Light travels in a straight line and its speed is affected, depending on the medium (air, glass, water) it is travelling through. When light travels from one medium to another (i.e. air to water) it is deflected (or bent) as the speed is affected. This is known as the Optical Refractive Index of Light.
Lenses are designed to work in air, so images taken underwater will inevitably be affected as light reflected from the subject travels through water, the dome port material, air inside the housing and the camera lens before reaching the camera’s image sensor. Also, when using a wide-angle lens and dome port, light is entering from a wide area and some enters the lens at a steep angle.
As a result, the wider the aperture is opened (smaller f-stop number), the more the image edges and corners will be blurred and distorted. This is known as aberration or distortion. In order to minimize image aberration and distortion, the aperture cannot be used wide-open. A smaller aperture (larger f-stop number) has to be used to obtain a more forgiving, deeper depth-of-field. Also, the iris on zoom lenses moves in/out slightly as the focal length changes and this also affects the sharpness of the image.
Simply put, internal correction lenses correct refracted (bent) light to that similar to light in air. The Internal Correction Lens is fitted to the camera lens and rectifies refracted light caused by the dome port and water.
Internal Correction Lenses have been designed to improve the resolving power at the edges and corners of images (to suppress field curvature and distortion) by a value of 2 f-stops. They make it possible to realize images with sharp edges and corners. How aperture values affect images when the values are different by two f-stops.
In the following are comparisons, the cameras settings are exactly the same except for aperture settings;
Image aberration can be corrected by controlling the amount of light entering the lens. In other words, aberration can be minimized by using a smaller aperture (higher f-stop). At the same time, the smaller the aperture used, the darker the image will become. The internal correction lenses rectify image aberration and distortion by a value of two f-stops.
The internal correction lenses are categorized as ‘convex meniscus lenses’ and compatible camera lenses compatible are limited.
There are two Internal Correction Lenses available: 77mm and 82mm diameters (refer to our exclusive-use system chart).
Camera: D810, Port: Fisheye Dome Port 240 + Extension Ring 40L + Extension Ring 20L
[Other common data] Camera used: EOS 5D MarkII, Port used: Fisheye Dome Port + Intermediate Ring 40 + Intermediate Ring 30L
When designing the lenses, SEA&SEA especially put weight on correcting curvature of field where corners and edges of an image are out-of-focus when the centre of image is in focus (and vice versa). The lenses have also been designed to correct distortion in which image corners and edges are stretched or compressed. The lenses have been designed to minimize field curvature and distortion.
The internal correction lenses are made of AR multi-coated glass which boasts high optical transmission. Because of this, the lenses do not adversely affect images at all. In almost all situations, the internal correction lenses correct field curvature and aberration and help reproduce images in detail. SEA&SEA recommends use of the lenses to all serious underwater photographers.