, but it’s worth knowing so that you don’t think your lens is broken when you find it’s no longer able to focus on something further than a few feet away. It’s for this reason that you can’t practically leave an extension tube in place all the time, so it’s something you need to carry in your bag and use when it makes sense to do so.
HOW DO EXTENSION TUBES AFFECT IMAGE QUALITY?
Unlike teleconverters, extension tubes have no optics in them at all so in some circumstances they have very little effect on image quality. The tricky thing about point is that every lens reacts very differently to using extension tubes, so it’s hard to deliver a sweeping answer to this question. When manufacturers design the optics inside a lens, they take into account things like barrel distortion and pincushion distortion, and try to correct for it as much as possible. Focusing a lens does move the optics inside a lens, so that means that the amount of distortion varies depending on how far away your subject is from the lens. Lens designers try as hard as they can to correct for distortion at the most important points in a lens’ focus range, but essentially what it means is that some lenses are sharper than others when they are used at the minimum focus distance. Take a macro lens for example; The designers know that it’s going to spend much of its time being focussed at the absolute closest focus point, so they correct distortions for that point. Often a macro lens is at its absolute sharpest when used at the MFD. Conversely, super telephoto lenses tend to be used for focussing on objects that are quite some distance from the lens, so the opposite is true.