Chapter 4 - ISO

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Photography :

Photography Chapter 4 – ISO CHAPTER 5 - LIGHT


Exposure determines the lightness or darkness of an image. Changes in exposure are measured in stops . An image that is too light and had colors that are washed out is overexposed. An image that is too dark is underexposed.


Review exposure by watching the demonstration in your e-text at the bottom of page 68. Equivalent exposures – different combinations of shutter speed and aperture settings that allow the same amount of light to expose the image. Selecting a faster shutter speed requires the selection of larger aperture opening to expose the image equally.


ISO is the level of sensitivity of a camera to available light. The lower the ISO number, the less sensitive it is to the light, while a higher ISO number increases the sensitivity of the camera.  Typically, ISO numbers start from 100-200 (Base ISO) and double as the setting go up. (Some cameras may start as low as 25.) So , the ISO sequence is: 100, 200, 400, 800, 1600, 3200, 6400 and etc . The important thing to understand, is that each step between the numbers effectively  doubles  the sensitivity of the sensor. So, ISO 200 is twice more sensitive than ISO 100, while ISO 400 is twice more sensitive than ISO 200. 


Higher ISO settings can cause the appearance of grain in film images, or noise in digital images. For this reason, always use the lowest number possible for the ISO setting. This will result in a smaller grain and finer detail. Only use higher ISO settings in low-light situations.


A digital camera’s sensor uses an array of millions of tiny light cavities or " photosites " to record an image. When you press your camera's shutter button and the exposure begins, each of these is uncovered to collect and store photons. Once the exposure finishes, the camera closes each of these photosites , and then tries to assess how many photons fell into each. 


Extreme overexposure can result in the sensor photosites becoming “blinded” by the light. This can cause the sensor to not be able to record any data. Therefore, it is always better to underexpose and image than to overexpose it.


In photography, a meter measures the amount of available light for any given ISO. An incident light meter measures the light striking the subject. A reflective light meter measures the light being reflected from the subject.


DSLR cameras have a built-in reflective light meter This exposure is correct. This exposure is one stop over-exposed. This exposure is one stop under-exposed.


Some situations can be difficult to meter. Light meters average the light and dark areas to determine the correct exposure. All white scenes give no dark areas to average. The scene can photograph quite gray. Bright light from windows can cause the subject to be underexposed.


Scenes that are difficult to meter can be fixed by using exposure compensation. Your camera most likely has an exposure compensation button. Use this button to deliberately under or overexpose images in difficult-to-meter situations. Review metering by watching the demonstration: Exposing scenes that are lighter or darker than average on the bottom of page 74 in the e-text.


Taking multiple shots of the same image using different exposures is known as bracketing . Bracketing is useful in difficult metering situations and to ensure a good image of important shots.


Ambient light is the light that is naturally present in a scene. Ambient, or natural light, is more realistic looking than light from a camera’s flash. Review Available Light Photography by clicking on the video at the bottom of page 77 in the e-text.


A histogram is a graph of light to dark tones in a photograph. The histogram can be found by pressing the info button on the camera. It can also be seen in photo editing software on the computer. Histograms are helpful for correctly exposing images.


HDR photography enhances images by using bracketing. HDR stands for High Dynamic Range . HDR photography uses bracketing to capture the best exposure of both shadows and highlights in a scene. The different images are combined into one image using photo editing software.


Filters change the appearance of an image by removing specific wavelengths of light. Filters can be made of glass, gel, or plastic. Review filters by watching Filters in the Digital Age at the bottom of page 86 in the e-text.


A neutral density filter simply reduces the amount of light coming into the camera. This is useful for long shutter speeds on bright, sunny days.


A circular polarizing filter reduces reflections from glass and water. It can also make the sky bluer.


Infrared light is light that is beyond the visible spectrum. Infrared photography can make scenes appear dream-like, turning blue skies dark and green leaves to white.


Digital SLR cameras have a filter that blocks infrared light. Therefore, one must convert a camera by removing this filter to be able to do infrared photography.


Light is measured in degrees Kelvin. Blues are considered “cool” colors, while reds and oranges are considered to be “warm” colors.


White balance (WB) is the process of removing unrealistic color casts, so that objects which appear white in person are rendered white in your photo. Proper camera white balance has to take into account the "color temperature" of the dominant light source, which refers to the relative warmth or coolness of white light. Our eyes are very good at judging what is white under different light sources, but digital cameras often have great difficulty with auto white balance (AWB) — and can create unsightly blue, orange, or even green color casts.


The most important factor in landscape or any outdoor photography is the time of day. The hour that occurs just before and during sunset is known as the “ golden hour ” because of the beautiful soft, warm light. This is the best time of day to take photos outside.

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