blacknet
Contributor
Dome port optics.
Dome port optics is an area of great confusion and much cloudiness, so I am going to attempt to help clear the mud some.
Ever wonder why there are flat ports and dome ports? The flat ports are primary used for close macro work while dome ports are used for wide angle work. Lets define the some basic terms for later on.
Refraction
1. The act of refracting, or the state of being refracted.
2. The change in the direction of ray of light, heat, or the
Like, when it enters obliquely a medium of a different
density from that through which it has previously moved.
Angle of refraction is the angle which a refracted ray makes with the
perpendicular to the surface separating the two media traversed
by the ray.
Radial Distortion is an alteration in magnification from the center of
the field to any point in the field measured in a radial direction
from the center of the field. Some radial distortion is inherent
in most optical systems, but can be reduced by proper design.
Chromatic aberration is the lens aberration resulting from the normal
increase in refractive index of all common materials toward the
blue end of the spectrum. The change in image size from one color
to another is known as lateral color or chromatic difference of
magnification.
Ok, now that we have definitions we can work with its easier to understand whats going on.
Flat ports:
Flat ports simply cannot correct distortions caused between the water and air. This is like putting a pencil in a glass of water, we see the pencil take on an odd angle, the same holds true with your flat port. The more off center things becomes the more distortion we see. With wide angle lens on a flat port we start to see radial distortion, remember that lens are not flat surfaces. At the edges of a flat port we start witnessing chromatic aberration. Different color light waves travels at their own unique speed, the edge of the port separates these colors, i.e. like a prism. As these colors falls onto the focal point the colors will overlap and lead to un-sharpness. This is similar to shooting into an aquarium or out of a glass window. Also by using geometry we can prove that a 20mm lens in flat port housing will turn into a virtual 35mm lens on land.
Dome ports:
Dome ports correct the problems we see with a flat port but bring on their unique problems, in most cases they are easy to work around. Dome ports will add an additional element to your lens and change the focal distance, the object shape and the object size; this is referred to as the virtual image or the secondary focal point.
The distance, size and shape of this virtual image are determined by several things; the magnification ratio and index of refraction of the dome port material, water and air.
We can use a simplified formula that states
AF" = 3R - 4t
Where R = (r1 + r2)/2 and the dome thickness t = r1 - r2
This is out viewable infinity point. Our lens will need to have a close focus of AF inches from the front of the dome or less to take good sharp photos. However, many lenses can not focus this close, we must add a diopter to the lens and this will change the focal length and the minimum focus distance.
The downside to dome ports is the curved field of focus. If we look at the hyperfocal distance we learn as the f-stop changes so does the depth of field. If the depth of field is very shallow our curved focus point will begin to loose focus at the edges. If we stop down to count for this curvature our demand for light increases so we have a trade off, sharpness around the edges or improper exposure. The lesser of the two evils, in most cases, would be the edge sharpness. Coincidently this is another reason to not attempt to shoot full frame images. NOTE see my other article on composition and the cropping section.
As usual I dont expect everyone to fully understand what I wrote so please ask questions! There are no stupid questions. You can either make a post about it or send me a private message.
Dome port optics is an area of great confusion and much cloudiness, so I am going to attempt to help clear the mud some.
Ever wonder why there are flat ports and dome ports? The flat ports are primary used for close macro work while dome ports are used for wide angle work. Lets define the some basic terms for later on.
Refraction
1. The act of refracting, or the state of being refracted.
2. The change in the direction of ray of light, heat, or the
Like, when it enters obliquely a medium of a different
density from that through which it has previously moved.
Angle of refraction is the angle which a refracted ray makes with the
perpendicular to the surface separating the two media traversed
by the ray.
Radial Distortion is an alteration in magnification from the center of
the field to any point in the field measured in a radial direction
from the center of the field. Some radial distortion is inherent
in most optical systems, but can be reduced by proper design.
Chromatic aberration is the lens aberration resulting from the normal
increase in refractive index of all common materials toward the
blue end of the spectrum. The change in image size from one color
to another is known as lateral color or chromatic difference of
magnification.
Ok, now that we have definitions we can work with its easier to understand whats going on.
Flat ports:
Flat ports simply cannot correct distortions caused between the water and air. This is like putting a pencil in a glass of water, we see the pencil take on an odd angle, the same holds true with your flat port. The more off center things becomes the more distortion we see. With wide angle lens on a flat port we start to see radial distortion, remember that lens are not flat surfaces. At the edges of a flat port we start witnessing chromatic aberration. Different color light waves travels at their own unique speed, the edge of the port separates these colors, i.e. like a prism. As these colors falls onto the focal point the colors will overlap and lead to un-sharpness. This is similar to shooting into an aquarium or out of a glass window. Also by using geometry we can prove that a 20mm lens in flat port housing will turn into a virtual 35mm lens on land.
Dome ports:
Dome ports correct the problems we see with a flat port but bring on their unique problems, in most cases they are easy to work around. Dome ports will add an additional element to your lens and change the focal distance, the object shape and the object size; this is referred to as the virtual image or the secondary focal point.
The distance, size and shape of this virtual image are determined by several things; the magnification ratio and index of refraction of the dome port material, water and air.
We can use a simplified formula that states
AF" = 3R - 4t
Where R = (r1 + r2)/2 and the dome thickness t = r1 - r2
This is out viewable infinity point. Our lens will need to have a close focus of AF inches from the front of the dome or less to take good sharp photos. However, many lenses can not focus this close, we must add a diopter to the lens and this will change the focal length and the minimum focus distance.
The downside to dome ports is the curved field of focus. If we look at the hyperfocal distance we learn as the f-stop changes so does the depth of field. If the depth of field is very shallow our curved focus point will begin to loose focus at the edges. If we stop down to count for this curvature our demand for light increases so we have a trade off, sharpness around the edges or improper exposure. The lesser of the two evils, in most cases, would be the edge sharpness. Coincidently this is another reason to not attempt to shoot full frame images. NOTE see my other article on composition and the cropping section.
As usual I dont expect everyone to fully understand what I wrote so please ask questions! There are no stupid questions. You can either make a post about it or send me a private message.
