Pentax K-X - Manual (Page 17)

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Shake Reduction (SR)
Shake Reduction (SR) on the a features a PENTAX original system
which uses magnetic force to move the image sensor at high speeds,
compensating camera shake.
The camera may generate some operating noise when it is shaken, such
as when changing the composition of a picture. This is normal and not a
malfunction.
The captured area (view angle) will differ between the a and 35 mm SLR
cameras even if the same lens is used because the format size for 35 mm film
and CMOS sensor are different.
Sizes for 35 mm film and CMOS sensor
35 mm film: 36×24 mm
a CMOS sensor: 23.6×15.8 mm
Angles of view being equal, the focal length of a lens used with a 35 mm camera
must be approximately 1.5 times longer than that of a. To obtain an angle of
view framing the same area, divide the focal length of the 35 mm lens by 1.5.
Example) To capture the same image as a 150 mm lens attached to a 35 mm
camera
150÷1.5=100
Use a 100 mm lens with the a.
Inversely, multiply the focal length of the lens used with the a by 1.5 to
determine the focal length for 35 mm cameras.
Example) If 300 mm lens is used with the a
300×1.5=450
Focal length is equivalent to a 450 mm lens on a 35 mm camera.