As with telescopes, the focal ratio of a lens is defined by the focal length divided by the aperture. A lens with a focal length of 200mm and an aperture of 71.4mm has a focal ratio of f/2.8. Focal ratio is represented by f-number, in this case the f-number is 2.8. The f-number of a lens is also called the f/stop. As we discussed in the previous section on aperture, camera lenses can change their effective aperture by closing down the diaphragm, and stopping down the lens. It is called "stopping down" because as the diaphragm or iris closes down, more light is prevented, or stopped, from reaching the sensor. As the lens is stopped down, the focal ratio gets "slower". In this case slower means it is slow to gather as much light as a larger aperture. A larger aperture is said to be "faster". You can take a picture faster with a larger aperture and use a shorter exposure. One thing that can be confusing about focal ratios and f-numbers is that a smaller number means a larger aperture for a given focal length. For example, for a 300mm lens, a focal ratio of f/4 means a larger aperture than a focal ratio of f/8. 8 is a larger number than 4, but it means a smaller aperture. The "faster" the lens' focal ratio, the larger the working aperture. A larger aperture collects more photons of light in a given amount of time. This is a good thing when photographing faint deep-sky objects. But the best f/stop to use for an astrophoto will be a compromise between recording faint detail in a reasonable exposure time at a faster focal ratio, and improving lens performance with a slower focal ratio. Lenses with a focal ratio of f/2.8 and lower are considered reasonably fast. Lenses from f/4 to f/5.6 are moderate. Once you get to f/8 a camera lens is considered slow. F/Stops and F-Numbers F/ratios are also known as f/stops for F-numbers in photography. Each f/stop is equal to a doubling or halving of the amount of light. For example, an f/ratio of f/4 lets in twice the amount of light as an f/ratio of f/5.6 and requires half the exposure. The full f/stop series, in one stop increments is:
F/stops to the left of this scale are considered "faster" than those to the right, with f/1 being the fastest focal ratio listed here. Other focal ratios also extend off both ends of this scale. Each of these f/stops is equal to a one-stop difference in light getting through. So every time you change the f/stop by one full increment, you also have to change the shutter speed, or exposure time, by doubling or halving the exposure to compensate. For example, at the same ISO, a 1 second exposure at f/5.6 would equal a 2 second exposure at f/8, or a 1/2 second exposure at f/4. All would be equivalent. Note that changing the ISO is not equivalent. This is a common misconception that we will cover in detail in section 307, ISO Speed and Sensitivity. Here is a list of equivalent exposures, all allowing the same amount of light to reach the sensor:
For simplicity in the short exposures, the higher shutter speeds are rounded off, such as 1/32nd sec is rounded to 1/30th sec, 1/64th to 1/60th, 1/128th to 1/125, 1/256th to 1/250th, 1/512th to 1/500th and 1/1024th to 1/1000th. The differences are so small as to be inconsequential. If you take a camera lens with a fixed focal length, and stop down the lens, and look at the lens from the front, into the camera, you will see the size of the hole made by the diaphragm blades gets smaller as the f-number gets bigger. f/32 uses a very small hole compared to f/2.8. f/32 is a "slow" aperture because the small hole lets in a lot less light over the same amount of exposure time as a larger hole. It's "slow" because it requires a longer exposure. More light gathered at a given focal ratio means better images for astrophotography because more signal is gathered. Signal is simply the photons of light from the object that we are interested in. We will discuss this in more detail in section 801, Signals and Noise, in the advanced imaging chapter.
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