Why shoot test charts at 45 degree angle instead of 90 degrees

[sanath_perera]

<p>When I shoot <a href="http://focustestchart.com/focus21.pdf">focus test chart </a>at 45 degree angle, the photos show back focusing but at 90 degrees there is no back or front focusing.</p>

<p>Why does instructions require shooting at 45 degree angle when in real life you would shoot at a 90 degree angle? What is the science behind it?</p>

<p> </p>

[William Michael]

<p>The Test Chart to test for Front / Back Focussing must be shot at an angle <strong>other than a right angle</strong>.<br>

If the test chart were shot at a right angle then there would be no DEPTH to the chart: the whole chart would be an "equal" distance from the camera's Focal Plane and parallel to it.<br>

If there were no DEPTH to the chart, the test could NOT ACCURATELY SHOW if there were Front Focussing or if there were Back Focussing: it could only show an error in focussing.<br>

If there is no DEPTH to the Chart the test could NOT RECKON to what degree (what distance) was any error.</p>

<p>45° is chosen, because it is easy to use.</p>

<p>WW </p>

[sanath_perera]

<p>Thanks for your replay.</p>

[michael_s10]

<p>I actually like the method used by the DataColor lens cal - the focus target is parallel to the sensor of the camera, but there's a ruler angled at 45 degrees off to the side. </p>

<p>There are a couple of tests that I can run with this setup.</p>

<p>1. I can test to make sure that there isn't a generic back or front focus issue that is independent of focus sensor placement by focusing on the larger parallel target.</p>

<p>2. I can then use the angled target to check individual focus sensors (on rare occasion it's possible to have a focus sensor as it appears in the view finder to not correlate exactly with the location that the camera is using to focus).</p>