你希望容纳到背景里的景色越多，得到平均曝光就越困难，因为焦距越短，进入机身的环境光就越多。
The more scenery you wish to include in your background, the tougher it will be to get an even exposure because of all the ambient light you're letting into your camera at wider focal lengths.
这在任何光圈和焦距上都适用，只不过光圈越小、焦距越短，景深越大，你能够拍摄的清晰长度就越大。
This works for all apertures and focal lengths, but the smaller the aperture and the shorter the focal length, and the greater the distance you shoot at, the greater the depth of field.
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