It's not that easy, for a few of reasons. One is the establishment (no better word for the mixture of subjectivity, entitlement, money, and fashion in the art w

It's not that easy, for a few of reasons. One is the establishment (no better word for the mixture of subjectivity, entitlement, money, and fashion in the art world). Rewriting art history is frowned upon unless it makes the genius artist look like a greater genius artist. Applying a scientific method doesn't work there. Another reason proving this is hard is that some artists do have a great ability to reproduce reality (this was really the most common way to measure an artist's ability before photography came along), but the economy of an artist's technique may reproduce some aspects of reality exactly, while other aspects are just suggested or completely ignored, so you'd need complete consistency. One more reason is that simply seeing a camera obscura image or a similar 2d image may change an artist's technique, they may become aware of things they hadn't considered before (this was the case with linear perspective, look up Brunelleschi's experiment - though nothing as elaborate as a camera obscura). For a camera obscura projection, lighting conditions are very specific, that alone might shift an artist's (mental) focus from being 100% descriptive to being very vague about contours. Basically, you can't "unsee it". Dutch artists from Vermeer's era are perfect candidates to have used lenses, because they had the technology. If you can find undeniable proof anywhere, it's probably there. Really the more proof adds up, the more ridiculous the naysayers appear. Some natural selection would do this field some good.