The image produced on the ground glass is thus reversed left to right from the actual scene. Some TLR models have interchangeable lenses, but most have a fixed lens. Due to the use of separate lenses for viewing and taking the picture, there will be a difference between what you see in the ground glass and the image recorded on the film when photographing a subject at a close proximity. This discrepancy is called parallax . Parallax compensation is made by simply moving the taking lens into the same position as the viewing lens.
Some cameras give guides for accurate parallax compensation.
Another factor to be aware of when focusing close with a twin lens with a bellows is that bellows extension causes a change in the amount of light hitting the film plane and thus requires exposure compensation. Most TLR cameras with bellows show the exposure factor on the ground glass.