焦距與景深

焦距越長，景深越淺。即使在相同光圈下，長焦鏡頭的可接受清晰範圍更窄。

拍攝距離

為了保持主體大小一致，使用長焦鏡頭時需要站得更遠。這會改變背景與主體的相對比例，使背景看起來更模糊。

壓縮效果 (Compression Effect)

長焦鏡頭會「壓縮」前景與背景的距離感，背景看似更近、更大，模糊也更顯著。

模糊圈 (Circle of Confusion)

長焦鏡頭在相同構圖下，背景的模糊圈會被放大，因此失焦效果更強烈。

Microsoft Copilot

Longer focal lengths (telephoto lenses) magnify the image more than wide-angle lenses. This magnification also enlarges the blur circles of out-of-focus objects.

Aperture: Changing your f-stop adjusts the angle of the light cone. A smaller aperture (higher f-number) makes the cone narrower, reducing the size of the blur circles and increasing the depth of field.

When your subject is very close to the lens, the light rays entering the camera are highly divergent. Even a tiny movement forward or backward from the focus point causes the light cone to widen rapidly. The Circle of Confusion grows large very quickly, resulting in a shallow depth of field.

For subjects far away, the light rays are more parallel. As you move away from the perfect focus plane, the light cone widens much more slowly.

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