Olympus OM manual focus lenses are extremely popular lenses for use on Olympus Four Thirds dSLR system, Micro Four Thirds system, Sony NEX E-mount camera system and Canon dSLRs because they are often readily available on Ebay, are small, light and compact, have an aperture ring, have great image quality and tend to have the common filter size of 49mm.
On most Olympus digital cameras they gain image stabilisation through the built-in IS in the camera body.
On Four Thirds and Micro Four Thirds cameras the field of view is half that when used on a full frame 35mm camera as the sensor size is a 2x crop. This is very handy when you want telephoto reach, but not so handy for the wide end.
Unfortunately, the price on these lenses has gone up over the past 10 years since Olympus discontinued production and collectors have sort these lenses.
The good news though, is that the lenses that are best used on the Four Thirds and Micro Four Thirds cameras tend to be the slower aperture prime lenses in the 21-200mm focal length range, and these are still at very reasonable prices – usually less than $US200.
In addition unless they are the super expensive APO versions, there will be purple fringing wide open, and are generally best used 1-2 stops down from maximum aperture, furthermore, the exposure system generally requires manual preset compensation for apertures wider than f/2.0.
The faster aperture, more expensive lenses often produce more ghosting due to the larger diameter rear element reflecting light rays back and forth to the shiny sensor.
Lenses with focal length 200mm or more get increasingly tougher to focus hand held when using a 2x crop sensor as you are at 400mm telephoto reach already.
Avoid legacy zoom lenses – the image quality is generally not as good, and image stabilisation becomes an issue as you have to manually enter a focal length, and this is not practical on a zoom lens with changing focal lengths.
In general, it is probably not worth spending a lot of money on these legacy lenses if a digital lens can be had for a similar price as the digital lens will usually give better image quality, have autofocus, allow the camera to control the aperture and meter more reliably, automatically optimise image stabilisation based on detected focal length, and record the lens settings in the EXIF data of the photo.
The following prices are average Ebay prices as at Feb 2010: