Coona Skies


Airy Disc Spot Sizes

The following table displays the spot size in microns for different f-ratios at various frequencies of light. The spot size is the diameter of the Airy disc. It indicates the smallest size a point source such as a star image will produce on your image detector (film, CCD, etc.). You can use this to work out if your imaging system is optimized for resolution. Of course you will also need to know the film grain or pixel size on your imaging detector.

The formula used is (spot size) = 2.44 * (wave length of light) * (f-ratio of optical system).

Please note that the above is the Rayleigh criterion and is theoretical. In reality, the best resolution achievable by your imaging system depends on both its quality as well as items that could be beyond your ability to control such as atmospheric conditions. The Rayleigh criterion is empirically derived and indicates the optical resolution acieivable with the human eye. The spot sizes indicated in the table are twice the Rayleigh criterion.

Spot size in microns for different f/ratio optical systems at different wavelengths of light.

f/ratio 300 nm 400 nm 500 nm 550 nm 600 nm 650 nm 700 nm 800 nm 900 nm 1000 nm
f/1 0.73 0.98 1.22 1.34 1.46 1.59 1.71 1.95 2.20 2.44
f/2 1.46 1.95 2.44 2.68 2.93 3.17 3.42 3.90 4.39 4.88
f/3 2.20 2.93 3.66 4.03 4.39 4.76 5.12 5.86 6.59 7.32
f/4 2.93 3.90 4.88 5.37 5.86 6.34 6.83 7.81 8.78 9.76
f/5 3.66 4.88 6.10 6.71 7.32 7.93 8.54 9.76 10.98 12.20
f/6 4.39 5.86 7.32 8.05 8.78 9.52 10.25 11.71 13.18 14.64
f/7 5.12 6.83 8.54 9.39 10.25 11.10 11.96 13.66 15.37 17.08
f/8 5.86 7.81 9.76 10.74 11.71 12.69 13.66 15.62 17.57 19.52
f/9 6.59 8.78 10.98 12.08 13.18 14.27 15.37 17.57 19.76 21.96
f/10 7.32 9.76 12.20 13.42 14.64 15.86 17.08 19.52 21.96 24.40
f/11 8.05 10.74 13.42 14.76 16.10 17.45 18.79 21.47 24.16 26.84
f/12 8.78 11.71 14.64 16.10 17.57 19.03 20.50 23.42 26.35 29.28
f/13 9.52 12.69 15.86 17.45 19.03 20.62 22.20 25.38 28.55 31.72
f/14 10.25 13.66 17.08 18.79 20.50 22.20 23.91 27.33 30.74 34.16
f/15 10.98 14.64 18.30 20.13 21.96 23.79 25.62 29.28 32.94 36.60
f/16 11.71 15.62 19.52 21.47 23.42 25.38 27.33 31.23 35.14 39.04
f/17 12.44 16.59 20.74 22.81 24.89 26.96 29.04 33.18 37.33 41.48
f/18 13.18 17.57 21.96 24.16 26.35 28.55 30.74 35.14 39.53 43.92
f/19 13.91 18.54 23.18 25.50 27.82 30.13 32.45 37.09 41.72 46.36
f/20 14.64 19.52 24.40 26.84 29.28 31.72 34.16 39.04 43.92 48.80
f/21 15.37 20.50 25.62 28.18 30.74 33.31 35.87 40.99 46.12 51.24
f/22 16.10 21.47 26.84 29.52 32.21 34.89 37.58 42.94 48.31 53.68
f/23 16.84 22.45 28.06 30.87 33.67 36.48 39.28 44.90 50.51 56.12
f/24 17.57 23.42 29.28 32.21 35.14 38.06 40.99 46.85 52.70 58.56
f/25 18.30 24.40 30.50 33.55 36.60 39.65 42.70 48.80 54.90 61.00