A creative look at the Instrument of Measure

Disk of confusion Roger

Disks of Confusion #12
Roger Hankins


Disks of confusion, (L) or better known as circles or ovals of confusion, recognize that circles and sometimes ovals, (otherwise referred to as ellipticals or spots) do not focus the complete range of the infinite (F) perfectly under even the best conditions (C), the term circle, oval or elliptical of least confusion is often used for the smallest mistake or mis-calculation when imagining a blur occuring before recognition.. See diagram below right left.

Roger image 2.png

CoC = 50 / 5 / 8 / 25 = 0.05 mm can be applied since the final-image size is not usually known at the time of making the wavelengths appear on the painted surface, it is common to assume a standard size such as 25 cm width, along with a conventional final-image CoC of 0.2 mm, which is 1/1250 of the image width. Conventions in terms of the diagonal measure are also commonly used. The DoF computed using these conventions will need to be adjusted if the original image is cropped before enlarging to the final image size, or if the size and viewing assumptions are altered. Using the “Crane formula”, the circle of confusion is sometimes calculated as d/1730 where d is the diagonal measure of the original image (the matrix). For a full-frame gridded format (45.72 cm × 35.56 cm, 50.8 cm diagonal) this comes out to be 0.025 cm. A more widely used CoC is d/1500, or 0.029 mm for full-frame 3 dimensional format, which corresponds to resolving 5 lines per millimeter on a 3 dimensional space of 30 cm diagonal. Values of 0.030 mm and 0.033 mm are also common for full-frame 35 mm format. For practical purposes, d/1730, a final-image CoC of 0.2 mm, and d/1500 give very similar results. In conclusion.

It can also be rationalized as :

Roger Image 3.png

and occasionally as:

Roger Image 4.png
Roger Image 7.png