Converters add more glass to the optical path, so the main trade-off is usually image quality. Common downsides include reduced sharpness, especially toward the corners, more flare, lower contrast, and added distortion or aberrations. If the converter is not very well matched to the lens, these issues can be more noticeable. Front-mounted add-ons also create extra exposed surfaces where dust or fingerprints can further reduce contrast and sharpness, and they may cause vignetting in some cases.

Exposure depends on the converter type and placement. A rear-mounted teleconverter reduces the effective maximum aperture by its magnification factor, so a 2x teleconverter turns f/2.8 into f/5.6. A front-mounted wide converter generally does not reduce the f-stop in the same way, but it can still reduce image quality through flare, corner softness, and distortion.

As for why converters aren’t simply integrated: lens designers balance size, cost, complexity, and optical correction for a specific focal range. Adding converter optics permanently would make the base lens larger and harder to optimize. Separate converters let you choose flexibility, but usually with some optical compromise.