lens - What are the drawbacks of an extender (wide or teleconverter)?

I'm asking a more general question, starting from a particular case with which I'm dealing these days: I'm considering buying a Wide Conversion Lens for my Panasonic HC-X900M camcorder (see the item here) but I'm wondering what drawbacks it introduces and why the Wide Converter (also Leica glass) wasn't "integrated" in the lens' design.

Or more generally, what drawbacks/disadvantages a Wide Conversion Lens or a Teleconverter brings to an optical system (lens)?

Answer

1. Reduction of light transmission. The specific amount depends on the specific design.


2. An additional set of refracting elements in the light path, thus an additional distortion / aberration / Image Quality reduction factor.


3. Observable decrease in sharpness at high contrast edges within image.


4. In the case of add-on extenders specifically: Dust between the main lens front element and the extender back element, hence reduction of contrast and sharpness. Essentially, 2 additional dust / fingerprint prone surfaces in light path.


5. Potential vignetting at corners, especially with telephoto extenders at maximum focal length.


6. Potential introduction of pincushion / barrel distortion at various focal lengths if the extender is not perfectly matched with (and designed specifically for) the main lens.