It's not just quite that simple if you're thinking of the 25mm. The screwmount isn't coupled to the rangefinder, so you just guess the distance and set it on the lens and use the hyperfocal distance, but it does come with a viewfinder. The M version is rangefinder coupled, but you might still need the viewfinder and it costs about £90. Once the M adaptor is on you just leave it in place, so no problems with that at all and don't let it influence your choice.