When viewing extended objects, the apparent brightness seen in the eyepiece is a function only of aperture and magnification, it is not related to focal ratio.
当观察扩展光源时，目镜中所看到的亮度只跟孔径和放大率有关，它跟焦比并没有关系。
A scene reproduced on cloth transparencies with various lights shining through the cloths to produce changes in effect, intended for viewing at a distance through an aperture.
透明幕上的画面：从孔中窥视到一段距离外部透明画面上的风景，通过变动透过画面的光线，可获得不同的画面效果。
Figure 6-9 shows the oven with the small aperture for viewing the radiation leaving the enclosure.
图6 - 9表示一个空腔，上面开著一个小孔，用来观察空腔发出的辐射。
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