essential skills: photoshop CS3
Format Compression Color modes Layers Transparency Uses
RAW No Unprocessed No No Master file
File formats
JPEG Yes RGB, CMYK,
Grayscale No No Internet and camera
format (compressed)
JPEG2000 Yes RGB, CMYK, No No Internet and archival
Grayscale
RGB, CMYK, Master file
Photoshop No Grayscale, Indexed Yes Yes (modified)
color
RGB, CMYK, Commercial printing
TIFF Yes Grayscale Yes Yes and generic camera
format (lossless)
GIF Yes Indexed color No Yes Internet graphics and
animations
Archival format for
DNG Yes Unprocessed No No storing original RAW
and metadata
JPEG2000 – This version of the JPEG format supports 16 Bits/Channel and alpha channels
and produces less image artifacts than the standard JPEG compression but uses a more complex
list of saving options than the standard JPEG format. Photoshop CS3 supports the file format
but it is not available as part of the ‘Save for Web’ options.
PSD (Photoshop Document) – This is the default format used by the Adobe image-editing
software. A Photoshop document is usually kept as the master file from which all other files
are produced depending on the requirements of the output device. The PSB format is another
version of PSD and is designed specifically for creating documents larger than 2GB.
TIFF (Tagged Image File Format) – This has been the industry standard for images destined
for publishing (magazines and books, etc.). TIFF uses a ‘lossless’ compression (no loss of image
data or quality) called ‘LZW compression’. Although preserving the quality of the image,
LZW compression is only capable of compressing images by a small amount. TIFF files now
support layers and transparency that can be read by other Adobe software products such as
InDesign.
GIF (Graphics Interchange Format) – This format is used for logos and images with a small
number of colors and is very popular with web professionals. It is capable of storing up to 256
colors, animation and areas of transparency. It is not generally used for photographic images.
DNG (Digital Negative Format) – The DNG format is a new archival file format that stores
both the RAW picture data as well as the metadata saved by the camera at the time of shooting.
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digital basics
Many work and education environments contain a mix of Windows and Macintosh
machines. Though both systems are far better at reading each other’s files than they
used to be, there are still occasions when you will have trouble when sharing files
between the two platforms. Use these tips to ensure that work that you save is available
for use in both environments.
1. Make sure that you always append your file names.
This means add the three-letter abbreviation of the file format you are using after the
name. So if you were saving a file named ‘Image1’ as a TIFF the saved file would be
‘Image1.tif ’, a JPEG version would be ‘Image1.jpg’ and a Photoshop file would be
‘Image1.psd’. Macintosh Photoshop users can force the program to ‘Always Append’ by
selecting this option in the ‘Saving Files’ section of Preferences.
2. Save TIFF files in the IBM version.
When saving TIFF files you are prompted to choose which platform you prefer to work
with; choose IBM if you want to share files. Macintosh machines can generally read
IBM (Windows) TIFFs, but the same is not true the other way around.
3. Macintosh users save images to be shared on Windows formatted disks.
If you are sharing images on a portable storage disk such as a Zip drive always use
media that are formatted for Windows. Macintosh drives can usually read the Windows
disks but Windows machines can’t read the Macintosh versions.
4. Try to keep file names to eight characters or less and don’t use spaces.
Older Windows machines and some web servers have difficulty reading file names
longer than eight characters. So just in case you happen to be trying to share with
a cantankerous old machine get into the habit of using short names – and always
appended of course. Use a hyphen or underscore instead of a space and use lower case
characters only (no capitals) if the images are destined for a web server that likes to say
‘no’.
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essential skills: photoshop CS3
Imaging files are huge. This is especially noticeable when you compare them with other digital
files such as those used for word processing. A text document that is 100 pages long can easily be
less than 1% the size of a file that contains a single 8 × 10 inch digital photograph. With files this
large it soon became obvious to the industry that some form of compression was needed to help
alleviate the need for us photographers to be continuously buying bigger and bigger hard drives.
What emerged was two different ways to compress pictures. Each enables you to squeeze large
image files into smaller spaces but one system does this with no loss of picture quality – lossless
compression – whereas the other enables greater space savings with the price of losing some of
your image's detail – lossy compression.
What is compression?
