Choosing ISO/ASA, Shutter Speed, and Aperture With Rebecca Scoggin McEntee The Camera Operates Like Eyes Camera Obscura: dark room Your Cornea is like the front element of a lens. Together with the lens they are the eye's focusing elements. The cornea takes widely diverging rays of light and bends them through the pupil, the round opening in the central portion of the colored iris. The iris and pupil act like the aperture of a camera. The iris is a muscle which allows adjustable control of the quantity of light entering the eye so that the eye can work in a range of viewing conditions, from dim to bright light. The retina is the sensory layer that lines the very back of our eyes. It acts like the imaging sensor chip in a digital camera . Light adjustment: Both the eye and the camera can adjust quantity of light entering the camera or eye. On a camera it is done with the aperture control built into the lens, while on the eye, it is done by making the iris of the eye smaller or larger. The eye is a subjective device. Eyes work in harmony with the brain to create the images perceived. The eye is the best automatic camera. The brain even chooses it’s film speed. If you are outside in bright light, you squint to protect your eye from getting too much light into the eye. If it is night time, your eyes are wider. Your brain is helping to choose which amount of light your eye needs to be ready to receive. In that way, you choose your own ASA or ISO. ASA/ISO • Old: American Standards Association (ASA) • New: International Organization for Standardization (ISO) • Based on earlier research work by Loyd Ancile Jones (1884–1954) the American Standards Association (now named ANSI) defined a new method to determine and specify film speeds of black-and-white negative films in 1943. The ISO difference in eyes: Eyes do not have definite ISO levels, they are flexible Eyes do have a great ability to naturally adjust to ambient light levels even under the most severe lighting conditions The human eye has special abilities: it can modify its own light sensitivity. After about 15 seconds in lower light, our bodies increase the level of rhodopsin in our retinas So eyes are around 600 times more sensitive at night than during the day Inside buildings, typically your ASA/ISO needs to be at a higher level, such as 800 or 1600. Even 3200 if you have a good camera that will keep good resolution in the dark. Outside in brighter light, you can choose a lower number such as 100 or 400, or something in between. Eye shutter speed • In a still camera a single “snapshot” of the scene relies on an exposure determined by aperture and shutter speed • Our own eye/brain combination refreshes every 1/50th of a second. Choose manual – “M” After ISO, choose Shutter, then Aperture Curtain shutters Leaf Shutters Some didital controls for ISO, shutter speed and aperture Electronic shutters just turn the sensor on and off to capture the exposure much like the brain records. The image builds up as light is captured by the sensor. Normally, after you have chosen your ISO, you will choose your shutter speed, depending on the action you might be photographing. With more experience you can choose aperture first, to establish depth of field. Which do you want? You choose a fast or slow shutter speed depending on whether you want to stop the action or not: (you can practice on camerasim.com) To stop action of talking subject, you may have to choose a shutter speed of 125. A sport will require and increase in shutter speed to 500 or more. Apertures are next: this is f.32 Once you have chosen your shutter speed, you look at your meter and watch the pluses and minuses move as you move the dial on the camera that controls them, to see which f.stop gets you closer to the mid range of the meter. The Light Meter in Camera • A light meter is an instrument inside your camera that tells you if the amount of light reaching the film will be enough or too much to properly expose your image. The light meter takes into account your shutter speed, film speed, and aperture settings. Light Meter Inside the Camera Depth of Field • Depth of field (DOF) is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image. • The higher the number on the f.stop, the more depth of field in general. Apertures and Depth of Field Various Depth of Field on same subject Now you know why and how to choose ISO, shutter speeds, and f. stops.