When choosing a specialist lens, first consider what you want to achieve
Extension Tubes
Inserting extension tubes between the body and the camera lens allows you to focus on shorter distances, which is useful when taking close-ups. Extension tubes contain no optical elements, so they don’t affect the image quality or sharpness in any way.
Extension tubes can be automatic or manual. Automatic let the camera control the lens, enabling Auto Focus. Their only drawback is the loss of lens aperture control.
Teleconverter
Used to extend the focal length and enlarge the object. A teleconverter is mounted between the camera and a photographic lens, whose last lens element must not be too close to the lens mount.
Teleconverters are typically made in multiple models (e.g. 1.4 ×, 2.0 ×), which indicates the focal length magnification and results in decreased light intensity. In some conditions, the lens does not have enough light for Auto Focus, in which case you have no choice but to focus manually.
Lensbaby
A special system designed to create innovative photographs with a touch of abstraction and blurring. Choosing what to focus on and the right moment is up to you.
Unlike normal lenses, Lensbaby lenses require aperture priority and fully manual mode. You have to find the sharpened “sweet spot” yourself and make sure you’ve got the right aperture. There are also interchangeable aperture disks.