Do you have a top rangefinder or a side rangefinder? The 150mm focal length is approximately 6 inch focal length. There were a few lenses made for the Graphic cameras of about 200mm focal length (that is eight inch focal length). If you try to use a 250mm (ten inch) lens you start to run out of bellows for the closer focusing distances. Also for portraiture, you will probably want a shutter that can synchronize flash. Tell us what you have for a budget and you may get some specific suggestions.