Let's start by clarifying the difference between a Lens Extender and an extension tube. Both make lenses longer – hence the confusing similarity of the names. But Extenders, also known as teleconverters, have lens elements inside to increase the effective focal length of lenses, while extension tubes have no optical elements and are used to increase the effective magnification of a lens simply by moving the lens optics further away from the camera. Extenders are typically used for distant subjects and extension tubes for close subjects.

To understand how extension tubes work, consider how close-focusing works. With many lenses, as you focus closer, the length of the lens increases. Or to put it another way, the optics inside the lens move away from the camera as the subject moves nearer. Some lenses have internal focusing, meaning that their elements move back and forth within the barrel and the length of the lens does not change, but the principle is the same.

The closest focusing distance is reached when the lens is extended as far as it will go or the elements are moved to their furthest point. This focusing distance varies considerably from lens to lens – for example, the Canon EF-M 22mm f/2 STM lens enables you to focus on a subject just 15cm from the camera's sensor, while the EF 800mm f/5.6L IS USM has a closest focusing distance of 6 metres. But the magnification of the subject (actually a reduction) stays within a narrower range, from about 0.1x to 0.3x.

Why do lenses stop extending? Partly because it costs more to design and manufacture lenses with longer extensions, partly because it is difficult to design a lens that gives high-performance results over a wide range of focusing distances.

Macro lenses such as the EF-M 28mm f/3.5 Macro IS STM, EF-S 35mm f/2.8 Macro IS STM, EF-S 60mm f/2.8 Macro USM, EF 100mm f/2.8 Macro USM, EF 100mm f/2.8L Macro IS USM and EF 180mm f/3.5L Macro USM focus all the way from infinity down to a distance that gives 1:1 (life-size) magnification. This means that the image of the subject on the sensor is the same size as the subject in real life, which should result in optimal detail when you're shooting small subjects close-up. If you use these lenses, you will find that the length changes considerably from one end of the focusing range to the other.

There are also some lenses such as the RF 35mm F1.8 Macro IS STM, RF 85mm F2 Macro IS STM and EF 50mm f/2.5 Compact Macro that enable you to focus much closer than a typical lens but by themselves don't enable life-size (1:1) reproduction – these particular lenses give 1:2 images (half life-size, or 0.5x magnification) and need accessories to enable closer focusing to achieve life-size (1:1) reproduction.

You can increase the magnification of a lens simply by moving it further from the camera. All you need is an extension tube, which fits between the camera and the lens. There are no optical elements inside the tube ? it is just a device to increase the length of the lens – and this means there is no distortion or loss of image quality.

Canon produces extension tubes in two different lengths ? 12mm and 25mm. The tubes have electrical contacts, which enable data transfer between the lens and camera to be maintained. It is possible to attach the 12mm tube to the 25mm tube to give a 37mm extension. Canon does not recommend this, because the data transfer may be affected, but if you're willing to experiment, acceptable results are possible. Note, though, that there will be some light loss at the closest focusing distances.

The Canon EF12 II and EF25 II Extension Tubes are not compatible with EF-M or RF lenses, which are based on different mounts, but can be used with most EF lenses (see below for the exceptions) and with EF-S lenses. (Note that it's the lens that matters, not the camera. To be clear, you can still use these extension tubes if you're using a compatible EF or EF-S lens on an 球探体育比分_欧洲杯足球网乐￥在线直播 System camera with an EF-球探体育比分_欧洲杯足球网乐￥在线直播 Mount Adapter or on an EOS M camera with an EF-EOS M Mount Adapter.)

Their predecessors, the Canon EF12 and EF25 Extension Tubes, were part of the EOS system from 1991 to September 2004, when Canon introduced the EF-S mount, designed for cameras with smaller, APS-C sensors. If you happen to get hold of them, these original EF extension tubes cannot be used with EF-S lenses, but since they have no optical elements they will work with the same EF lenses as the Mark II tubes.

The Canon EF12 II and EF25 II Extension Tubes are not compatible with the following lenses:

? EF 15mm f/2.8 Fisheye (now discontinued)?

? EF 8-15mm f/4L Fisheye USM

? EF 14mm f/2.8L II USM

? MP-E 65mm f/2.8 1-5x Macro

? TS-E 17mm f/4L

In addition, the EF25 II tube is not compatible with the following lenses:

? EF 20mm f/2.8 USM

? EF 24mm f/1.4L USM (now discontinued)

? EF 24mm f/1.4L II USM

? TS-E 45mm f/2.8 (now discontinued)?

? EF 11-24mm f/4L USM

There are also some limitations to using the EF12 II and EF25 II Extension Tubes with the following lenses – notably, they can't be used at the wide angle setting:

? EF 16-35mm f/2.8L III USM

? EF 24-105mm f/4L IS II USM

? EF 200-400mm f/4L IS USM Extender 1.4x

? EF-S 10-18mm f/4.5-5.6 IS STM

? EF-S 10-22mm f/3.5-4.5 USM

? EF-S 15-85mm f/3.5-5.6 IS USM

? EF-S 17-55mm f/2.8 IS USM

In addition, there are slight exposure problems with some combinations of cameras and lenses. The instruction leaflet provided with the extension tubes gives some guidance on this, but the best advice is to shoot some test frames with the camera and lens you intend to use and make a note of any exposure compensation that may be required. Keep this information safe for future reference.