- Image File Size
- Popular File Formats
- Choosing a Format
- Additional Digital Photography Terms: Essential Elements of Photo-Editing
- Step-by-Step Photo-Editing
In Part I of this publication series, we discussed the basics of taking quality digital images from a photography standpoint, or the image capture. Now we will turn our attention to the terms used in digital imagery.
Digital picture making can be described as a three-step process.
- Image Capture involves using a digital camera or scanner to capture light information, convert it into a series of numbers, and then store these numbers in a picture file.
- Image Processing involves using software to manipulate the numbers that were created in the capture process. The purpose of this step is to enhance the original picture, with adjustment of brightness, contrast, color, sharpness, cropping and other image attributes.
- Image Output involves converting the numbers in the picture file back into a form that can be recognized as the picture originally taken. This can be done using any number of digital printing devices.
During each of these steps, understanding of digital terms and how they are applied is essential.
In general, the size of an image file depends on:
- the number of pixels in the image, or its resolution.
- the amount of compression used to store it.
Digital photographs are made up of small squares called pixels (picture elements). Similar to a painting made with dabs of paint with different color to create a cohesive whole drawing, the computer and printer can use pixels to display or print photographs. To do so, the computer divides the screen of printed page into a grid containing hundreds of thousands or millions of pixels. The computer or printer then uses the values stored in the digital photograph’s file to specify the brightness and color of each pixel in this grid and form a painting by number. Controlling or addressing a grid of individual pixels in this way is called bit mapping, and digital images are called bit-maps.
Pixels are arranged in horizontal rows and vertical columns (such as 640 x 480).
The size of a digital photograph is specified in one of two ways: by its dimensions in pixels or by the total number of pixels it contains. For example, the same image can be said to have 1600 x 1200 pixels or to contain 1.92 million pixels (1600 multiplied by 1200).
Resolution is the amount of information in a digitally captured image measured in pixels. The higher the number of pixels, the higher the resolution of the picture, and thus the more photographic the image will be. A low-resolution digital image (at 640 x 480 pixels), for instance, may look excellent when displayed on the Internet, but it can appear fuzzy when printed or enlarged.
By comparison, high-resolution images such as those at 1280 x 1024 pixels contain enough pictorial information (sharp contrasts, rich colors and picture details) to look good on the Internet as well as when printed or enlarged. A high-resolution camera will be capable of capturing images in the 1280 x 960 range and above. These are sometimes referred to as “mega-pixel” cameras. It means that the picture is made up of more than one million pixels. This is one of the major aspects that control the price of a camera. Generally, the higher the resolution, the higher the cost of the camera.
Many cameras give you the choice of at least two resolutions — a high resolution such as 1280 x 1024 and a low resolution of 640 x 480. A lower resolution is useful because it allows you to store more pictures in the available memory, and the image will still print well at 4 x 6 inches (but not larger; see Resolution and Image Size). The larger resolutions take a lot more room in memory but allow for printing at sizes up to 8 x 10 inches.
Resolution must be considered during both the Image Capture (taking pictures with a camera) and Image Output (displaying/printing pictures). There are two aspects of capture and output resolution, spatial resolution and brightness resolution (also referred to as bit depth).
In general, spatial resolution refers to the number of pixels in the image file. In a high-resolution image, the picture would have to be magnified to see individual pixels. In lower resolution images, it may be possible to see individual pixels with the naked eye. The higher the spatial resolution, the greater the number of pixels in the picture and, correspondingly, the smaller the individual pixels will be. As a result, images having higher spatial resolution will have greater picture detail.
The same image taken at different resolutions:
Brightness resolution refers to the number of brightness levels that can be recorded in any given pixel. The greater the brightness resolution, the greater the number of levels that can be included in the picture file.
In black-and-white images, levels are seen as shades of gray. In color images, levels will be seen as specific color hues.
As a general rule, brightness resolution for black-and-white picture information should be at least 8 bits. Brightness resolution for full color pictures information should be at least 24 bits.
