Digital Darkroom P 207

Transcription

1 Digital Darkroom P 207 Digital Photographic Terms, Definitions and Hand Outs Instructor: Stephen Grote

2 Raster Pixel based Each individual pixel in the image must be mapped to a specific location, with a specific color. Raster applications such as Photoshop create large file sizes and are very demanding on computer resources such as memory, hard drive space and processor speed. Raster based files can only be scaled down without having to use interpolation where the computer has to guess the colors of the missing pixels. Vector Geometric description with a color tag (small file sizes and scaleable). Vector based applications scale easily in either direction, larger or smaller. Illustrator, InDesign, CorelDraw! and Quark Express are examples of vector based programs. Pixel Dots used to display a bitmapped image in a grid on a computer screen. Each pixel is assigned its own color or shade. Pixels are the smallest components of a digital image. Each pixel must have its own coordinates that cannot be moved. DPI dots per inch used to measure the quality of a printer and halftone screens LPI lines per inch printers use line screens PPI pixels per inch used to measure resolution of a digital file Bitmap The screen image in Photoshop is a bitmap, which is a geometric arrangement (or mapping) of a layer of dots of different shades or colors on a rectangular grid. Each dot in a bit map is called a Pixel. Resolution The number of pixels an image contains as measured in pixels per inch. The more pixels per inch an image displays the smoother the curves become and the smoother the color or grayscale transitions appear. Gamut The range of colors your scanner, monitor and/or printer can reproduce. Scanners have the largest gamut, then monitors and finally the smallest gamut belongs to printers. Printers are the limiting factor, because their dyes and pigment inks cannot reproduce the same range of light and optic colors as light driven RGB devices. Dyes vs. Pigment Inks Dyes are non-permanent solutions that give a wider color gamut and can faithfully reproduce most colors. We say they are not permanent because they fade very fast compared to actual ink pigments. However, we often use pigment inks due to their archival stability and resistance to fading. Pigment inks have larger particles that sometimes will not permeate the super coating on many paper surfaces (they will sit on top of the paper surface instead of being absorbed into the paper) which causes smearing and/or a dusty effect. Profile A snapshot of a device at a given point in time. Usually developed from running spectral data gathered from a spectrophotometer through various algorithms in programs such as GreyTag MacBeth s Profile Maker, EyeOne Profiler or Monaco Color. Spectrophotometer an instrument that measures the characteristics of light reflected from or transmitted through an object, which is interpreted as spectral data.

3 8-bit (24-bit color depth) vs. 16-bit (48-bit color depth) You must choose which mode to scan and work in while in Photoshop. An 8-bit file has a lot of information, but a 16-bit file has a lot more information and smoother tonal renditions. We commonly call Photoshop s image modes 8-bits per channel or 16-bits per channel. Channels in Photoshop are the separate parts of the light spectrum that make up all colors for example, RGB mode channels are Red, Green and Blue. When we have 8-bits of information per channel times three channels (Red, Green and Blue) we work in 24-bit color depth when in 8-bit RGB mode. Photoshop also offers 16- bits of information per channel where we have 48-bit color depth in 16-bit RGB mode (16-bit of information per channel times three channels R, G and B). We prefer to work in 16-bit mode when possible. It is easy to convert to fewer colors by going from16-bits to 8-bit. However, we cannot add colors when we go from 8-bit to 16- bit. We are stuck with the 8-bit color palette. Even though we have the ability to interpolate information from 8-bits to 16-bits, it does not add any additional color or benefits since the originally only held 8-bits of information. In computers and in photography math is very important, but it all hinges on the math surrounding the number 2. For each channel (Red, Green and Blue) there are either 8-bits or 16-bits of information. Currently we have 8-bits of information per channel times three channels so we work in 24-bit color when in RGB mode. When Photoshop works with16- bits of information per channel we have 48-bit color in RGB mode. We get the bit level of information by using basic exponents. In an 8-bit per channel mode we have 2 raised to the 8 th power bits of information for every pixel in every channel. 2 8 equals 256 pieces of information. Multiplying that by the channels gives us 256 for the red channel, 256 for the green channel, and 256 for the blue channel (256x256x256 or ) which equals 16,777,216 possible colors for each individual pixel. 16-bit starts to become mind numbing equals 65,536 pieces of information per channel. Multiplying it out as 65,536 x 65,536 x 65,536 or 65,536 3 equals a whopping 281,474,976,710,656 possible color combinations for each pixel. Needless to say, with that amount of additional information 16-bit has much smoother tonal renditions and many more possible colors, but will produce astoundingly larger files. 16-bit files force us to have much more powerful computers with fast processors, tons of Random Access Memory (RAM) and Read Only Memory (ROM) such as large hard drives holding many gigabits of information. Photoshop s implementation of 16-bit information is actually limited to 15-bits of information and a locator tag. 15-bits of information is a major upgrade from 8-bit and the 16-bit mode (15+1) should be used whenever possible.

