[Neroli]

根據dvfever果本書, 做到淺景深有兩個方法
一是用大光圈, 二是zoom到最遠
另外, f值=鏡頭焦距/光圈口徑, 而f值越細,即光圈越大,
換言之,光圈大小是同鏡頭焦距是成反比的
那麼, 那兩個方法就不是茅楯的嗎?
因為要做到大光圈, 焦距應該是越細越好

[hytong]

Because of the smaller CCD/CMOS sensor size. Do not expect good depth of field control with consumer level DC.

f number is a ratio. Using the same shutter speed, a 35mm lens @ f2.8 allows exactly the same amount of light passes through as of a 400mm lens @ f2.8.

光圈 is the actual opening (光圈口徑). For the same focal length. The smaller the f number the larger the opening.

On a (variable-aperture) zoom lens, with the fixed max. physical opening. F number is smaller at the wide angle end larger at the tele end. This is your case.

Assuming lens-to-subject distance remains constant., at the same aperture, wide angle offers more depth of field than a tele.

Believe it or not, using the same f number, for the same object size, depth of field remains the same. The only difference is the perspective (viewing angle).

http://www.luminous-landscape.com/tutorials/understanding-series/dof.shtml

Because of minimum lens diameter requirement. In terms of optics, it is easier (both cost & labour - less expensive) to make a large aperture wide angle lens (e.g. 35mm f2) than a large aperture tele lens (e.g. 400mm f2.8).