Resolution and Image Size
Surprisingly, the number of pixels in an image does not by-itself indicate sharpness or even size. The same number of pixels can occupy a small or large area on the screen or printout. As they are spread over a larger area, the perceived sharpness falls (from the same viewing distance). Conversely, if they are squeezed into a smaller area, perceived sharpness increases. The images on high-resolution screens or printouts look sharper only because the available pixels are grouped into a much smaller area, not because there are more pixels. The smaller an image is displayed from a given file, the sharper is will appear. When enlarged too much, sharpness begins to fade and eventually the square pixels will begin to show, and the image becomes pixelated.
In the image output phase, resolution is typically described in dots-per-inch (DPI), and refers to the size of the spots created by an output device. Many different types of output devices have many different output resolution capabilities. There is a relationship between the resolution of the image bitmap file (that was created in the capture stage) and the resolution of the output device. The final size of an image depends on the resolution of the output device.
The brightness information for each pixel in the image bitmap file will be used to determine the number, size and color of the dots of ink that will be laid down on paper when the picture is printed. You can see how this happens by studying the table below that lists a number of image resolutions found on photo CD disks. An image with 768 x 512 pixels will fit on the screen set to a resolution of 800 x 600. It will be displayed a little more than 10 x 7 inches. That same image, printed on a 300 dpi printer is reduced to about 2.5 x 1.7 inches. On a 1500 dpi printer, the image is further reduced to about 0.5 x 0.3 inch.
|Original||14" Screen @ 72 ppi||Printer 300 dpi||Printer 1500 dpi|
To make an image larger or smaller for a given output device, you must resize it in a photo-editing program or by the application you are printing it with. Resizing is done by interpolation, a method used by the software. When an image is made larger, the program adds extra pixels, and the color of each new pixel is determined by the colors of its neighbors. When an image is made smaller, the program deletes some pixels.
Resizing and Resampling Images
In graphics software, the resample command is used to increase or decrease the size and/or the resolution of a bitmap-based image. An image is up-sampled to increase the resolution by adding new pixels. An image is down-sampled to decrease the resolution by deleting pixels.
Text and graphic art should be resized because they are simple and hard-edged images. Resizing duplicates or deletes pixels while resampling smooths out rough spots by estimating how the “missing” pixels should appear and filling them with the appropriate color.
- When you are changing the on-screen size of an image resample, modify the pixel dimensions, and set the resolution to 72.
- When you are changing the print size of an image, leave the resample box unselected and
either adjust the print dimensions or the resolution to get the results you need.
- As you increase the print dimensions, resolution decreases (thus, print quality decreases).
- As you increase resolution, the print size must decrease based on the amount of pixel data available.
If you do not have enough pixel data to get the print size and resolution you need, then you will have to resample the image and the graphics software will create new pixels. This often causes poorer quality print results, so you need to decide on a trade-off — the loss in quality from the reduced resolution or the loss in quality from resampling.
By applying compression to the file, the file shrinks to a fraction of its original size. You can now fit more images onto the memory card because they have been compressed and take up less space. More importantly, the smaller file size also means the file can be sent over the Web considerably faster. Compression is a function of the camera’s internal programming to take a picture and store it in less space than it would normally fill. During compression, data that is duplicated or that has no value is eliminated or saved in a shorter form, greatly reducing a file’s size. For example, if large areas of a meadow are the same shade of green, only the value for one pixel needs to be saved along with the locations of the other pixels with the same color. When the image is then edited or displayed, the compression process if reversed.
Two forms of compression are used in digital cameras.
Lossless compression is a compression scheme in which no bits of information are permanently lost. Lossless compression does not provide much compression and files remain very large. For this reason, it is used mainly where detail is extremely important, as it is when you plan to make large prints. Lossless compression is offered by some digital cameras in the form of TIFF and RAW files formats (see Popular File Formats). For most types of data, lossless compression techniques can reduce the space needed by only about 50 percent.
Lossy compression is a compression scheme in which some bits of information are permanently lost during compression and decompression of an image. Because lossless compression is not practical in many cases, all popular digital cameras offer a lossy compression. This process degrades images to some degree, and the more they are compressed, the more degraded they become. In many situations, such as posting images on the Web or making small to medium size prints, the image degradation is not obvious. If the image is enlarged, however, the degradation is apparent. JPEG has the ability to achieve much greater compression. It can reduce file sizes to about 5 percent of their normal size.
