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Infobank

Lenses: Extenders

Extenders, generally known as teleconvertors, increase the effective focal length of lenses. Canon Extenders are available in two strengths, 1.4x and 2x. As the name suggests, the 1.4x Extender extends the focal length of your lens by a factor of 1.4, and the 2x by a factor of 2.

Extenders are a relatively cheap and convenient way of enhancing telephoto capabilities. But, increased focal length comes at a cost − maximum aperture. The 1.4x Extender causes a decrease of one stop in the maximum aperture of the lens, while the 2x Extender causes a loss of two stops. This means you gain extra focal length at the expense of losing some light. For example, if you attach a 1.4x Extender to an EF70-200mm f/2.8L IS USM lens, for example, it will become a 98-280mm f/4 lens.

A Formula 1 racing car shot with an EF500mm lens, and an EF500mm lens fitted with a 1.4x EF1.4x Extender.

Using an Extender to increase the focal length of a lens is very useful when you are not able to move closer to your subject.

Not all Canon lenses accept Canon Extenders. This is due to their construction. Extenders have a protruding front element that will not fit into the rear of many EF lenses. Compatible lenses have a recessed rear element, which creates space for the front element of the Extender.

Maximum aperture

When the camera focuses a lens, it does so at the maximum aperture of a lens. This is the widest setting the aperture can have, letting through the most light. For example, the EF70-200mm f/2.8L IS USM lens has a maximum aperture of f/2.8, while the EF300mm f/4L IS USM lens has a maximum aperture of f/4.

When you add an Extender to a lens, the maximum aperture is decreased. A 1.4x extender will decrease it by one stop, while a 2x Extender will decrease it by two stops. This means that for the EF300mm f/4 lens with a 1.4x Extender the maximum effective aperture will be f/5.6, while with a 2x Extender, it will be f/8.

Autofocus with Extenders

Apart from reduced maximum aperture, another potential drawback of using Extenders is loss of autofocus. On most cameras, autofocus stops when the maximum effective aperture drops below f/5.6, irrespective of the amount of light available. However, there are some EOS models that allow autofocusing at maximum apertures down to f/8.

With the introduction of the EOS 3 came a new high density, high precision autofocus sensor. Designed to focus in the most demanding situations, it has 45 sensors, split into two types − horizontally sensitive points and vertically sensitive points. These points are arranged so that in certain areas the horizontal and vertical sensors form a cross-type sensor, sensitive to both horizontal and vertical lines. Each of these cross-type sensors is actually made up of four sensors − a pair of horizontal line sensors and a pair of vertical line sensors.

All earlier cameras have sensors that are only sensitive to light from a lens with a maximum aperture of f/5.6 or wider. The EOS 3, and some later models, allow autofocusing with a maximum aperture of f/8, though only with the centre focusing point.

All EF lenses have a maximum aperture of f/5.6 or wider, so all cameras will autofocus with any autofocus lens. The f/8 autofocusing ability is only needed when you are using a lens and Extender combination that reduces the maximum aperture of the lens to f/8 or smaller (see table). Some lens and Extender combinations mean that the maximum aperture of the lens is maintained at f/5.6 or wider and so autofocus remains unaffected.

Which cameras?

The cameras that feature the 45-point focusing system are: the EOS 3 and EOS 1V film cameras, and all the EOS 1D and 1Ds series digital cameras. All other cameras feature either 1, 3, 5, 7 or 9 focusing points and lack the central point vertical line sensor to operate with lenses having an effective aperture of f/8. You can still use Extenders with these cameras, but you might have to focus the lens manually.

Autofocus speed

Even if autofocus still functions when you fit an Extender to one of the compatible lenses, the autofocus speed may be reduced.

45-point focusing:

Black
Vertical line sensors operational with lenses having an f/5.6 or greater aperture.

Blue
Cross-type sensors operational with lenses having an f/2.8 or greater aperture Vertical line sensors operational with lenses having an f/5.6 or greater aperture.

Red
Cross-type sensor operational with lenses having an f/4 of greater aperture Vertical line sensor operational with lenses having an effective aperture of f/8 or greater.

What you might notice looking at this is that of the seven cross type sensors, six (the blue ones) are only functional with a small number of lenses, mainly the fast prime lenses and the f/2.8 zoom lenses. If you are not using one of those lenses, then those cross-type sensors perform no function in autofocusing your lens.

  Extender 1.4x Extender 2x
  f/5.6 f/8 f/5.6 f/8
EF135mm f/2L USM
EF180mm f/3.5L Macro USM 1 1 ×
EF200mm f/2.8L II USM
EF300mm f/2.8L IS USM
EF300mm f/4L IS USM × 2
EF400mm f/2.8L IS USM
EF400mm f/4 DO IS USM x 2
EF400mm f/5.6L USM × 2 × ×
EF500mm f/4L IS USM × 2
EF600mm f/4L IS USM × 2
EF1200mm f/5.6L USM × × × ×
EF70-200mm f/2.x L IS USM
EF70-200 f/2.8L USM 3 3 3 3
EF70-200mm f/4L USM × 2
EF100-400mm f/4.5-5.6L IS × 2 × ×

Table legend:

• AF is possible.

× AF is not possible.

1 To autofocus, use the range from 0.8m to infinity.

2 AF is possible, but only using the central focusing point.

3 AF is only possible using the central focusing point of an EOS camera with more than one focusing point.

f/8 cameras are the EOS 3 and 1V film cameras, and the EOS 1 series digital cameras. All other EOS models are f/5.6 cameras.

Mark III Extenders

In late 2010 Canon announced it was updating the EF1.4x and EF2x Extenders to Mark III versions. These new Extenders match the high image quality of the new lenses they were announced with (EF300mm f/2.8L IS USM II and EF400mm f/2.8L IS USM II) and ensure the fastest and most accurate autofocus performance possible.

Each Mark III Extender features a new integrated micro-processor chip to provide complete communication between lens and camera and ensure that the focusing can be completed as quickly as possible. Note that so that the Extender-Lens combination is reported correctly the Extender should be fitted to the lens first before attaching the whole combination to the camera.