Aperture Digital Photography Fundamentals

K Apple Computer, Inc.

2005 Apple Computer, Inc. All rights reserved. No part of this publication may be reproduced or transmitted for commercial purposes, such as selling copies of this publication or for providing paid for support services. Every effort has been made to ensure that the information in this manual is accurate. Apple is not responsible for printing or clerical errors. The Apple logo is a trademark of Apple Computer, Inc., registered in the U.S. and other countries. Use of the keyboard Apple logo (Option-Shift-K) for commercial purposes without the prior written consent of Apple may constitute trademark infringement and unfair competition in violation of federal and state laws. Apple, the Apple logo, Apple Cinema Display and ColorSync are trademarks of Apple Computer, Inc., registered in the U.S. and other countries. Aperture is a trademark of Apple Computer, Inc.

Contents

Preface Chapter 1
An Introduction to Digital Photography Fundamentals 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 Understanding RAW, JPEG, and TIFF RAW Why Shoot RAW Files? JPEG TIFF Shooting Tips Reducing Camera Shake Minimizing Red-Eye in Your Photos Reducing Digital Noise How Digital Images Are Displayed The Human Eyes Subjective View of Color Understanding How the Eye Sees Light and Color Sources of Light The Color Temperature of Light How White Balance Establishes Color Temperature

Chapter 2

Chapter 45 47
Measuring the Intensity of Light Bracketing the Exposure of an Image Understanding How a Digital Image Is Displayed Additive vs. Subtractive Color Understanding Color Gamut Displaying Images Onscreen The Importance of Color Calibrating Your Display Apple Cinema Displays Are Proof Perfect Displaying Images in Print Printer Types Understanding Resolution Demystifying Resolution Learning About Pixels Learning About Bit Depth How Resolution Measurement Changes from Device to Device Mapping Resolution from Camera to Printer Camera Resolution Display Resolution About the Differences Between CRT and Flat-Panel Display Resolutions Printer Resolution Calculating Color and Understanding Floating Point Learning About Bit Depth and Quantization Learning About the Relationship Between Floating Point and Bit Depth Understanding How Aperture Uses Floating Point Credits

Appendix

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. The following chapters explain how your camera captures a digital image, how images are displayed onscreen and in print, and how cameras, displays, and printers measure image resolution.

Understanding Lens Speed

A lenss speed is determined by the maximum amount of light the lens is capable of transmittingthe largest f-stop value. When a lens is capable of transmitting more light than other lenses of the same focal length, that lens is referred to as fast. Fast lenses allow photographers to shoot at higher shutter speeds in low-light conditions. For example, lenses with maximum f-stop values between 1.0 and 2.8 are considered fast. Depth of Field Depth of field is the area of the image that appears in focus from foreground to background and is determined by a combination of the opening of the aperture and the focal length of the lens. A small aperture setting results in greater depth of field. Controlling depth of field is one of the easiest ways for a photographer to compose the image. By limiting the depth of field of an image, the photographer can turn the attention of the viewer on the subject in focus. Often, limiting the depth of field of an image helps eliminate clutter in the background. On the other hand, when shooting a landscape, you want the image to have great depth of field. Limiting the depth of field to the foreground would not make sense.
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

The shutter is a complicated mechanism that precisely controls the duration of time that light passing through the lens remains in contact with the digital image sensor. The cameras shutter is activated by the shutter release button. Prior to the digital age, the shutter remained closed to prevent the film from being exposed. Depending on the type of digital image sensor, a mechanical shutter may not be necessary. Rather than a shutter revealing light to initiate a chemical reaction in the film, the digital image sensor may simply be turned on and off.
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. For example, 1/60 of a second is half as much exposure time as 1/30 of a second, but about twice as much as 1/125 of a second. Photographers often use shutter speeds to convey or freeze motion. A fast-moving object, such as a car, tends to blur when shot with a slow shutter speed like 1/8. On the other hand, a fast shutter speed, such as 1/1000, appears to freeze the blades of a helicopter while its flying.