File format is the manner in which a file is constructed or in which a scanned picture is saved. A format is the pattern of ones and zeros in a file as created by a program using a specific set of rules to write that pattern. Another program must use the same or very similar rules to successfully read or interpret that file. Many software products can insert or import a picture from a file if it is saved in a file format the program supports (uses the same rules).
File formats are important in the use of digital cameras. They enable the camera to process photographic images in a more efficient manner. Different file formats meet many varying needs, allowing images to be manipulated and modified in many different programs or applications. The following provides a summary of some different file formats and how they are used.
Joint Photographic Experts Group is a compression method that reduced file size by eliminating redundant or unnecessary image data. This is the most popular format for photographic images. In fact, most cameras save their images in this format unless you specify otherwise.
A JPEG image is stored using lossy compression, and the amount of compression can vary. This allows you to choose between lower compression and higher image quality or greater compression and poorer quality. The only reason to choose higher compression is because it creates smaller files so you can store more images, and it is easier to send them by e-mail or post them on the Web. Most cameras offer two or three choices equivalent to good, better and best, although names vary. JPEG is a very efficient, true-color, compressed image format. Although it uses lossy compression, it has the capability of showing more colors than GIF (more than 256 colors).
Graphics Interchange Format is a compression method used for line art. This is the most common image format on the World Wide Web that uses lossless compression. When converting an image to GIF format, you have the option to have the image display any number of colors up to 256.
Tagged Image File Format has been widely accepted and supported as an image format. It is an uncompressed format. Most recently developed digital cameras offer a choice to save images as TIFF files.
RAW format stores the data directly from the image sensor without processing it first. This data contains everything captured by the camera. In addition to the digitized raw sensor data, the RAW format also records color and other information that is applied during processing to enhance color accuracy and other aspects of image quality. In addition, the original raw data can be processed with other software. This is unlike a JPEG image, where data are permanently changed or deleted during processing in the camera and can never be recovered.
Exchangeable Image File Format, a variation of JPEG, is used by almost all digital cameras to record extra interchange information to image files as pictures are taken. The type of information stored in a file varies by camera model, but it can include such information as date and time a photo was taken, resolution, camera settings used for the shot, amount of compression applied, color information, whether or not the flash was fired, shutter speed and name of camera owner.
Most photo manipulation software released after 2001 supports the reading of EXIF information, but it you want to ensure that the information is retained within your picture files, you must use software that supports EXIF. The EXIF information is lost if you save a picture in a program that does not support it. Specialized software can also extract and edit files with EXIF format.
If your camera lets you choose an image format or compression ratio, always choose those that give you the highest quality. If you decide later that you can use a smaller image or greater compression, you can do so to a copy of the image using a photo-editing program. If you take the image at a lower quality setting, you can never really improve it very much or get a large, sharp print if you want one. The only difficulty with this approach has to do with file sizes. The highest quality images can be 15 or more megabytes in size. These are almost impossible to send to anyone and slow to open, edit and save on even a powerful desktop computer. In addition, when you take images of this quality, you often must wait a long time between shots because the camera is tied up processing the last image you took. Most photographers compromise and take pictures in the highest quality JPEG format. Even these images can be 2 to 5 megabytes in size.
When you open an image to work on it, you should first save it so you are working on a copy, preserving an unchanged original. Save it in a loss-free format such as TIFF. If you want a specific format for the finished image, save it in that format as the final step. In particular, do not repeatedly close, open and resave JPEG original images. Every time you open one of these files and then save it again, the image is compressed. As you go through a series of saves and reopens, the image becomes more and more degraded. (An image is compressed only once during a single session, regardless of how many times it is saved.) Also, when you save an image as a JPEG, the image on the screen will not reflect the compression unless you close the files and then open the saved version.
The physical size of a compressed image will vary depending on the subject matter. A pure white wall will compress well, but a large tree with many leaves (adding detail) will not. Less compression (Fine, Super High Quality, etc., camera modes) gives better images used for larger prints, but you cannot store as many images. More compression (Normal, Basic, Standard camera modes) stores more images and makes the images better for making smaller prints, posting on a Web page or sending as e-mail attachments. The compression can be of different ratios such as 1:2, 1:4, 1:11, 1:19, and so on. (These ratios are sometimes referred to in the camera with terms such as “Good,” “Fine,” “Better,” “Best,” “Normal,” etc., by the manufacturers.)
