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Addendum
Page 248
There are various opinions whether a chart should be viewed at 45° from the lens
centerline axis, or at 30°, or any angle or even flat. The reality is that it should give
good results at any angle between 30° and 60°. Chart-1 and Chart-2 were design for
viewing at 45° from the lens’ centerline axis. The measurements on the viewing
surface were corrected for accurate reading when viewed at a 45° angle. The Charts
could have been designed for viewing at 30°, giving more viewable depth of field
for both front and back. However, I opted for the 45° because the charts could fit on
a 8 ½” x 11” sheet of paper (Letter size).
I chose a focusing area shaped as a circle. The bottom half is black and the top half
is white. When viewed perpendicularly or flat, it has an oval shape, because the
chart is meant to be viewed at 45°. When viewed at 45°, the focusing area appears
as a perfect circle. That also helps verifying that the lens is at a 45° angle..
Chart No. 2 viewed perpendicularly or flat.