Using Reciprocity to Compose Your Image
You can adjust the aperture setting and shutter speed to create several different correctly exposed images. The relationship between the aperture and shutter is known as reciprocity. Reciprocity gives the photographer control over the depth of field of the image, which controls the area of the image that remains in focus. This is the easiest way to control what part of the image you want the viewer to pay attention to. For example, opening the lens aperture by one stop and decreasing the shutter speed by one stop results in the same exposure. Closing the aperture by one stop and increasing the shutter speed by one stop achieves the same exposure as well. Therefore, f4 at 1/90 of a second is equal to f5.6 at 1/45 of a second. The reason is that the cameras aperture setting and shutter speed combine to create the correct exposure of an image.

Digital Image Sensor

When the reflective light from the photographed subject passes through the lens and aperture, the image is captured by the digital image sensor. A digital image sensor is the computer chip inside the camera that consists of millions of individual elements capable of capturing light. The light-sensitive elements transform light energy to voltage values based on the intensity of the light. The voltage values are then converted to digital data by an analog-to-digital converter (ADC) chip. This process is referred to as analog-todigital conversion. The digital numbers corresponding to the voltage values for each element combine to create the tonal and color values of the image.
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.) A process known as color interpolation is employed to ascertain the additional color values for each element.
Bayer pattern color filter array
Common Types of Digital Image Sensors There are two types of digital image sensors typically used: a charge-coupled device (CCD) and a complementary metal oxide semiconductor (CMOS). CCD CCD sensors were originally developed for video cameras. CCD sensors record the image pixel by pixel and row by row. The voltage information from each element in the row is passed on prior to descending to the next row. Only one row is active at a time. The CCD does not convert the voltage information into digital data itself. Additional circuitry is added to the camera to digitize the voltage information prior to transferring the data to the storage device.
Bayer RGB pattern on CCD sensor Voltage values are collected row by row. Each element records only one color.

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. Advances have been made in recent years in the sensitivity and speed of CMOS sensors, making them the most common type of digital image sensor found in professional DSLRs.
Bayer RGB pattern on CMOS sensor Voltage values for each element are created simultaneously.
Megapixels A cameras resolution capability is measured in megapixels. This measurement is based on the number of millions of pixels of image information that can be captured by the light-sensitive elements on the digital image sensor. Thus, a 15 megapixel camera is capable of capturing 15 million pixels of information. ISO Traditionally, the International Standards Organization (ISO) has provided a benchmark rating of the relative sensitivity of film. The higher the ISO rating, the more lightsensitive a particular film is. Higher ISO films require less light to record an image. The ISO rating has been redefined for digital cameras, indicating the image sensors sensitivity to light. Most DSLRs have ISO settings from 100 to 3200 ISO. Unfortunately, at higher ISO settings (400 ISO and above), some cameras have difficulty maintaining consistent exposure for every single pixel in the image. To increase the sensitivity of the digital image sensor in these situations, the camera amplifies the voltage received from each image sensor element prior to converting the signal to a digital value. As the voltage signals from each element are amplified, so are anomalies within solid dark colors. This results in sporadic pixels with incorrect bright color values, also known as digital noise. For more information on digital noise, see Reducing Digital Noise on page 25.

Why Shoot RAW Files?

