What they said. Uncoated lenses generally give negatives of lower contrast. This is often visible as more "open" shadows (more shadow detail, sometimes,) and more flare around highlights. Because of the lower contrast and greater flare or ghosting of highlights, some will interpret the negatives as being "softer" (not as sharp,) as a negative from a more modern, coated lens. In general, I think that's true: older lenses sometimes aren't as sharp as modern lenses. Sometimes, with the simpler designs (designs having fewer glass elements, such as tessars,) sharpness is actually quite high. You can use a yellow filter to increase contrast if the lens is adequately sharp otherwise, and get negatives with what most would consider "normal" contrast. I also fancy that a high-quality, modern coated glass filter over the lens also helps with flare, although you absolutely still need to shade the lens from direct sunlight (or, really, strong direct light from any source.)
Personally, I don't care for uncoated lenses for negatives I intend to enlarge but with a yellow filter, and if adequately sharp, they're marvelous for negatives to be contact printed.
Mike
