Questions On Focimetry

[Deepak]

Can any member of tghis optiboard university explain to my why the image of something viewed through a prism is displaced toward the prism apex,but when a prism is placed on focimeter,the target image is displaced towardthe prism base/?
Secondly,why is cyliner axis is read at higher reading on focimeter when axis should be at loverpower orweaker mearidianm.
Deepak

[chip anderson]

Deepak:

Can't answer it all but the higher the power the more critical (and accurate) a lensometer (focometer for you limeys) is. Therefore check axis on the highest distance power read.

Chip

[Darryl Meister]

Essentially, because of the eyepiece of the device, you are seeing an inverted image of the target, which is why the image appears to be displaced towards the base of the prism. The eyepiece is basically a keplerian telescope, which inverts images.

Also, for your cylinder question... A typical focimeter target has two sets of crosshairs, which are 90 degrees apart (crossed at right angles). When you bring one crosshair into focus, you are actually neutralizing the power through the other crosshair, 90 degrees away. This occurs because a line (crosshair) aligned along the axis meridian of a cylinder is expanded width-wise by the blur produced by the cylinder meridian, since no error or magnification occurs along the sphere (or axis) meridian. Consequently, the line becomes “fatter” and blurred looking. However, a line (crosshair) aligned along the cylinder meridian is expanded length-wise by the blur produced by this meridian. Consequently, the line does not become blurred looking, since it is only being "stretched" along its length.

Best regards,
Darryl