Aperture Digital Photography Fundamentals
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1 Preface 5 Contents An Introduction to Digital Photography Fundamentals Chapter 1 7 7 8 9 11 11 12 14 14 15 16 17 17 20 20 21 21 21 22 22 22 22 23 25 How Digital Cameras Capture Images Types of Digital Cameras Digital Single-Lens Reflex (DSLR) Digital Rangefinder Camera Components and Concepts Lens Understanding Lens Multiplication with DSLRs Understanding Digital Zoom Aperture Understanding Lens Speed Shutter Using Reciprocity to Compose Your Image Digital Image Sensor Memory Card External Flash Un
31 32 33 33 34 34 35 35 36 36 Measuring the Intensity of Light Bracketing the Exposure of an Image Understanding How a Digital Image Is Displayed Additive vs.
Preface An Introduction to Digital Photography Fundamentals This document explains digital terminology for the professional photographer who is new to computers and digital photography. Aperture is a powerful digital photography application designed to help you produce the best images possible. However, many factors outside of Aperture can affect the quality of your images. Being mindful of all these factors can help prevent undesirable results.
1 How Digital Cameras Capture Images 1 If you’ve previously shot film and are new to digital media, this chapter is for you. Here you’ll find basic information about the types of digital cameras, camera components and concepts, and shooting tips. People take photographs for many different reasons.
Digital Single-Lens Reflex (DSLR) This camera is named for the reflexing mirror that allows you to frame the image through the lens prior to capturing the image. As light passes through the DSLR camera’s lens, it falls onto a reflexing mirror and then passes through a prism to the viewfinder. The viewfinder image corresponds to the actual image area.
Digital Rangefinder There are two classes of digital rangefinder cameras: coincident rangefinder and point-and-shoot. Coincident Rangefinder Unlike DLSR cameras, the coincident rangefinder does not provide the photographer with the ability to view the subject through the lens. Instead, the coincident rangefinder employs a mirror or prism that uses triangulation to unite the images seen through the viewfinder and a secondary window to bring the subject into focus.
Digital Point-and-Shoot This is a lightweight digital camera, aptly named after the two steps required of the photographer to capture an image. Basically, point-and-shoot cameras require pointing the camera and taking the picture without manually adjusting settings such as the aperture, shutter speed, focus, and other settings that professional photographers routinely set on more sophisticated cameras. Of course, some point-and-shoot digital cameras do include adjustable aperture and shutter settings.
Camera Components and Concepts The basic components of a DSLR are described below. (Most of the components in a rangefinder are also found in a DSLR.)  Lens  Aperture  Shutter  Digital image sensor  Memory card  External flash Lens A lens is a series of sophisticated elements, usually glass, constructed to refract and focus the reflective light from a scene at a specific point—the digital image sensor.
Focal Length An important attribute of a lens, besides its quality, is its focal length. Focal length is technically defined as the distance from the part of the optical path where the light rays converge to the point where the light rays passing through the lens are focused onto the image plane—or the digital image sensor. This distance is usually measured in millimeters. From a practical point of view, focal length can be thought of as the amount of magnification of the lens.
Lens Types Although there are many varieties of lenses, common lens types include telephoto, wideangle, zoom, and prime. All of these lenses perform the same basic function: they capture the reflective light from the subject and focus it on the image sensor. However, the way they transmit the light differs. Note: Although there are several subcategories and hybrids of these lens types, these are the most basic. Telephoto A telephoto lens is a lens with a long focal length that magnifies the subject.
Zoom A zoom lens, also known as an optical zoom lens, has the mechanical capacity to change its focal length. A zoom lens can be extremely convenient, because many zoom lenses can change their focal lengths from wide-angle to standard and from standard to zoom. This eliminates the need to carry and change multiple lenses while shooting a subject or project. However, because of the movement between focal lengths, the f-stops aren’t always entirely accurate.
f-stop The photographer adjusts the opening of the aperture by setting the f-stop. An f-stop is a ratio of the focal length of the lens to the diameter of the opening of the aperture. For example, a 50 mm lens with an aperture opened up to a diameter of 12.5 mm results in an f-stop of f4 (50 ÷ 12.5 = 4). Therefore, the larger the numerical value of the f-stop, the smaller the opening of the aperture. The speed of a lens is determined by its largest f-stop value (smallest number).
Telephoto lenses (with long focal lengths) tend to have shallow focus when the aperture is opened all the way, limiting the depth of field of an image. Wide-angle lenses (with short focal lengths) tend to create images with great depth of field regardless of the aperture setting. Shallow depth of field Only the foreground is in focus. Great depth of field The image is in focus from the foreground to the background.
Shutter Speed Shutter speed refers to the amount of time the shutter is open or the digital image sensor is activated. The exposure of the image is determined by the combination of shutter speed and the opening of the aperture. Shutter speeds are displayed as fractions of a second, such as 1/8 or 1/250. Shutter speed increments are similar to aperture settings, as each incremental setting either halves or doubles the time of the previous one.