4 RGB Red, Green, Blue Monitor emissions and true light colors. Additive colors, if you add them all together you get white light. RGB is Photoshop s native mode and is what Photoshop artists and photographers use. Do all your editing and/or work in RGB mode. Most photographic printing devices are RGB devices including our Epson printers (an 99-percent of all other inkjet printers). RGB input printers automatically convert to their ink cycle when you tell them to print. CMYK Cyan, Magenta, Yellow, Black The three subtractive primary colors (cyan, magenta and yellow) are the primary inks used in the four-color printing process; however when mixed together they form a muddy brown. The fourth ink, black, is then introduced to produce a rich black when mixed in small quantities with the other three colors. Black ink may also be introduced to print single color half-tone screens such as B&W photographs or to provide a single color, dark, easily readable text color. Ink printing colors for four-color presses. This mode is for prepress workers to convert colors on the screen to simulate what will print as ink pigment on the press. Photographers should use the CMYK mode sparingly and only when necessary for prepress work. When converting to CMYK it is best to convert to LAB Color then convert to CMYK to preserve the color. Complimentary Colors Additive Color Subtractive Color Red Green Blue Cyan Magenta Yellow Transmissive, Reflective and Emissive Light Transmissive light passes through an object or substance and its color is rendered (glass, film, etc.) scanners Reflective light is reflected off the subject rendering its color (printed pages, solid objects) printers Emissive light is generated by the object (TV, computer monitor, artificial lighting such as household lights, etc.) monitors

5 Scanning You should always scan for your output device resolution at the final print size. In other words, if you know the printer is a photo quality printer (i.e. Epson/Canon/HP inkjet) the resolution should be at least 300 dpi at the final print size which we will say is normally 11 x14 for this class. This will make a rather large file size in a raster based program such as Photoshop, but we should all get used to working on larger file sizes for higher quality images. Please note that while vector programs scale well (both increasing and decreasing image sizes), raster programs do not scale up well. Raster files will only scale down. Once a raster file has been scaled down it throws away any unnecessary file information and cannot be scaled back up without interpolation (the program guessing what information should be there) thereby decreasing image/file quality. Output Devices and their Resolution Internet or Web Usage Laser Printers Fiery Printers InkJet Printers Specialized Printers 72 dpi is the de facto standard (although PCs handle 96 dpi it is very hard to tell any discernable difference on the screen) Most Laser Printers fall between 88 dpi and 150 dpi. Set 150 as your standard for laser devices. The Fiery is a high end color laser printer. Set your resolution to be 150 dpi. The Epson, Canon, HP, etc. devices will easily handle 300 dpi or more, although you are wasting your time and computer resources (processor power, memory and storage space) if you scan at more than 300 dpi at the final print size. Many inkjet printers have maximum 250 dpi print range so all the info above 250 dpi is wasted. There are many specialized printers that will accept a higher ppi/dpi level. One example is the Fuji LightJet for photographic output which needs 400 dpi at the final output size. Printers that make photographic negatives or transparencies almost always require 600 dpi or higher to give smooth tonal renditions without pixilation. Please do not expect to scan at higher resolutions just to try to try to impress someone because large file sizes tend to crash computers and leave you with nothing to show for all your hard work.