All digital picture files store information about the color, brightness and position of the pixels
that make up the image. Compression systems reorder and rationalize the way in which this
information is stored. The result is a file that is optimized and therefore reduced in size. Large
space savings can be made by identifying patterns of color, texture and brightness within
images and storing these patterns once, and then simply referencing them for the rest of the
image. This pattern recognition and file optimization is known as compression.
The compression and decompression process, or CODEC, contains three stages:
1. The original image is compressed using an algorithm to optimize the file.
2. This version of the file becomes the one that is stored on your hard drive or web site.
3. The compressed file is decompressed ready for viewing or editing.
If the decompressed file is exactly the same as the original after decompression, then the
process is called ‘lossless’. If some image information is lost along the way then it is said to be
‘lossy’. Lossless systems typically can reduce files to about 60% of their original size, whereas
lossy compression can reduce images to less than 1%.
There is no doubt that if you want to save space and maintain the absolute quality of
the image then the only choice is the lossless system. A good example of this would be
photographers, or illustrators, archiving original pictures. The integrity of the image in this
circumstance is more important than the extra space it takes to store it.
On the other hand (no matter how much it goes against the grain), sometimes the
circumstances dictate the need for smaller file sizes even if some image quality is lost along the
way. Initially you might think that any system that degrades the image is not worth using, and
in most circumstances, I would have to agree with you. But sometimes the image quality and
the file size have to be balanced. In the case of images on the web they need to be incredibly
small so that they can be transmitted quickly over slow telephone lines. Here some loss in
quality is preferable to images that take 4 or 5 minutes to appear on the page. This said, I
always store images in a lossless format on my own computer and only use a lossy format
when it is absolutely crucial to do so.
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digital basics
Compression Original Compressed Lossy/
Format
amount file size file size Lossless
JPEG Minimum 0.14MB Lossy
JPEG Maximum 2.86MB Lossy
JPEG2000 Minimum 0.07MB Lossy
JPEG2000 Maximum 2.54MB Lossy
JPEG2000 – 20.0MB 5.40MB Lossless
(PSD file)
TIFF LZW 10.30MB Lossless Comparing the
TIFF Zip 10.10MB Lossless compression abilities
TIFF None 27.30MB Lossless of different file types
and settings using the
PSD – 20.00MB Lossless same 20MB base file
The term lossy means that some of the image’s quality is lost in the compression process. The amount
and type of compression used determines the look of the end result. Standard JPEG and JPEG2000
display different types of ‘artifacts’ or areas where compression is apparent. The level of acceptable
artifacts and practical file sizes will depend on the required outcome for the picture.
To help ensure that you have the best balance of file size and image quality make sure that you:
Use the Save for Web or Save As > JPEG2000 features – both contain a post-compression
preview option.
Always examine the compressed image at a magnification of 100% or greater so that
unacceptable artifacts will be obvious.
Typical artifacts resulting from applying too much compression to a photo
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essential skills: photoshop CS3
Resolution
Resolution is a term that is used to specify the size of a pixel, a dot of colored light on a monitor
or a dot of ink on the printed page. There are usually two resolutions at play at any one time –
the resolution of the digital file and that of the output device. We can talk about capture size,
image resolution, monitor resolution and printer resolution. They are all different, but they all
come into play when handling a single digital image that is to be printed. Various resolutions
can be quoted as we move through the chain of processes involved in creating a digital print (in
the example below the total number of pixels remains constant throughout the chain of events).
An image captured at a resolution greater than 3000ppi is displayed at 100ppi on a high-resolution
monitor. Using Photoshop, the image resolution is lowered to 256ppi (the pixel dimensions remain
the same). The image is then printed using an inkjet printer with a printer resolution of 1440dpi
(dots per inch). The different resolutions associated with this chain of events are:
Capture size > Display resolution > Image resolution > Output device resolution
Image sensor
The sensor to the right creates an image file with 5 million
pixels or 5 megapixels (2560 × 1920 pixels). The resolution
assigned to the image file by the capture device may be a print
or monitor resolution. Either way it has no bearing on the file
size, which is determined by the total number of pixels.
The monitor resolution (the size of its display pixels, e.g. 1024
× 768) is defined by its resolution setting (approximately 100
pixels for every linear inch or 10,000 pixels for every square inch
in a high definition TFT display). The image pixels (different
than the display pixels) can be viewed in a variety of sizes by
zooming in and out of the image using image-editing software.
