As far as I know, it is necessary to multiply the focal length by the crop factor. My Sony Nex 5 has a crop factor = 1.5 For lens Helios 77M-4 1.8/50 focal length will 50 * 1.5 = 75
Is this true for Sony Nex? And also for M39 lenses?
The focal length will be exactly what it says on the tin. A 50mm lens will be a 50mm lens, whether it's an M39-mount or a native Sony E-mount. What you would calculate using the crop factor is the 35mm full-frame equivalent field of view. That is, a 50mm lens mounted on a camera with a crop factor of 1.5 will give you the same field of view as a 75mm lens mounted on a 35mm (135 format or full-frame digital) would have.
If you have experience shooting with a 35mm film camera, the easiest way to think about things is that most of your focal lengths move up one notch in their use-cases. So you'd use a 35mm lens on your NEX (or on any 1.5-1.6 crop factor camera) where you would have used a 50mm on a 35mm camera; the 50mm nudges into the space the 70-85mm lenses used to occupy; an 85mm would be used in place of a 135mm, and so forth. The focal length hasn't changed, but you're only using a 16x24mm crop out of the centre of a 24x36mm frame.