The 36x24mm 135 film format was most popular for highend consumer cameras for decades, but the difficulty of making large sensors made smaller formats more common in digital cameras. The result is a variety of sensor formats – and lenses designed to cover each. However, mirrorless bodies allow mounting lenses designed for various non-native formats. One would expect lenses to work best using the sensors they were designed for, but there are many potentially good reasons to use lenses designed for one format on a sensor of another format. This paper explores how lens behavior changes as lenses are used on non-native sensor formats, either directly or with the addition of optical elements that have the side-effect of adjusting coverage: rear-mounted focal reducers and tele-converters.