I get a lot of emails asking me about lens choices. Some people get so confused. I was recently asked what the best portrait lens was in a specific price range. I responded that the 85mm 1.8 is a great choice. The reply was, well, with my camera, that would make it a 136mm lens, and that is too long I think. That got me to thinking.....it has been hashed out on internet forums, magazines, trade shows over and over again, but no one is grasping it correctly. Your camera does NOT change the focal length of a lens.
There is a lot of talk about Full Frame cameras. I hate to be the bearer of news that will go against what everyone in the world has believed in for so long, but all Digital SLR cameras are full frame. They may just not have the same size frame. There are 35mm sized sensors, there are APS-C sensors, there are sensors between 35mm and APS-C, there are sensors smaller, there are medium format sensors that are much larger than even a 35mm sized sensor. But they are all full frame. Now a few, some of the newer offerings from Nikon (D2X) and from some smaller companies that do, in effect, crop the sensor some. The D2X does this to improve speed. It's APS-C sensor is cropped down to a lower resolution and this increases the frames per second it can shoot. There are new cameras that will adjust the crop of the sensor to give you a HDTV orientation, or a 4x3 or a 5x7 crop. But none of these effect your lens focal length.
Before diving into digital "lens conversion", let's look back at film. As digital was emerging, there were two main consumer film sizes. APS and 35mm. APS was not as popular, and actually looked to be dying, and 35mm had been the standard for so long. The step up from 35mm was medium format. For this story, we will look at the 645 size, or 6cmx4.5cm.
Every camera type had a standard lens. With 35mm, the normal lens was a 50mm. If you kept both eyes open, it was very close to equal what you saw through the camera and with your naked eye (45mm was almost the exact as the naked eye). But you put a 50mm lens on a APS camera, things appeared bigger. Not really. The size of the film is not as big as the size of a 35mm frame, so the view you got was actually coming from the center portion of the lens. In effect cropping the lens. But....it was still a 50mm lens. You put a 50mm lens on a APS camera, it was what you expected. Now put a 50mm lens on a medium format camera and it acts as a wide angle lens. It will show much more area than on a 35mm frame, but it is still a 50mm lens. If you wanted an image to look the same from camera format to camera format, on your APS camera, a 35mm would be a good choice, 50mm for 35mm, and 80mm for medium format, but the lens focal lengths do not change.
In the digital age, that is the same. So you have an Olympus system that states 2x lens conversion. Incorrect. The frame is smaller than a 35mm frame, but it is not magnifying the lens at all, so a 100mm lens is still a 100mm lens, but compare a shot from an Olympus E-300 to that of a Canon 1Ds with a 200mm lens on it, you will at first think, that the Olympus really is doubling the lens. But it is not. The 100mm lens is acting like a 100mm lens from telephoto compression to DOF. What is commonly known as Field of View Crop, which is close to accurate, the smaller sensor is only using part of the lens. Now some manufacturers have made lenses that take this into account, but they are not "new" lenses. In fact, they are the same type lenses that were made for film cameras, but ground down to a smaller physical size. This makes them smaller and lighter and unusable on cameras with larger sensors. What makes a 50mm lens a 50mm lens is the fact that a certain part of the lens is 50mm from the sensor or film plane. So, if you could cut your lens and camera in half and measure from a specific point in the lens to the sensor you would find it the same distance in mm as your lens focal length. It does not matter what lens or what camera. On a 1Ds, a 30D, a Rebel Xt, a 50mm lens will always measure 50mm. So when you throw that 300mm lens on your Rebel, do not think that you are effectively shooting a lens that is 480mm. Your 300mm lens is still just a 300mm lens.