Pentax 26755 Operation Manual - Page 18

16 About the Zoom Lens Aperture Zoom lenses whose maximum aperture varies depending on the focal length have a [Minimum length] aperture index for the shortest focal length and a [Maximum length] aperture index for the longest focal length. To set the aperture to A [Auto], turn the aperture ring to the white [Short focal length index] at the center (regardless of the focal length) as shown in the illustration. For the longer focal lengths, if you want to set an aperture other than A, align the aperture ring with the long focal length's aperture. For example, with a 28-105mm zoom lens set to 105mm, set the f/8 aperture by aligning the aperture ring's f/8 to the [Long focal length aperture index] mark on the left of the white index. * When setting the aperture manually at a middle focal length, use the aperture index for the short and long focal lengths as a reference.