Portrait mode chooses a large aperture (a small 'f' number) which will make the depth of field (the amount of your shot in focus) smaller.

人像模式用一个大光圈(f值小)，而让景深(清晰对焦的那部分)变小。

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The f-stop, otherwise known as the aperture, determines your depth of field (the area in focus) and how much light can pass through the lens to reach the sensor.

f制光圈(或称光圈)决定相机的景深(对焦区域)以及有多少光能通过镜头到达传感器。

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F is a gradient field. Why?

为什么要说F是一个梯度场？

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The F-35 has already seen plenty of field time in the US, with more than 500 flights already in 2012, and it's set to make its way to the UK armed forces next week and the Netherlands later this year, but while the aircraft is quite familiar to the pilots tasked with flying it, the public hasn't had an opportunity to experience Lockheed's latest airborne warrior.

ENGADGET: Lockheed Martin F-35 Lightning II stealth fighter cockpit demonstrator hands-on (video)