Each light-sensitive element on a digital image sensor is fitted with either a red, green, or blue filter, corresponding to a color channel in a pixel in the image that is captured. There are roughly twice as many green filters as blue and red to accommodate how the eye perceives color. This color arrangement is also known as the Bayer pattern color filter array. (For more information on how the eye perceives color, see “Understanding How the Eye Sees Light and Color” on page 29.
CMOS CMOS sensors are capable of recording the entire image provided by the light-sensitive elements in parallel (essentially all at once), resulting in a higher rate of data transfer to the storage device. Additional circuitry is added to each individual element to convert the voltage information to digital data. A tiny colored microlens is fitted on each element to increase its ability to interpret the color of light.
Memory Card After the digital image sensor has captured the image, the camera employs a series of processes to optimize the image. Many of these processes are based on camera settings established by the photographer prior to taking the shot, such as the ISO setting. After image processing, the camera stores the digital information in a file. The type of digital file created varies depending on the camera’s manufacturer.
Understanding RAW, JPEG, and TIFF It’s important to understand the differences between image file types. RAW, JPEG, and TIFF file types are described below. RAW A camera’s RAW file is an uninterpreted, bit-for-bit digital image recorded by the camera when the image is captured. Along with the pixels in the image, the RAW file also contains data about how the image was shot, such as the time of day, the exposure settings, and the camera and lens type. This information is also known as metadata.
JPEG JPEG (Joint Photographic Experts Group) is a popular image file format that lets you create highly compressed image files. The amount of compression used can be varied. Less compression results in a higher-quality image. When you shoot JPEG images, your camera converts the RAW image file into an 8-bit JPEG file (with 8 bits per color channel) prior to saving it to the memory card. In order to accomplish this, the camera has to compress the image, losing image data in the process.
You can eliminate camera shake by using a tripod or by increasing the shutter speed to a value higher than the focal length. For example, if you’re shooting at a focal length equivalent to 100 mm, you should set your shutter speed to 1/100 of a second or faster. The digital image sensor will capture the image before the movement of the lens has time to register additional light information on the sensor.
There are a few ways to minimize or eliminate red-eye in your pictures. Some cameras provide a red-eye reduction feature that fires a preflash, forcing the irises in your subject’s eyes to close before you take the picture. The main problem with this method is that it often forces subjects to involuntarily close their eyes before the image is taken, and it doesn’t always completely eliminate the red-eye effect.
Reducing Digital Noise Digital noise is the polka-dot effect in images with long exposures or images shot at high ISO settings in low-light situations. The effect is most noticeable in images shot in low-light situations. Many consider digital noise to be a synonym for film grain. Although the causes are the same, the effects are quite different. Some film photographers purposely shoot images with enhanced grain for artistic effect.
2 How Digital Images Are Displayed 2 Having a basic understanding of how light is captured, stored, and displayed onscreen and in print can help you achieve the image you intended to create. It isn’t necessary to understand the physics of light and color to appreciate that the colors in an image look realistic.
The subjective nature of visual perception should not necessarily be viewed as a handicap. If anything, it may be a blessing. Many challenges in photography come from the fact that the technology is so unforgivingly objective. A common example of this is the issue of white balance. Both film stocks and digital image sensors are designed to interpret white under specific conditions. Outdoor light (daylight) contains a lot more blue light than indoor (incandescent) light bulbs and candlelight.
Understanding How the Eye Sees Light and Color Digital image sensors and the human eye perceive color in similar ways. One of the remarkable things about human vision is the incredible range it has. A healthy eye can see in very bright sunlight and in nearly total darkness. If you have spent much time working with a camera, you know how amazing this range is. Film that works well outdoors is nearly useless indoors, and vice versa.
Sources of Light Prior to the invention of electric lights, electromagnetic energy originated from only a few sources. Even today, the sun is the primary source of light. Fire and candlelight provided evening light for thousands of years, though considerably weaker than modern electric lights. Newer sources of light include incandescent light bulbs, fluorescent light tubes, cathode-ray tubes (CRTs), liquid crystal displays (LCDs), lightemitting diodes (LEDs), and some phosphorescent materials.
With the invention of color film came a whole new set of considerations. In addition to correctly exposing the image, photographers had to take into account the various color tints different light sources cast across their film emulsion. Film manufacturers improved the situation by developing film emulsions rated for daylight and tungsten lamp color temperature ranges.
Cameras with sophisticated light meters can be set to meter, or test, specific areas of the scene. Most DSLRs allow you to choose the portion of the viewfinder to meter. These meter settings include, but are not limited to: Â Evaluative: Evaluative metering operates by dividing the frame into several small segments, taking a reading from each individual segment, and processing the average of the total segments to recommend the best exposure value for the overall image.