There are many reasons to capture images as RAW files rather than JPEG files. However, its important to note that RAW image files require additional work to achieve the color balance youre looking for, whereas JPEG files are color-balanced by the camera for you. JPEG files are also smaller than RAW image files, requiring less storage space. The advantages to shooting RAW files are: Increased bit depth allows for more color-correction head room. The JPEG format is limited to 8 bits per color channel. RAW images store 16 bits per channel, with 12 to 14 bits per channel of color information. Although it may sound confusing, this means you can do significantly more color correction without degrading the image or introducing color noise. (For more information about bit depth, see Learning About Bit Depth on page 38.) After the RAW file is decoded, you work with the most accurate and basic data about an image. You control the white balance, color interpolation, and gamma correction aspects of the image during post-production rather than when shooting. The image file isnt compressed, as JPEG files are, which means that no image data is lost. Most cameras are capable of and do shoot color outside the gamut range of JPEG (both Adobe RGB 1998 and sRGB), which means color clipping occurs when you shoot JPEG files. RAW files preserve the cameras original image gamut, allowing Aperture to make image adjustments that take advantage of the full range of captured colors. RAW files give you control of noise reduction (luminance and color separation) and sharpening after capture. JPEG noise reduction and sharpening are permanently applied to the image according to the settings on the camera.
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. JPEG images are commonly used for online viewing.
TIFF (Tag Image File Format) is a widely used bitmapped graphics file format capable of storing 8 or 16 bits per color channel. Like JPEG files, TIFF files are converted from RAW files. If your camera does not have an option to shoot TIFF files, you can shoot RAW files and then convert them to TIFF files using software. TIFF files can have greater bit depths than JPEG files, allowing them to retain more color information. In addition, TIFF files can use lossless compression, meaning that although the file gets a little smaller, no information is lost. The end result is greater image quality. For these reasons, printing is commonly done from TIFF files.

Shooting Tips

Here are some tips for dealing with common photography issues.

Reducing Camera Shake

Camera shake is caused by a combination of the photographers hand movements or inability to keep the camera still, slow shutter speed, and long focal length. Camera shake results in a blurred image. The focal length of the lens, combined with a slow shutter speed, creates a situation in which the shutter speed is too slow to freeze the image before the camera moves significantly.
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 youre 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.
Note: Some lenses have image stabilization features that allow the photographer to shoot at a shutter speed whose value is lower than the focal length of the lens.
Minimizing Red-Eye in Your Photos
Red-eye is the phenomenon where people have glowing red eyes in photographs. This is caused by the close proximity of the flash (especially built-in flash) to the camera lens, which causes light from the subject to be reflected directly back at the camera. When the flash fires, the light reflects off the blood in the capillaries in the back of the subjects eyes and back into the camera lens. People with blue eyes are particularly susceptible to the red-eye phenomenon because they have less pigment to absorb the light.
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 subjects 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 doesnt always completely eliminate the red-eye effect. A more effective method is to use an external flash via the cameras hot-shoe mount or, better yet, with an extension bracket. An external flash radically changes the angle of the flash, preventing the lens from capturing the reflection of the blood in the back of your subjects eyes. While you can also fix the red-eye effect using Aperture, there is no way to accurately reproduce the original color of your subjects eyes. Preventing the problem before it occurs is the preferred solution.

External flash unit

Light enters the eye at different angles, diffusing as it leaves the eye.

Built-in flash

Light enters the eye and bounces straight back into the camera, causing 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. However, digital noise detracts from the image because of the sporadic bright pixels within solid colors, and lacks the aesthetic qualities of enlarged film grain.

100 ISO

200 ISO

400 ISO

800 ISO

1600 ISO

3200 ISO
You can reduce digital noise by taking your photographs at ISO settings between 100 and 400. The 400 ISO setting provides more exposure latitude, but even 400 ISO exhibits a little noticeable digital noise. If your subject is not moving and you cant use a flash, using a tripod can allow you to shoot successfully with low ISO settings. Many DSLR models come with a noise-reduction feature. If you turn on the noisereduction feature, it is automatically activated when you shoot long exposures. The camera color corrects at the pixel level, processing the image as its shot. The main negative aspect to digital noise reduction on the camera is the significant lag time required for the image to process between shots. One way to avoid this lag time between shots is to keep the noise-reduction feature on your camera off and use the Aperture Noise Reduction adjustment controls after youve imported your images.
How Digital Images Are Displayed
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 isnt necessary to understand the physics of light and color to appreciate that the colors in an image look realistic. How do you know a sunset is orange, the sky is blue, and the grass green? And exactly how orange is the sunset? What kind of orange is it? Its easy enough to verbally describe your perception of colors, but how do you choose a white balance that conveys the color orange most accurately? This chapter explains how to faithfully reproduce the color you capture with your camera onscreen and in your prints. This chapter covers: The Human Eyes Subjective View of Color (p. 27) Understanding How the Eye Sees Light and Color (p. 29) Sources of Light (p. 30) Understanding How a Digital Image Is Displayed (p. 33)

