I've been shooting 35mm film for a few months and am now planning on getting a Micro 4/3 Olympus camera soon. One question I have is whether or not I can use my existing M42 mount lenses on the new Micro 4/3 camera at the same focal length (50mm). I know you can buy adapters to connect the lens, but have also heard that it has the effect of essentially doubling the focal length (in this case to about 100mm). Just wanted to check whether this was the case and if so, is there any possible way to reduce the length back to normal? Would be nice to keep using my existing lenses if possible.
The focal length stays at 50mm, but the field of view will be that which you would get from a 100mm lens on 35mm film. Depth of field characteristics at a given aperture will be that of a 50mm lens.
There are special adapters, called focal reducers or speed boosters, that shorten the effective focal length (like the opposite of a teleconverter). Since these are optical devices, they will interfere with other lens characteristics, but not necessarily in a disadvantageous way (in some cases, resolution and contrast will actually improve, and you gain some lens speed). These come in a wide variety of prices and qualities, and are specific to a front and rear lens mount just as glassless adapters are. Hint: If you have legacy lenses with more than one mount, eg Canon FD and M42, look into possibilities like getting the FD mount version and using a M42 to FD adapter on top of it.
Take a look at this diagram I found on the web:
Assume this is your film camera and your 50mm lens, and you are photographing this red "tree". (It's an arrow, but let's pretend it's a tree.) Your lens projects the light rays onto the film plane, and projects the tree there (rendered blue in the diagram). The lens projects an inverted image. Note the height of the blue tree that is recorded on the film.
Now, if you remove your lens from this film camera and hold it in mid-air, attached to nothing, the lens is still working. It's still projecting the image of the tree onto some imaginary plane in mid-air, where the projected tree is still exactly the same size as was recorded on film. This is a property of the lens. There is no camera/film/sensor, so the projected image is not being recorded anywhere, but it is there on an imaginary plane in mid-air.
Now, mount this lens on a Micro Four Thirds camera. The lens continues to project the image of the tree onto the "film plane" where the digital sensor now is. (The lens adapter ensures that the lens is positioned perfectly so as to align the projected image with the plane where the sensor is.) Again, the projected tree is the same height as always - this is a property of the lens. But the sensor in your Micro Four Thirds camera has smaller dimensions than a frame of 35mm film, and it's not big enough to fit the full height of the tree. Let's say the top of the tree is chopped off.
So it's the same lens, with the same focal length (that's a property of the lens), but now you have a smaller field of view, by virtue of a smaller sensor capturing the image projected by the lens. You can't avoid this.* We are talking about physical aspects of the lens and sensor. And in fact, because the sensor is essentially half the size of a frame of 35mm film, you now have a field of view just as if you had used a 100mm lens on your film camera to photograph that tree, similarly chopping off the top of the tree. You have a crop factor of 2, and we can say that your 50mm lens has an equivalent focal length of 100mm. That is really just a way of communicating the new field of view.
(* Well, actually, you can do something about it, but only by using a lens adapter with glass elements in it, which is not so common, and often not desireable.)