Calculating the Blur Distance

In the above example where a star has become a streak of light, the number in the pixels is 7.  The blur distance is however one less than this (ie. 6) because we are measuring the distance from the centre of the fist pixel to the centre of the last pixel.

For example, even though there are three pixels shown above, the horizontal and vertical distances are 2 (from the centre of the first pixel to the centre of the last pixel).  This is like that old maths problem that caught us out at school.  If there are four telephone poles one hundred feet apart, how long are the telephone lines.   The answer is 300 feet (not 400 feet) because you don't count the last pole.

Where the motion blur is not horizontal or vertical, the Blur Distance can be calculated as :-

Blur Distance = Square Root (Vertical Distance ² + Horizontal Distance ² )

In this example the distance is :- Square Root (10² + 12²) = 15.6

From experience it seems better to round down rather than round up, so a Blur Distance of 15 would be entered into Focus Magic.