Lyagukh
lvl.2
Flight distance : 68179 ft
Ukraine
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Let4ik Posted at 1-8 19:32
If the distance from the front lens to the dome a few millimeters, then there is no problem. When the distance is longer, distortions begin. The image with a flat dome will be 33% narrower than the real one. The spherical dome refracts light without distortion.
It depends not as much on the distance you mentioned but on the lens focal distance. Consider all three kinds of distortions:
- refraction will in effect increase the focal length by approximately 25% - you see the same effect through your mask (and in a typical mask there is more than a few millimeters between your eye and the glass).
- radial distortion (pincusion) will be present with flat port and less so with spherical (or rather hemisperical) dome. It's effect is really noticeable for wide lenses unless we are talking about professional use for which OP is not suitable anyway. Also it can be corrected to some extent in post with one of several plugins for Photoshop
- chromatic aberration - roughly the same logic as for radial distortion
On the other hand hemispherical domes introduce problems with focusing. The port itself works like an additional lens element (and that only underwater) which creates it's own "virtual image". And you should focus on that virtual image, not the real thing. That's one reason why original underwater housing for GoPro Hero had poor underwater performance because GoPro has fixed focus and those housings had dome ports. Many divers including yours truly were forced to replace the dome with flat one. Even if your camera allows focusing, image in the corners may still be out of focus with dome ports meaning you want to increase DOF by stopping down your lens - and underwater there is never too much light
All in all, unless you want to shoot with really wide lens, flat dome is IMHO acceptable for amateur underwater use. |
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