see Optical Density
Optical density is a measure used for filters to describe how much light of a blocked wavelength still passes the filter
ODn = 10^-n = 1/10^n
OD1 = 10^-1 = 1/10^1 = 0.1 = 10% light passes
OD2 = 10^-2 = 1/10^2 = 0.01 = 1% light passes
OD3 = 10^-3 = 1/10^3 = 0.001 = 0.1% light passes
OD4 = 10^-4 = 1/10^4 = 0.0001 = 0.01% light passes
For a camera of 256 intensity levels this equals 2.6 intesity values. This is not much stronger than the sensor noise.
OD3 entspricht 0.1 Prozent.
For a camera of 256 intensity levels this equals 0.26 intesity values. This is less than the sensor noise.
The higher the number the more expensive the filter.
higher quality filters have OD3
Filters ilters for outdoor use might need OD4 or higher to block too intensive sunlight.
so you can generater an OD6 filter by two OD3 filters. However, the optics should be prepared for the thicker glass then.
The optical path length is proportional to the time light needs to travel between two points
[table caption=”sample OPLs for BK7″ width=”400″ colwidth=”100|100|100|100″ colalign=”left|left|left”]
wavelength,index of refraction,phys. distance, OPL
orthographic lenses are a type of fisheye lenses
An orthographic / hemispherical fisheye, uses a parallel projection of a hemisphere onto a plane. The resulting image will obviously be circular. The widest view angle is 180 degrees. At +/-90 degrees extreme distortion will occur. Due to this heavy distortion it’s less often used than f-Theta / angular fisheye.