Tutorial
Cameras

1. The depth-of-field preview button lets you see what a scene looks like at the programmed aperture. It is useful for some close-up shots, where one would want to obtain an accurate sense of the depth-of-field of the resulting photograph. On a normal single-lens reflex (SLR) camera, the diaphragm of the aperture is fully opened during focussing and framing to allow for maximum brightness. What is seen through the viewfinder will therefore darken when the preview button is pressed.

   [John P. Scchaefer, Basic Techniques of Photography: An Ansel Adams Guide. Little, Brown, and Company, Toronto, 1992.]

2. There are several circumstances where it might be useful to move the back-plane of a view-camera with respect to the lensboard. It can be used to provide an effective increase in the depth-of-field, or to alter the apparent perspective of a scene, as illustrated in the following figure.

   

3. A longer focal length results in a larger image recorded on the film. This is perhaps easiest to explain for objects at infinity -- the image plane should in this case be placed at a distance, d, from the camera equal to the focal length. The longer focal length will have a larger distance between the lens and camera, and therefore a correspondingly larger image.