平均焦距
Mean focal length
以上为机器翻译结果，长、整句建议使用 人工翻译 。
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.
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