Chapter 2 How Digital Images Are Displayed
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. The range of human sight comes from three different parts of the eye: Pupil or iris: The pupil (also known as the iris) contracts and expands depending on the amount of light entering the eye. Rod cells in the retina: One of the two different types of cells that sense light. Rod cells perceive levels of brightness (but not color) and work best in low light. Cone cells in the retina: One of the two different types of cells that sense light. Cone cells can perceive color in bright light. Just as digital image sensors have light-sensitive elements that read red, green, and blue light, the eye has three kinds of cone cells, each sensitive to a different part of the visible electromagnetic spectrum: Cone R: Perceives colors with red hues with wavelengths in the visible spectrum roughly between 600700 nanometers (nm). Cone G: Perceives colors with green hues with wavelengths in the visible spectrum roughly between 500600 nm. Cone B: Perceives colors with blue hues with wavelengths in the visible spectrum roughly between 400500 nm. The human eye has roughly twice as many green cone cells as red and blue cone cells. This color arrangement is similar to the arrangement of color elements on a digital image sensor. (For more information about how digital image sensors capture images, see Digital Image Sensor on page 17.) The color the eye sees in a scene depends on which cells are stimulated. Blue light, for example, stimulates the blue-sensitive cones, which the brain then interprets as blue. The brain interprets combinations of responses from multiple cones at once and secondary colors are seen as a result. For example, red light and blue light stimulate both the red cones and blue cones, respectively, and the brain interprets this combination as magenta (red + blue). If all three types of cone cells are stimulated by an equal amount of light, the eye sees white or some neutral shade of gray. Cones are more spread out in the eye than rods. Also, they are much less light-sensitive, so they arent even active unless the brightness of a scene or object is beyond a certain threshold. The result is that low-light situations tend to look monochromatic (like black and white), whereas brighter scenes are detected by the cones and thus seen in full color.

The Importance of Color Calibrating Your Display
Its 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 wont reproduce faithfully in print if your display isnt calibrated. Theyll also look different when viewed on other displays. Calibrating your display allows ColorSync to adjust your image for consistent viewing results. Calibrating involves attaching an optical device to your screen that evaluates your screen for luminance and color temperature. There are several companies that manufacture color-calibration tools. The tools can be expensive and can vary greatly in quality, so make sure you do an adequate amount of research before you make your purchase. For a list of available color-calibration tools and devices, see the Mac Products Guide at http://guide.apple.com.
Apple Cinema Displays Are Proof Perfect
Apple Cinema Displays are so good at displaying color that you can use them in a SWOP-certified soft-proofing workflow. Display-based proofing systems Remote Director 2.0 from Integrated Color Solutions, Inc. and Matchprint Virtual Proofing System-LCD from Kodak Polychrome Graphics both have Specifications for Web Offset Publications (SWOP) certification. The prestigious SWOP certification means you can use Remote Director 2.0 to approve jobs for press production onscreen without the need for paper proofs, providing significant time and cost savings for print professionals. Certified systems are capable of producing proofs visually identical to the SWOP Certified Press Proof as defined in ANSI CGATS TR 001, Graphic Technology. Integrated Color Solutions, Inc. and Kodak Polychrome Graphics chose Apple flatpanel displays because they are capable of providing the luminance and color gamut necessary to create an onscreen proof that has the same brightness and feel as paper. Note: Your Apple Cinema Displays must be color-calibrated to achieve accurate results when soft-proofing your images.
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. For additional information about image quality in print, see Chapter 3, Understanding Resolution, on page 37.

