Finding rectilinear wide-angle lenses for full-frame Nikon SLR cameras that work well underwater is a challenge. In order to understand why, it is important to understand a little bit of dome port theory. 

为全画幅单反寻找一款合适在水下拍摄的无畸变直线广角镜头是非常有挑战性的事情。 为什么会这样说呢，我们先了解一下光学理论。

When a lens is placed behind a dome, and the dome is then immersed in water, the dome generates a virtual image just in front of the port itself. This virtual images is what is actually captured onto the camera’s sensor. As the dome is curved, so too are the corners of the virtual image.

当镜头放在镜头罩后面，然后随镜头罩一同浸入水中时，镜头罩前端生成一个虚像。这个虚像实际上是捕捉到了相机传感器上的成像。由于镜头罩(玻璃罩/亚克力罩)表面是弯曲的，所以虚像的角落也是弯曲的。

This causes two primary effects:

1.The lens must focus on this virtual image (rather than the actual scene), which means that it must be capable of very close focus.

2.As the virtual image is not flat, but is being recorded onto the flat sensor’s surface, it can be difficult for there to be sufficient depth of field to allow for the “closer” corners and the “further away” center of the image to both be in focus.

这造成了两个主要的影响:

1.镜头必须对虚像对焦而不是对实物，因此这意味着它必须具备近距离对焦能力。

2.由于虚像不是平面的，而是被记录到平面传感器的表面上，因此很难有足够的景深来允许“更近”的角和“更远”的图像中心都对上焦。

Cameras with large sensors have a considerably less inherent depth of field (DOF) than smaller ones. Hence people will often describe an image as having soft corners, effectively saying that the lens and dome combination did not have sufficient DOF to get both the center portion and the corners in focus. DOF is controlled by aperture, and the smaller the aperture, the greater the depth of field. The center is normally in focus, as the autofocus points tends to be concentrated around the center of the sensor, so it is the corners that suffer. In addition, the camera’s viewfinder does not show a DOF preview, so it cannot be seen until the picture has been taken.

物理性来讲，大尺寸的传感器的摄像机的固有景深（DOF）比较小的摄像机要小得多。因此，人们在评价一张照片角落不够清晰，实际是在说镜头和镜头罩搭配没有让对焦中心和角落有足够的景深。景深由光圈控制，光圈越小，景深越大。中心通常是对焦清晰的，因为自动对焦点通常集中在传感器中心，所以角落会有一定的影响。此外，相机的取景器不显示景深，因此在拍摄照片时无法看到景深情况。

Given that this compromise exists, there are additional factors that affect corner sharpness. The first of these is the diameter of the dome and secondly, linked directly to this, is the curvature of the dome. Optical theory says that larger domes generate a virtual image that is further from the lens. Given that most domes are spherical, the amount of curvature is hence more as it approached the edges of the dome than it is in the center. Hence, larger domes tend to also produce a less curved virtual image than a smaller dome as the lens only “sees” the center and hence less curved part of the dome.

鉴于这种折中情况，还有其他因素会影响边角的清晰度。第一是水下镜头罩（玻璃罩/亚克力罩）的直径；第二个是镜头罩的曲率。光学理论认为: 更大的镜头罩会产生离镜头更远的虚像。由于大多数镜头罩是半球形的，因此，曲率数值最大是在镜头罩的边缘，而不是在最中心。因此，较大的镜头罩对比小一点的镜头，可产生完全变形更小的虚像，因为镜头只“看到了”中心，更是没有太变形的中心。

   这个图表（是由Alex Mustard）提供，显示了虚像到镜头的距离，及镜头罩大小之间的关系

It is possible to use a “less curved” dome to try and reduce the curvature of the dome, Typically, they are actually sections of a large dome cut down. Whilst this can help reduce the curvature, it is important to remember that the diameter of the port determines the position of the virtual image. Hence, although the curve of the virtual image will be less, these domes are typically smaller in diameter so the image will be closer to the lens and it still may not be able to focus.

有可能使用一个不太弯曲的镜头罩来尝试减少曲率，通常，他们实际上是一个大镜头罩消减的一部分。虽然这有助于减少曲率，但镜头罩的直径大小决定了虚像的位置。因此，尽管虚像的曲线会更小，但这些镜头罩的直径通常更小，因此虚像会更接近镜头，有无法对焦的可能性。

 

Lens designs vary. The position of the lens’s elements are fixed with a prime lens, but move in a zoom lens. This movement changes the distance between the lens’s optics and the virtual image. Hence some zoom lenses will work better than others. Actually, this is also true with prime lenses too, especially given that close focus ability is inherent (or absent) by design rather than by type or focal length. There is no in evidence in underwater imaging that suggests that prime lenses inherently perform any better than zooms or will necessarily allow the use of smaller dome ports. It is all down to the individual lens’s design.

镜头的设计有所不同。镜头元件的位置在定焦镜头钟是固定的，但在变焦镜头中可以移动。这个运动改变了镜头的光学原件和虚像之间的距离。因此，一些变焦镜头将比其他镜头可以更好的工作。实际上，对于定焦镜头也是如此，特别是考虑到近距离对焦能力是固有的，而不是由镜头的类型或焦距决定的。在水下成像技术中，没有任何证据表明定焦镜头在本质上比变焦镜头表现得更好，也没有证据表明定焦镜头一定要使用小一点的镜头罩。这完全取决于每个镜头的设计特征。

Traditionally, better/more expensive lenses tend to have larger apertures. This is due to them normally having larger lens elements. For underwater use behind a dome, this is somewhat of a “red herring” as large apertures also have a very shallow DOF and corner sharpness will be adversely affected. Underwater photographer should try and avoid the trap of assuming that an f1.8 lens will definitely outperform an f4 one.

常理来说，更好/更贵的镜头往往有更大的光圈。这是由于它们通常有更大的光学元件。在水下使用镜头罩时，这会变成一个陷阱。因为更大的光学元件也同时具备非常浅的景深，图像角落的锐利度在水下镜头罩后也会有不利的影响。水下摄影师应该尽量避免这样的陷阱，即认为F1.8镜头肯定会优于F4镜头。

有些主体的角落不需要太锐利

Lastly some types of subject, while benefitting from the rectilinear look, do not need sharp corners! A good example are bigger creatures in the blue (or green) where there is no detail in the corners that draws the viewers eyes away to notice any softness.

最后，水下有些物体不需要非常清晰的角落锐利度！ 一个很好的例子是在海洋中较大的游动生物，其角落没有任何细节需要来吸引观众的目光，从而注意到其最终锐利度。

文章译自 ： 

Review: Nikon FX wide-angle lenses   by Adam Hanlon

https://wetpixel.com/articles/review-nikon-fx-wide-angle-lenses