Pentax K-7 Sensor shift (Composition adjust)

One of the features of the Pentax K-7 is that you are able to move the sensor by small amounts along the focal plane.  The effect of this is to adjust the position of the image formed by the lens on the sensor.  This differs slightly from moving the camera and therefore the lens and sensor.  Figure 1 shows a lens and sensor positioned so that light from point (A) goes through the center of the lens and reaches the sensor at it’s center (A1). In this scenario point (B) is obscured by point (A) and none of it’s reflected light reaches the lens.  Now if we were to move only the sensor using the composition adjustment feature of the K-7 as in Figure 2 The image formed by the lens is relocated on the sensor but because the lens has not moved the relationship of the elements in the scene remain unchanged and point (B) remains obscured.  Now if we were to reposition the entire camera as in figure 3 the new position of the lens allows some reflected light from point (B) to reach the sensor.  So even though (A1) reaches the senor at the same point in figure 2&3 the image formed differs.

A simple way of demonstrating this effect is to close one eye and hold a piece of paper edge on in front of the other eye.  What you will see is a very thin slice of paper, now swap which eye is open and closed while not moving the paper.  Now you will see a partial side view of the paper, just as if you repositioned a camera.  Of course the distance between your eyes should be greater than the amount the sensor is able to shift so the effect is exaggerated.

Here is the effect demonstrated in practice.

This is the neutral position for the scene

Sensor shifted left

Sensor Shifted Right

Notice how even though the position of the element within the frame changes the relationship does not.  The ball appears to just visually touch the right hand edge of the blocks in all three.

Camera shifted right

Notice how in this instance the right side of the block is now revealed.

So the composition adjust of the Pentax K-7 allows you to modify the framing of a scene while preserving the relationship of the elements of the scene.  It also allows rotation of the sensor to further aid in composition.  This feature is found under “Rec. Menu 2”


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