Artifacts
As in any analog process, every optical "pass" degraded the picture, just like a photocopy of a photocopy. Also, since a new, different piece of negative was exposed and printed, matching the exact colors of the original was a problem. For economic reasons, especially in the 1950s and later in TV series produced on film, printer work was limited to only the parts of a scene needing the effect. The original footage was spliced mid-shot with the optically-printed portion, resulting in an obvious change in image quality when the transition occurs.
Other problematic artifacts depend on the effect attempted, most often alignment inaccuracies in matte work. For this reason, shots intended to be manipulated via optical printer were often shot on larger film formats than the rest of the project. Larger film (such as the Vistavision format) has better clarity and less grain when reprinted and also reduces alignment problems.
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