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4. Parallax

Looking through the eyepiece, a surveyor sees two images simultaneously: the sighted target and the crosshairs. The target may be hundreds of feet away while the crosshairs are a few inches in front of the surveyor. However, both images must come to focus on the back of the observer’s eye or a condition called parallax will exist.

If the image and crosshairs were in perfect focus, we would have the situation shown in Figure E-8.

Figure E-8
Image and Crosshairs Focused Perfectly


However, our eyes have a depth of field, like a camera lens, which together with our mind, compensates for varying focal distances of multiple objects. As long as the focal differences fall within the depth of field, the images will appear clear and in focus, Figure E-9.

Figure E-9
Image and Crosshairs Slightly Misfocused


Look out the window: objects at similar distances will appear in focus. That’s the combination of your eye optics and mind compensating for multiple objects at varying distances. Concentrating on closer objects causes farther ones go out of focus; concentrating on farther objects causes closer ones to get blurry. That occurs because not all object distances can be accommodated by the eyes’ depth of field.

If we take the situation in Figure E-9 and move the eye slightly, there will be an apparent movement of the crosshairs on the object, Figure E-10. That’s because the two images do not come to focus on the back of the eye so do not move together. This shift is called parallax.

Figure E-10
Apparent Movement When Eye is Shifted


If there is no apparent movement of crosshairs on the target, then there is no parallax - both images are perfectly focused. That's how we test for parallax: focus on the object, then shift your eye. No movement, no parallax.

When using instruments, we don’t want parallax present because it can introduce errors in our sightings or readings. How do we eliminate parallax? The simplest way is:

(1) Adjust the main focusing knob until all images are out of focus.

(2) Using the crosshairs focusing ring, bring the crosshairs into a sharp clear image.

(3) Focus on an object on using the main focusing knob and check for parallax - it should be gone.

By the way: if when you first look through the telescope and can focus on an object but cannot see the crosshairs, use the crosshair focus ring to bring them into view. Then proceed with the paralax clearing method.

Parallax is affected by the geometry of the observer's eye. If a glasses-wearing surveyor clears parallax with glasses on, she may experience parallax with her glasses off and vice versa. Different people will have different parallax conditions: one surveying student checking another's reading may get a different value due to parallax difference. Before taking the first reading of the day, the instrument operator should check for and remove parallax. Every time the instrument operator changes, parallax should be cleared.