The resolution of the digital file is adjusted to 256 pixels per
inch (ppi). Each pixel is allocated a size of 1/256th of an inch.
Because the digital file is 2560 pixels wide this will create a print
that is 10 inches wide if printed (256 × 10 = 2560).
Note > Increasing the document size further will start to
lower the resolution below an acceptable level (the pixels
will become large enough to see with the naked eye).
The image is printed on an inkjet printer using a printing resolution of 1440 dots per inch.
Many colored dots of ink are used to render a single image pixel (see ‘Dpi and ppi’).
30
digital basics
Understanding resolution
Resolution is perhaps the most important, and the most confusing, subject in digital imaging. It
is important because it is linked to quality. It is confusing because the term ‘resolution’ is used
to describe at what quality the image is captured, displayed or output through various devices.
10 pixels per inch 20 pixels per inch 40 pixels per inch
Increasing the total number of pixels in an image at the capture or scanning stage increases both the
quality of the image and its file size. It is ‘resolution’ that determines how large or small the pixels
appear in the final printed image. The greater the image resolution the smaller the pixels, and the
greater the apparent sharpness of the final image. Resolution is stated in ‘pixels per inch’ or ‘ppi’.
Note > With the USA dominating digital photography, measurements in inches rather
than centimeters are commonly used – 1 inch equals exactly 2.54 centimeters.
The images to the right have the same pixel
dimensions (300 × 300) but different resolutions.
The large image has a resolution half that of the
small one. A digital image can be made to appear
bigger or smaller without changing the total number
of pixels, e.g. a small print or a big poster. This is
because a pixel has no fixed size. The pixel size can be
modified by the image-editing software to change the
document size. Increasing the resolution of the image
file decreases the size of the pixels and therefore the
output size of the file.
Note > When talking about the ‘size’ of a digital
image it is important to clarify whether it is
the pixel dimensions or the document size
(measured in inches or centimeters) that are
being referred to.
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essential skills: photoshop CS3
If manufacturers of software and hardware were to agree that dots were round and pixels were
square it might help users differentiate between the various resolutions that are often quoted.
If this was the case the resolution of a digital image file would always be quoted in ‘pixels per
inch’, but this is not the case.
At the scanning stage some manufacturers use the term dpi instead of ‘ppi’. When scanning,
‘ppi’ and ‘dpi’ are essentially the same and the terms are interchangeable, e.g. if you scan at
300dpi you get an image that is 300ppi.
When working in Photoshop image resolution is always stated in ppi. You will usually only
encounter dpi again when discussing the monitor or printer resolution. The resolutions used to
capture, display or print the image are usually different to the image resolution itself.
Note > Just in case you thought this differentiation between ppi and dpi is entirely logical
– it isn’t. The industry uses the two terms to describe resolution, ‘pixels per inch’ (ppi) and
‘dots per inch’ (dpi), indiscriminately. Sometimes even the manufacturers of the software
and hardware can’t make up their minds which of the two they should be using, e.g. Adobe
refer to image resolution as ppi in Photoshop and dpi in InDesign – such is the
non-standardized nomenclature that remains in digital imaging.
When we use the measurement ‘ppi’ or ‘pixels per inch’ we are referring to a linear inch, not a
square inch (ignore the surface area and look at the length).
2 inch × 2inch file @ 1ppi = 4 pixels 2 inch × 2 inch file @ 2ppi = 16 pixels
File size, however, is directly linked to the total number of pixels covering the entire surface area
of the digital image. Doubling the image output dimensions or image resolution quadruples the
total number of pixels and the associated file size in kilobytes or megabytes.
Note > Handling files with excessive pixel dimensions for your output needs will slow down
every stage of your digital image process, including scanning, saving, opening, editing and
printing. Extra pixels above and beyond what your output device needs will not lead to extra
quality. Quality is limited or ‘capped’ by the capability of the output device.
32
digital basics
Scanning resolution is rarely the same as the resolution you require to print out your image. If
you are going to create a print larger than the original you are scanning, the scanning resolution
will be greater than the output resolution, e.g. a 35mm negative would have to be scanned at
1200ppi if a 6 × 4 inch commerical print is required. If the print you require is smaller than the
original, the scanning resolution will be smaller than the output resolution.
