In July 2009 I sold the body of my Olympus E-510, and the buyer went out looking for a lens. He sent me some messages that suggested that a clarification could be useful, so here it is. Maybe it'll be of use to other people too. Most of this page applies equally well to other modern DSLRs, though usually the lens mount is different. Many other models will also take lenses for 35 mm film cameras without an adaptor.
The Olympus is a relatively typical modern digital SLR (DSLR), and it takes interchangeable lenses. You can use quite a few different kinds of lens with it. Here's an overview, in decreasing order of desirability.
The Olympus DSLRs are designed to use the so-called four-thirds lens mount, and these lenses will give you by far the best results. At the time of writing, lenses are available from Olympus, Panasonic/Leica and Sigma. These lenses are among the best available, though many reports suggest that the Sigma lenses aren't quite of the same quality as the others. Olympus offers three categories: standard, high-grade and super high-grade. They differ markedly in price and specifications, but even the cheaper ones have very good image quality. Andrzej Wrotniak maintains a list of currently available four-thirds lenses.
Prices for four-thirds lenses start at about $100 for a used standard lens (14-42 or similar, and f/3.5 to 5.6; there have been several models). These lenses are better than the price suggests, since many people buy them with the camera and then upgrade to a lens with better specs (wider aperture, bigger zoom range). Currently the most expensive lens is the Sigma EX DG HSM APO 300-800 long telephoto, which sells for about $US 10,000. The Wrotniak list gives you a rough idea, but the prices aren't always up to date.
You can use almost any lens from other cameras on an Olympus DSLR. You need an adaptor, which you can get on eBay for prices in the order of $10 to $20. I have personally used Pentax M42 screw thread lenses (and still do), and also Olympus OM (35 mm) series lenses. I know that you can use Nikon lenses too, but I haven't done so. The only make I'm not sure about is Canon, for which I haven't been able to find adaptors.
The picture quality you get with these lenses depends on the lens itself, of course. In some cases, it can be as good as, if not better than, the Olympus lenses. But in every case you lose functionality: the four-thirds lenses have autofocus and automatic exposure support. With the adaptors, you lose most of this support, so you have to focus manually. You can still use the camera's automatic exposure system. You have to set the aperture manually, and the camera will set the appropriate shutter speed.
When using lenses from other cameras, the focal length remains unchanged. If you use an Olympus OM 50 mm f/2 standard lens, available quite cheaply from time to time, it will still be 50 mm. But on the OM, that's a standard focal length; on the E series DSLRs it's a medium telephoto lens. The “standard” focal length is about 25 mm, and you can't find cheap, good lenses from 35 mm SLRs in that focal length range.
One warning: most lenses are designed to focus at full aperture, and the camera tells it when to stop down to the selected aperture. This doesn't work with the adaptors. Most of these lenses also have a manual stop-down lever, but they don't all do so. You need this lever: otherwise you can only use the lens at full aperture.
The camera can determine when an object is in focus even when the lens doesn't support autofocus, but there are adaptors available on eBay which do so. They're mainly electronics that fools the camera into believing that the lens is autofocus, but they're available both in combination with a lens adaptor and also by themselves for attaching to an existing lens.
A supplementary lens is one that is designed to be used in conjunction with another lens. There are two basic kinds:
“Teleconverters”, which connect between the lens and the camera, and which increase the focal length by a fixed factor, typically 1.4x or 2x. They also increase the aperture by the same factor, so a 150 mm f/4 lens with a 2x teleconverter becomes a 300 mm f/8 lens.
Teleconverters can be of high quality—Olympus makes two of them—but the increase in aperture is a serious disadvantage. I've discussed some of this on my telephoto lens comparison page.
Lenses mounted in front of the main lens. These get screwed into the filter thread, which isn't really designed to hold anything that heavy. Like teleconverters, they multiply the focal length by a fixed factor. Unlike teleconverters, that factor can be less than 1 (i.e. a 0.45x wide angle lens). I suspect that most of these adaptors are designed for video cameras, and the quality can be terrible. There's one particularly bad example in my telephoto lens comparison page. I'd personally recommend steering well clear of this kind of lens.
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