Referred to as ‘extenders’ by some manufacturers, the magnification offered by 1.4x and 2x converters is irresistible for bird photographers seeking to increase the size of the subject in the frame. However, they do have disadvantages. They reduce the amount of light reaching the sensor by a significant amount, which affects autofocus speed and aperture unless you increase the ISO. In addition, they cause deterioration in image quality. The effect is barely noticeable with some lens/converter combinations but ruinous with others. Try out all the combinations and see what works for you.
Above: The long ‘reach’ of a 600mm lens and 1.4x converter were needed to achieve good image size and definition with this Balearic Warbler, which is a tiny bird
Right: A section of the image showing how camera file size, and the magnification of a super telephoto lens, allow for cropping and enlargement