The smaller the original the higher the scanning resolution.
1˝
4˝
Magnification × output resolution = scanning resolution
scanning resolution = 4 × 300ppi = 1200ppi
To calculate the correct file size and scanning resolution for the job in hand you can:
Either: Go to ‘File > New’ in Photoshop, type in the document size, resolution and mode you
require and then make a note of the number of megabytes you require from the scanning process.
Then adjust the scanning software resolution until the required number of megabytes is captured.
Or: Multiply the magnification factor (original size to output size) by the output resolution (as
dictated by the output device) to find the scanning resolution (not so difficult as it sounds!).
Size and mode Output device resolution
100ppi 240ppi 300ppi
screen inkjet commercial
8 × 10 RGB 2.29MB 13.20MB 20.60MB
8 × 10 Grayscale 781K 4.39MB 6.87MB
File size
5 × 7 RGB 1.00MB 5.77MB 9.01MB
5 × 7 Grayscale 342K 1.92MB 3.00MB
4 × 6 RGB 703K 3.96MB 6.18MB
4 × 6 Grayscale 234K 1.32MB 2.06MB
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essential skills: photoshop CS3
Before you adjust the size of the image you have to know how to determine the size you need.
Six- and eight-megapixel digital cameras are currently the affordable ‘end’ of professional digital
capture. The image resolution produced by these digital cameras is not directly comparable to
35mm film capture but the images produced can satisfy most of the requirements associated
with professional 35mm image capture. DSLRs using full frame sensors can match medium
format film cameras for quality.
Six megapixel cameras capture images with pixel dimensions of around 3000 × 2000 (6 million
pixels or 6 ‘megapixels’). The resulting file size of around 17MB (1 megapixel translates to nearly
3 megabytes of data) is suitable for an image in a commercial magazine that would nearly fill the
page. Ten-megapixel cameras are capable of producing files that can be used to illustrate double-
page spreads in magazines with just a small amount of resampling (see Interpolation).
Typical standard-resolution monitor: 1024 × 768 pixels
Typical full-page magazine illustration: 3400 × 2500 (8.5 million pixels)
High-resolution TFT monitor: 100ppi
High quality inkjet print: 240ppi
Magazine quality printing requirements: 300ppi
Full-screen image: 2.25MB (1024 × 768)
Postcard-sized inkjet print: 4MB
10 × 8 inch inkjet print: 13.2MB
Full-page magazine image at commercial resolution: 20MB
Note > Remember to double the above file sizes if you intend to edit in 16-bit per
channel mode.
A 20MB file will usually suffice if you are not sure of the intended use of the digital file.
Thirty-five-millimeter film scanned with a scanning resolution of 2300 will produce a
20.3MB file (2173 pixels × 3260 pixels).
34
digital basics
Before retouching and enhancement takes place, determine if the ‘image size’ needs to be
scaled for the intended output (the capture resolution will probably require changing to output
resolution). This will ensure that optimum image quality and computer operating speed are
maintained. To control image size go to ‘Image > Image Size’ in Photoshop.
Image size is described in three ways:
Pixel dimensions (the number of pixels determines the file size in terms of kilobytes).
Print size (output dimensions in inches or centimeters).
Resolution (measured in pixels per inch or ppi).
If one is altered it will affect or impact on one or both of the others, e.g. increasing the print
size must either lower the resolution or increase the pixel dimensions and file size. The image
size is usually changed for the following reasons:
Resolution is changed to match the requirements of the print output device.
Print output dimensions are changed to match display requirements.
When changing an image’s size a decision can be made to retain the proportions of the image
and/or the pixel dimensions. These are controlled by the following:
If ‘Constrain Proportions’ is selected the proportional dimensions between image width
and image height are linked. If either one is altered the other is adjusted automatically and
proportionally. If this is not selected the width or height can be adjusted independently of
the other and may lead to a distorted image.
If ‘Resample Image’ is selected (use with caution, see ‘Resampling’) adjusting the
dimensions or resolution of the image will allow the file size to be increased or decreased to
accommodate the changes. Pixels are either removed or added. If deselected the print size
and resolution are linked to prevent resampling. Changing width, height or resolution will
change the other two. Pixel dimensions and file size remain constant.
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