Based on one of Ansel Adams' well-known teaching tools, the Visualizer tells you what focal length you need to match the image size you want. This saves you the need of trying different lenses, and it also makes tripod repositioning easier.
It is a black plastic panel with a precision-cut window matched to a specific camera format. An attached tape measure is calibrated in a full range of lens focal lengths. You simply look through the Visualizer's window from your established shooting distance, moving the panel in and out until the subject fills the window just the way you want. Then you "measure" the distance between your eye and the Visualizer with the calibrated tape; the needed focal length is clearly marked. Available for formats from 35mm to 8x10, this Visualizer is for 4x5" and 8x10" formats.