**Posts:**90**Joined:**Fri Mar 17, 2017 10:12 pm

Mario Belamaric wrote:I thought diffraction steps in at higher f-stops (11 and up).

Rayleigh's Criterion is a mathematical way of putting a number on the smallest resolvable detail when an imaging system is limited by diffraction. For 4K or HD, F/8-F/11 could be okay. The 12K sensor pixel pitch is only 2.2μm. It can't resolve 12K fully if the smallest possible light spot falls across more than 2-3 pixels. Diffraction sets a resolvable limit.

Rayleigh's Criterion:

θ=1.22λ/D

Where θ is in radians, λ is wavelength of green light; .55μm , D is pixel pitch; 2.2μm.

The diameter of the airy disk is approximately 2-3x the pixel pitch. This is because the pixel pitch corresponds to the spacing between the center of two adjacent pixels, the smallest resolvable detail, and the diameter would cover at least two pixels. F/stop is 1/2 the inverse of numerical aperture (NA), because θ is in radians.

We write this as the smallest resolvable f/stop:

f/stop = 2D/2.44λ

That is, two pixels covered and 2 radians, make 1 numerical aperture, the inverse of which is the f/stop.

For the UMP12K, that's 2 x 2.2 / 2.44 x .55 = f/3.3

For the Cine12K, that's 2 x 2.9 / 2.44 x .55 = f/4.3

For the Pocket 6K that's 2 x 3.8/ 2.44 x .55 = f/5.7

For the UMP4.6G2 that's 2 x 5.5/ 2.44 x. 55 = f/8.2

For the BMCC6K, that's 2 x 6.0 / 2.44 x .55 = f/8.9 (Winner, Chicken Dinner)

If we accept 3 pixels covered by the airy disk instead of just two, we can stop down to get:

For the UMP12K, that's 3 x 2.2 / 2.44 x .55 = f/4.9

For the Cine12K, that's 3 x 2.9 / 2.44 x .55 = f/6.5

For the Pocket 6K that's 3 x 3.8/ 2.44 x .55 = f/8.5

For the UMP4.6G2 that's 3 x 5.5/ 2.44 x. 55 = f/f12.3

For the BMCC6K, that's 3 x 6.0 / 2.44 x .55 = f/13.4

As you can see, diffraction sets the limit on the smallest physical detail. As the pixel pitch gets more densely packed, the sensor will out-resolve the lens at smaller f/stops openings.