最近看了一部關於吉姆?弗雷澤鏡頭的紀錄片，描述此鏡頭有極大的景深，可以同時對焦極遠和極近的物體，想具體瞭解一下這種鏡頭到底是如何做到的。

超景深百萬像素的視頻監控鏡頭

…昏過去。

你們能認真的把專利看完麼，答案不都在那裡了

The primary lens is a very wide-angle lens...it has a very large depth of field...throws a wide image circle...on a pellicle element (which takes the normal place of film or a
sensor) ...This second lens can zoom in on the projected wide-angle image, giving an apparently longer focal length ...

他使用了一個具有較大成像圓的大廣角鏡頭，將「具有較大景深」的影像投影在一張薄膜上，然後再用一個變焦鏡頭去捕捉薄膜上的影像，來實現變焦並控制光圈。

這道理簡單到不能再簡單，因為前面的廣角鏡頭完成了3D-&>2D的投影轉換，接下來的變焦實質是對一個2D平面的翻拍，所以不存在景深的問題。通過後面變焦鏡頭的「放大」，也可以獲得類似長焦的效果，但由於是對2D影像的放大，所以景深沒問題。

至於 @jonny lee 提到的稜鏡，專利裏也說了：

There is also a system of prisms in the system that allows the lens to be aimed without altering the cameras orientation.

就是為了在不變換機位的情況下讓鏡頭可以指向別處，同景深沒什麼關係

不是我說的輕巧，原文就是這麼表述的，「Theres nothing too very special or revolutionary about that.」，「By itself, that has also been done before,」，「Again, thats not novel in itself.」
同看了這個紀律片，感覺很震撼，潛在的價值太大了。

翻牆搜到的一段回答：

The detailed explanation is available in the patent (US5727236), which is available at Google Patents, among other places.

A simplified explanation is that he developed a system that uses two lenses. The primary lens is a very wide-angle lens, and like all wide-angle lenses, it has a very large depth of field for any given aperture (and the aperture is kept fairly large in Fraziers design). Thats what gives the deep-focus "magic"; a very wide-angle lens. Theres nothing too very special or revolutionary about that.

This particular wide-angle lens, though, also throws a wide image circle. In other words, it makes a very big picture for its focal length. Again, nothing spectacular there; covering a large film frame with a short focal length lens is something optical designers have been doing for quite some time.

The image that the wide-angle lens throws isnt used directly by the camera, though. Its focused on a pellicle element (which takes the normal place of film or a sensor) and viewed through a separate lens behind the wide-angle lens. This second lens can zoom in on the projected wide-angle image, giving an apparently longer focal length and allowing the aperture to be varied as necessary for photography/filming exposure while maintaining essentially the "baked-in" depth of field of the primary wide-angle lens. By itself, that has also been done before, as explained in the "prior art" section of the patent.

There is also a system of prisms in the system that allows the lens to be aimed without altering the cameras orientation. That part of the system is not too very different from an adjustable periscope. Again, thats not novel in itself.

Its the entire system taken together — using a large image circle from a very wide angle lens at an optimal wide aperture for that lens, taking a picture of that lenss picture through a separate lens, and having a system for articulating and orienting the two-lens system — that was patentable and award-winning. (However, it seems he blew the patent protection, at least in some jurisdictions, by poor choice of promotional methods. See the Wikipedia article for more details and links.)

我猜測是運用光學折射公式趨向極限區域的特性運用，直白說：如何讓有一定角差的兩根光線射入界面同一點，透過的兩根折射線的角差越小，也就實現了聚焦的物像點的間距縮小，再對物像放大成像，每個透鏡都要遵循以上規律

剛看了紀錄片，的確感到震撼。

應用到安保監控系統，景深範圍可以達到200米。。。。。。

我試了一下，即使用一個超廣角鏡頭，也不可能獲得一個大光圈的大景深照片。而且，從透鏡的遠離來看，透鏡只有一個焦點，離開焦點以外的圖像必然會不清晰。所以不存在什麼3d轉2d就會清晰的事情。既然2d本身不清晰，再怎麼去用變焦拍，也不會清晰。那麼弗雷澤產生如此大景深的鏡頭都是怎麼弄出來的？困惑
這個節目我也看了，後來想了想覺得應該是先用一個大景深的鏡頭把近距離和遠距離的物體成像於像面，然後用景深小的鏡頭拍這個像面的特寫，這樣應該可行吧？
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