From: Sandy King ^lt;*[email protected]*>

Date: 08/08/04-05:53:21 PM Z

Message-id: <a0602040dbd3c6d3e1564@[192.168.2.4]>

Date: 08/08/04-05:53:21 PM Z

Message-id: <a0602040dbd3c6d3e1564@[192.168.2.4]>

Let me send this again with a proper subject line.

I am at something of a impasse with the following problem and hope

that someone with a better understanding of mathematics can provide

some assistance.

The issues is this. I am trying to figure out how to convert angular

measurements of point discrimination to linear measurements for the

purpose of creating custom Circle of Confusion tables for minimum and

critical thresholds of human vision for 1 minute of arc and 25

seconds of arc. I would like to create the tables for a common

observing distance of 25 cm, or 10 inches.

John B. Williams, in Image Clarity: Theory of High-Resolution

Photography, gives the following formula for the conversion of

angular to linear measurements.

l = Dtana

Where l is the linear measurement, D is the observing distance and a

is the angular size.

He provides the answer for this equation for a distance of 25 cm as a

linear measurement of 0.07mm, or 70 microns, and continues by noting

that the tangent of one minute of arc is roughly 0.00029. I have

worked this problem backwards several ways and still have not been

able to figure out how he determined that the tangent of one minute

of arc is 0.00029.

Help appreciated from all competent persons.

Sandy King

Received on Mon Aug 9 12:16:17 2004

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