6 File Types Few things are as confusing in the world of the digital image as the huge number of file types. As is often the case, it is crucial to know the path to the end result of a project. It is not advisable to change types in midstream, only change a file type at the end of the project when necessary. What follows is a list of the most common file formats and some description of their application. This list is far from exhaustive. Since most readers of this document will be photographers, the list below will reflect a bias in that direction. Only bitmap (or raster) formats composed of pixels are considered here. Many other types are used with vector based images composed of points at specific locations and the paths between them in either 2 or 3 dimensional spaces. Drawing programs such as Illustrator and CorelDraw or 3D modeling programs like 3D Studio Max, Maya, or AutoCAD are used to create vector images. Photoshop (.psd) Simply stated, the.psd format is the only file type that can implement layer features and all of the image modes in Adobe Photoshop (grayscale, RGB, CMYK, etc.). As a general rule, the.psd file should be used until you have a good reason to convert to something else. The most common progression is from.psd to another type appropriate to the end use of the image. As examples, many workers convert.psd to.tif or.eps for prepress applications or.psd to.jpg or.gif for the Internet. With the proper software, the.psd file can move between Windows and Macintosh operating systems. Tagged Information File Format or TIFF (.tif) The.tif file is the standard of the prepress industry. Most bitmap image programs open a TIFF file. They are stable, compressible (with lossless LZW compression) and transportable from program to program and, like the.psd file, between Windows and Macintosh operating systems. TIFF files can save channels containing masking and color separation information. Page composition programs such as Adobe InDesign and Quark Xpress are all.tif enabled. Joint Photographic Experts Group or JPEG Format (.jpg or.jpeg) The most common uses of the JPEG format are online and digital camera applications. An 8-bit file with 24-bit color depth (16.7 million colors) format, the.jpg file employs a lossy compression scheme. This means a compromise must be considered between file size and image quality. JPEG enabled software (like Photoshop) provides a multi-step choice between size and quality. Small files are a significant issue on the Internet. Every time a JPEG file is saved it is recompressed and looses some of its fidelity. The higher the quality number the less the compression, the small the number the more compression. More compression achieves smaller file sizes, however, color and overall image quality suffer. The more times the file is saved (and compressed) the more quality loss it experiences. Opening the file does not affect quality, only saving it. If you are going to send a file via please send it as a JPEG at 72 ppi for viewing on screen. It is generally better to send the file as either 7 or 10 inches on the long dimension to match current standards (e.g. 5x7 or 8x10) at 72 ppi. Smaller files transfer faster over the Internet and do no clog up inboxes.

7 Compuserve Graphic Interchange Format (.gif) The most common uses of the GIF format are on the Internet. An 8-bit (256 color limitation) format, the.gif file sacrifices color depth to achieve a smaller file size. Small files are a significant issue on the Internet since they transfer faster. 8-bit files will be dithered (composed of a dot pattern) and show a coarse tonal transition. Grayscale images are not affected as much as color images. Encapsulated Postscript (.eps) The EPS file is a remarkably versatile format that may be used with either raster (pixels) or vector (points and paths) based imagery. It requires that the output device: printer, film recorder, etc. be Postscript enabled. Reasons to Use Adjustment Layers 1. An adjustment layer only affects the layers beneath it. 2. Adjustment layers are non-destructive, they do not affect the original file pixels. a. They can be turned on and off easily. b. They can be infinitely edited or tweaked for the desired result without resulting in file or layer degradation. 3. Adjustment layers take up very little space (RAM, Hard Drive and Swap File resources) compared to full layers or duplicate layers. 4. Every Adjustment layer comes with it own Layer Mask for editing and/or selectively applying the effect. The opacity can be changed for the entire layer or changed via the opacity of the brush strokes applied to the layer mask. Ways to Adjust Color 1. Levels can be an adjustment layer 2. Curves can be an adjustment layer 3. Color Balance can be an adjustment layer 4. Variations 5. Hue/Saturation can be an adjustment layer Ways to Adjust Contrast All can be used (and should be used) as adjustment layers 1. Curves 2. Levels 3. Brightness/Contrast

8 Ways to Convert to B&W 1. Grayscale (Direct) 2. Convert to LAB Color, then Convert to Grayscale 3. Desaturate (will discard all color info) 4. Hue/Saturation Adjustment Layer (then use the saturation slider selectively) 5. Channel Mixer Adjustment Layer (check monotone) then play with sliders to achieve the effect you want. please note that you are still in RGB mode. 6. Calculations (to make a new channel), then Convert to Grayscale discarding all other channels. The preferred methods are the Channel Mixer and Calculations. Global vs. Local Corrections There are two major types of corrections: Global and Local Global corrections affect the entire image. They should be made before beginning any serious work on the image. These types of corrections include overall color adjustments, contrast adjustments, etc. Local corrections only affect a small portion of the image. They are things that only adjust a certain part of an image, i.e. adjustments made with a selection inside the image. Most of the time selections can be achieved via channels, paths or layer masks to help isolate them from the rest of the image. Most of time selections are made on separate layers, often duplicate or copy layers to give the user an undo avenue. Workflow Workflow is a buzz word. Everyone in the digital photographic, graphic design and prepress community is using it and it is important to understand why. Workflow refers to the process, from beginning to end, for achieving a consistent result. It is a set of specific steps taken in a specific order. In our process workflow refers to the steps we take with cameras and/or scanning the image through the final output (whether printing or on screen). Each step must be taken in a specific order and the same variables changed every time. Having color spaces, color, file resolution, image size, file type, printing resolution, etc. match is the key to achieving our desired consistency.