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The Sea & Sea Internal Correction Lens is a unique optical tool designed to correct image abberation and distortion caused on wide angle lenses shooting behind a dome port. This lens attachs directly to the lens on your camera and is designed for Full Frame cameras behind large dome ports.
When shooting wide angle underwater it is common practice to stop down the aperture to F8 or higher in order to product the best overall photo with sharp corners. It is commonly known that larger (smaller number) aperture causes distortion and blurring at the edges / corners of an underwater photo due to refration differences in water vs air. This lens has been carefully designed to correct that difference and improve blurring and distortion. Curvature & distortion are improved by approximately 2 F-Stop values when using the Internal Correction Lens.
These lenses are available in either 77mm or 82mm sizes, and screw directly onto the front threads of your camera lens (like a filter). They are designed to work with large diameter dome ports (8" and up).
Taken by professional underwater photographer Andy Sallmon who says, "Here’s a great white shark image shot at F/7.1 with the Internal Correction lens and a Canon 16-35mm F/2.8 II. The shark's nose is at the edge of the frame where aberration is usually terrible with this lens at any f-stop wider than F/11. Even though I would never have tried this normally using this lens at F/7.1, the edge sharpness here on the shark's nose is maintained very well by the internal correction lens. So on this lens, F/7.1 = the same edge/corner sharpness that would have required F/11 without the internal correction lens.
The following images have been taken with the same camera, lens, housing set up and camera settings, the only difference between each comparison would be the removal of the Internal Correction Lens.
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.
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.
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. We recommend the use of these lenses to all serious underwater photographers.
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