Understanding How a Digital Image Is Displayed Photographers display their digital images in two basic ways: onscreen or in print. The method by which an image is displayed onscreen and the way it is displayed as a print hanging on a wall are completely different. Computers, televisions, and video and digital still cameras create color images by combining red, green, and blue (RGB) primary colors emitted from a light source. This approach is based on the additive color theory.
Understanding Color Gamut In 1931, a group of scientists and intellectuals who called themselves the Commission Internationale de l’Eclairage (CIE) had the goal of defining standards for color. Using as much objectivity as is possible with this highly subjective topic, they developed a coordinate system for categorizing the world of colors. According to this system, every hue the eye can see can be described in terms of x and y coordinates.
The Importance of Color Calibrating Your Display It’s incredibly important to color calibrate your display or displays to ensure that the color on your screen matches the color you intend to output to print or to the web. Your digital workflow depends on successful color calibration, from capturing to displaying to printing. The adjustments you make to your digital image won’t reproduce faithfully in print if your display isn’t calibrated. They’ll also look different when viewed on other displays.
Displaying Images in Print Displaying images in print requires converting the color from the RGB color space to CMYK. The reason for this is that printed images need to reflect light from external light sources to be viewed. Images are usually printed on white paper, so no white ink is necessary. Darker colors are created by adding colors together, whereas lighter colors are produced by reducing the color mix.
3 Understanding Resolution 3 The concept of resolution often confuses people. Cameras, displays, and printers measure resolution in different ways. Resolution describes how much detail an image can hold. This section explains image resolution and shows how understanding image resolution can help you create better digital images. This chapter covers: Â Demystifying Resolution (p. 37) Â How Resolution Measurement Changes from Device to Device (p. 40) Â Mapping Resolution from Camera to Printer (p.
Learning About Bit Depth Bit depth describes the number of tonal values or shades of a color each channel in a pixel is capable of displaying. Increasing the bit depth of color channels in an image’s pixels exponentially increases the number of colors each pixel can express. The initial bit depth of an image is controlled by your camera.
Here’s a practical example of bit depth. To understand the effect of bit depth on an image, look at the picture of the girl below, which is an 8-bit grayscale image. Her eye is used to illustrate the effects that lower bit depths have on the resolution of the image. 1 bit 2 possible values 2 bits 4 possible values 4 bits 16 possible values 8 bits 256 possible values Formats like JPEG use 24 bits per pixel: 8 bits for the red channel, 8 bits for the green channel, and 8 bits for the blue channel.
The following example illustrates how increasing the bit depth of a pixel increases the number of color values it can represent. Increasing the bit depth by 1 bit doubles the number of possible color values.
Mapping Resolution from Camera to Printer Tracking the changing units of measurement from camera to display to printer is confusing. But without an understanding of how resolution changes between devices, you can inadvertently compromise the quality of your images. Camera Resolution A camera’s potential resolution is measured in megapixels (the number of millions of pixels used to record the image). The larger the number of megapixels, the more information is stored in the image.
Display Resolution The maximum number of pixels that can appear on a display’s screen determines its maximum resolution. Most displays have a variety of resolution settings from which to choose. For example, the 23-inch Apple Cinema HD Display has resolution settings from a minimum of 640 x 480 to a maximum of 1920 x 1200 pixels. As a photographer, you will want to operate your display at its maximum resolution setting. This ensures that you see as much of the image as possible on your screen.
Calculating Color and Understanding Floating Point As you’ve learned, digital devices translate color into numbers. Aperture calculates color using floating point, a type of calculation that allows calculations to be performed at a very high resolution with a minimum of error. Learning About Bit Depth and Quantization When you capture an image using a digital image sensor, the analog voltage values have to be converted to digital values that can be processed and then stored.
Learning About the Relationship Between Floating Point and Bit Depth When you make multiple adjustments to a digital image, the adjustments are mathematically calculated to create the result. Just as with analog-to-digital conversions, there can be quantization errors when adjustments are calculated. For example, consider the following calculation: 3 ÷ 2 = 1.5. Note that for the answer to be accurate, a decimal point had to be added for an extra level of precision.
Understanding How Aperture Uses Floating Point Internally, Aperture uses floating-point calculations to minimize quantization errors when image adjustments are processed. Floating-point calculations can represent an enormous range of values with very high precision, so when adjustments are applied to an image, the resulting pixel values are as accurate as possible. Often, multiple adjustments to an image create colors outside the gamut of the current working color space.
Appendix Credits Photography by Norbert Wu (pages 41 and 43) Copyright 2005 Norbert Wu http://www.norbertwu.com Photography by Matthew Birdsell (pages 9 and 16) Copyright 2005 Matthew Birdsell http://www.matthewbirdsell.