Printer Types

The following printer types are divided into two groups: personal printers and professional printers. Personal Printers There are two basic types of printers that are affordable for most photographers. Inkjet: Inkjet printers create images by spraying little ink droplets onto the paper. Inkjet printers are capable of placing the microscopic droplets on the paper with great precision, resulting in high-resolution photographs. There are two methods of applying the ink to the paper. One technique involves heating the ink to a temperature warm enough to allow the ink to drip. The second method involves vibrating a tiny valve filled with ink, forcing it to fling a droplet onto the page. Dye sublimation: Dye sublimation printers create images by heating colored ribbon to a gaseous state, bonding the ink to the paper. The ribbon is a plastic material that makes the print nearly waterproof and difficult to tear. The incredible durability of dye sublimation prints gives them a longevity that cannot be surpassed by any other medium. The quality of inkjet printers has improved remarkably in the past few years, making their resolution and color gamut superior to those of dye sublimation printers. Professional Printers There are two basic types of printers employed for professional use. Unlike personal printers, these printers are relatively expensive. Offset press: Offset presses are used for high-volume printing for items such as magazines and brochures. Offset printing presses deposit ink in lines of halftone dots to produce images on the page. The printer uses a fixed drum to roll the image onto the paper. RA-4: RA-4 printers are capable of printing digital files on traditional photographic paper. They use a series of colored lights to expose the paper, which blends the colors together to produce continuous-tone prints. Due to their expense and size, most photo-direct printers are only available at professional photo labs.

Understanding Resolution

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: Demystifying Resolution (p. 37) How Resolution Measurement Changes from Device to Device (p. 40) Mapping Resolution from Camera to Printer (p. 41) Calculating Color and Understanding Floating Point (p. 43)

Demystifying Resolution

An images resolution is determined by the images pixel count and the bit depth of each pixel.

Learning About Pixels

A pixel is the smallest discernible element in an image. Each pixel displays one color. A pixels color and brightness range is determined by its bit depth. For more information, see Learning About Bit Depth on page 38. Pixels are grouped together to create the illusion of an image. On color displays, three color elements (one red, one green, and one blue) combine to form a pixel. As the number of pixels increases, the images detail becomes sharper, more clearly representing the original subject. Therefore, the higher the pixel count, the more likely the displayed image will look like the original subject. Because so many pixels fit in even a small image, pixel count is often expressed in megapixels (millions of pixels). For example, 1,500,000 pixels equals 1.5 megapixels.

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 images pixels exponentially increases the number of colors each pixel can express. The initial bit depth of an image is controlled by your camera. Many cameras offer several file settings; for example, DSLR cameras usually have two settings, allowing the photographer to shoot an 8-bit JPEG file (with 8 bits per color channel) or a 16-bit RAW image file (with 12 to 14 bits per color channel). Image file types use static bit depths. JPEG, RAW, and TIFF all have different bit depths. As you can see in the table below, the file type you shoot your images in dramatically impacts the tones visible in your images.
Bit depth per color channel Possible tonal values per color channel 16,384 65,536 JPEG, some TIFF Most RAW Some RAW Some TIFF Nearest equivalent file type
Note: The bit depth of an image file is uniform (each pixel in the image has the same number of bits) and is initially determined according to how the image was captured.

Although an 8-bit color channel cant display the color value represented by 167.5, floating-point calculations can use this value to create a more accurate final color.
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. In fact, some adjustments are calculated in different color spaces. Floating point permits color calculations that preserve, in an intermediate color space, the colors that would otherwise be clipped. When its time to print the image, the output file has to be within the gamut range of the printer. A pixels tonal values can be processed with incredible accuracy and then rounded to the output bit depth, whether onscreen or print, as necessary. The accuracy is most noticeable when rendering the darker shades and shadows of the image. The bottom line is that image processing using floating-point calculations helps produce extremely high image quality. For more information about color gamut, see Understanding Color Gamut on page 34.

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.com