Converters or extenders, as they are called at Canon, do not necessarily enjoy the best reputation. But they are a small, lightweight and above all comparatively inexpensive solution when you need more focal length. So a little fact check is in order.

What does a converter do?

In general, a converter only enlarges the image that the lens produces. This results in a tighter cropping. This corresponds to an extension of the focal length. Common converters extend the focal length by a factor of 1.4 or 2, so a 2x converter turns a 300 mm lens into a veritable 600 mm supertele lens. However, since there is nothing for free in technology, the use of a converter unfortunately also has its downsides. So let’s take a brief look at the pros and cons.

Pro converter

  • They can be combined with many lenses. So by using a converter you can easily multiply your focal length range
  • They are so small and light that they still find a place somewhere in every photo backpack. So you can always carry them with you
  • A good converter is not exactly cheap, but when you consider that telephoto lenses in particular are very expensive, the costs are kept within reasonable limits

Contra converter

  • The speed of your basic lens is reduced by one f-stop with 1.4x converters and by two f-stops with 2x converters
  • The enlargement of the image leads in principle to a deterioration of the image quality (sharpness, resolving power, contrast)

Trade-off

The pros probably do not need to be discussed further. They obviously speak for the use of converters. The cons, however, need to be examined a little more closely.

Lens speed

Reducing the speed of the basic lens is indeed problematic. Unfortunately, this effect means that using a converter requires a lens with a high initial aperture. The problem here is not so much the lower light output. With digital cameras, this can be compensated for by using a higher ISO number without much loss of quality . The real showstopper is the autofocus. For fast and precise focusing, it needs the highest possible initial aperture. The lower the initial aperture, the slower it works. At some point, it won’t work at all (many Canons stop at an initial aperture of f/8).

Therefore, the use of basic lenses with a high initial aperture (2.8 or 4) is more than desirable if you want to use the autofocus. Unfortunately, this puts the price argument into perspective, since lenses with high initial apertures are not available at bargain prices.

In addition, Canon extenders mechanically only fit certain L-lenses (i.e. the particularly expensive ones), since the front lens extends very far into the barrel of the basic lens.

Image quality

The main argument against using converters is the poorer image quality compared to the basic lens without a converter.

This is basically correct. However, from our experience, the deterioration in image quality is acceptable if the basic lens is of high quality and the lens and converter are well matched. This is quite to be expected if the lens and converter are from the same manufacturer. It should also be remembered that only the center of the image is used by the base lens, which usually provides better image quality than the peripheral areas.

We are not afraid to use even the 70-200 mm / 1:2.8 with a 2x converter. So far, none of our photo agencies has rejected a photo taken with this combination for reasons of poor technical quality. This is always the main criteria for us.

In some situations, we wouldn’t have been able to get a usable photo at all without a converter. The small kingfisher was taken with 1000 mm focal length (and the photo is even cropped a little bit). This was only feasible thanks to a converter.

Kingfisher

Useful lenses for the use of a converter

I had already written that the basic lenses should have a high initial aperture and good overall image quality. However, another aspect is sometimes overlooked. Converters are basically intended for use with long focal length lenses. That’s why they are also called teleconverters. They are simply not designed for wide-angle or even wide-angle zoom lenses.

Conclusion

Converters are a matter of taste. Nevertheless, we are happy to use them without much hesitation.

As usual, please feel free to leave a comment. I’m looking forward to it.