为啥长焦大光圈镜头贵

In optics,  the f-number is the reciprocal [倒数] of the relative aperture (the aperture diameter divided by focal length).

aperture: 1. narrow opening; 2. (size of an) adjustable opening for admitting light into a camera

都是f/1.8，200mm的是50mm的直径的4倍。

In some contexts, especially in photography and astronomy, aperture refers to the diameter of the aperture stop rather than the physical stop or the opening itself. For example, in a telescope, the aperture stop is typically the edges of the objective lens or mirror (or of the mount that holds it). One then speaks of a telescope as having, for example, a 100-centimeter aperture. Note that the aperture stop is not necessarily the smallest stop in the system. Magnification and demagnification by lenses and other elements can cause a relatively large stop to be the aperture stop for the system. In astrophotography, the aperture may be given as a linear measure (for example in inches or mm) or as the dimensionless ratio between that measure and the focal length. In other photography, it is usually given as a ratio.

The f-number of an optical system such as a camera lens is the ratio of the system's focal length to the diameter of the entrance pupil ("clear aperture"). It is also known as the focal ratio, f-ratio, or f-stop, and is very important in photography. It is a dimensionless number that is a quantitative measure of lens speed; increasing the f-number is referred to as stopping down. The f-number is commonly indicated using a lower-case hooked f with the format f/N, where N is the f-number.

The T-Stop, on the other hand, is an actual measurement of the light getting through the lens. While f/4 might meter differently on half a dozen different photography lenses, T4, on the other hand is identical across the board. At least, it is when it comes to quality known brands of cinema lenses. 拍电影的时候可能频繁更换镜头。人们不愿看到场景(”镜头“)忽明忽暗。

F-Stops vs T-Stops - What are they? and when do you use them?

Latin camera, meaning 'chamber' or 'room'

在一间黑暗的小屋朝阳的墙上开一个小孔，人对着小孔站在屋外，屋里相对的墙上就出现了一个倒立的人影。为什么呢？墨家解释说，光穿过小孔如射箭一样，是直线行进的，人的头部遮住了上面的光，成影在下边，人的足部遮住了下面的光，成影在上边，就形成了倒立的影。小孔成像

A pinhole camera is a simple camera without a lens but with a tiny aperture /ˈæpətʃə/ [窄孔; 隙缝; 光圈] (the so-called pinhole) - effectively a light-proof [防光] box with a small hole in one side. Light from a scene passes through the aperture and projects an inverted image on the opposite side of the box, which is known as the camera obscura [dark] effect.

A pinhole is a tiny hole. A keyhole is the hole in a lock that you put the key in. Keyhole surgery is a surgical technique in which the surgeon inserts the instruments through small cuts in the patient's body, using as a guide an image provided by equipment inserted into the patient's body.

The camera obscura or pinhole image is a natural optical phenomenon. Early known descriptions are found in the Chinese Mozi writings and the Aristotelian [亚里士多德] Problems.

Mozi (/ˈmouˈtsiː/; Chinese: 墨子; pinyin: Mòzǐ; Wade–Giles: Mo Tzu /ˈmouˈtsuː/; Latinized as Micius /ˈmisiəs/; circa [about] 470 – c. 391 BC), original name Mo Di (墨翟), was a Chinese philosopher who founded the school of Mohism during the Hundred Schools [学派] of Thought period (early portion of the Warring States period of c.475–221 BC).

The first known description of pinhole photography is found in the 1856 book The Stereoscope by Scottish inventor David Brewster, including the description of the idea as "a camera without lenses, and with only a pin-hole". According to inventor William Kennedy Dickson, the first experiments directed at moving pictures by Thomas Edison and his researchers took place around 1887 and involved "microscopic pin-point photographs, placed on a cylindrical shell".

The largest pinhole camera was created from an airplane hangar measuring 13.71 x 48.76 x 24.38 m (45 x 160 x 80 ft). The camera produced a photograph on canvas measuring 9.62 X 33.83 m. Aperture: 6mm. Exposure time: 35 minutes.

A hangar is a large building in which aircraft are kept. A hanger is a curved piece of wood or metal with a hook on top, used for hanging clothes on. 一个是a，一个是e.

六级/考研单词: optic, reciprocal, diameter, astronomy, physics, telescope, lens, mount, magnify, meter, brand, chamber, invert, insert, equip, portion, accord, cylinder, airplane, canvas, curve, clothe

永诺镜头YN100mm F2N | YN85mm F1.8 | camera