- Reduction of light by 1 or more stops, dependingtransmission. The specific amount depends on the specific design.
- An additional set of refracting elements in the light path, thus an additional distortion / aberration / Image Quality reduction factor.
- Observable decrease in sharpness at high contrast edges within image.
- 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.
- Potential vignetting at corners, especially with telephoto extenders at maximum focal length.
- 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.