These are the essential terms used in digital photography. For some additional ones, refer to the list below.
With Digital Zoom, the image is magnified by spacing out the pixels electronically and, as such, is very little different from blowing up an image to a larger size using a software program on a computer.
Macro focus is the camera’s ability to focus on objects close to the camera. — closer than about 12 inches. When used in conjunction with a zoom lens, macro can allow the photographer to fill the viewfinder with small objects. With some cameras, a dime can nearly fill the frame. This is useful with flowers, insects and other such small items.
Mega = 1 million. Megabyte = 1 million bytes. One byte is a measure of information.
Memory cards are small memory modules that can be inserted into the camera to hold images. When the card is full, it can be removed and another card inserted. The memory on these cards is non-volatile, that is, they don’t lose their images when they are removed from the camera. The images can later be downloaded from the card and, when the images are erased from the card, it is ready to be reused. Examples of popular memory cards are Smartmedia and Compact Flash.
NiMH stands for Nickel Metal Hydride, a type of rechargeable battery that produces sufficient current for digital cameras. They are rechargeable like Nicad batteries, but they are safe to throw away in landfills. They also do not have the “memory” effect that Nicad batteries have shown to exhibit. They need chargers specifically made for NiMH batteries.
This is a zoom lens that achieves its magnification by moving parts of its lens forward or backward. This usually takes place inside the camera, out of view, but it can often be heard as the sound of a small motor and other mechanical parts.
Some lenses have the ability to change their magnification level through a range of focal lengths (wide angle through telephoto). This is called a zoom lens, as seen on most all camcorders. With digital cameras, the range varies by brand and model, but ranges in the area of 35 mm to 115 mm are fairly common. This range is often referred to as “3x” (three times) zoom (3 x 35 = 105, so the telephoto setting is about three times the wide angle setting). The best cameras achieve this by moving various elements in the lens. This is referred to as optical zoom. Cameras using optical zoom are generally larger than those without because of the more complicated and thus larger lens elements.
Once a photograph is in digital form, you can store it on your system and then edit or manipulate it with a photo-editing program. Digital image manipulation offers almost limitless possibilities, which can be overwhelming for most people who are not professional art designers. Anyone, however, can learn and apply some basic photo-editing techniques, which can improve the quality of your digital images. In most cases digital photographs benefit from adjustment of color, brightness and contrast. In some cases, you improve an image by eliminating or reducing flaws. In other cases, you adjust an image for other purposes, perhaps to make it smaller for e-mailing or posting on a Web site. You may want to convert the original image to another format. Finally, you might take an image to a new place, making it something it never was. Here are some basic ways of image manipulation:
- Reduce the size of the photograph to make it smaller for posting on the Web or e-mailing.
- Crop the photograph to emphasize the key part.
- Adjust brightness and contrast to improve the image.
- Correct the color balance.
- Adjust color saturation.
- Use filters to sharpen the photograph.
- Convert color images to black and white.
- Retouch dirt specks and scratches.
- Remove/add backgrounds.
- Add text to photographs.
- Add layers to photographs.
- Convert the photograph to another format.
Adobe Photoshop® Limited Edition was chosen as the photo-editing software in this bulletin. You may have different software that you use. In that case, the tool menu will be different, although the basic techniques are similar.
You may have a full Adobe Photoshop version, perhaps higher or lower versions of the program. Your screen may appear somewhat different, and you may have more menu options, but the tools you will be working with should be identical. You will need a few digital images to work with. We have selected images with names that describe the content, such as “Lantana,” “Vinca,” etc. You may choose to use different names for your files.
Click on 'File,' then 'Open' from the drop-down menu. Select the name of the file you will be working on. For this example, it is 'Lantana.jpg.'
Resizing and Resampling Files
Adjusting Color Balance
Adjusting Brightness and Contrast
Adjusting Hue and Saturation
You may want to convert the image to black and white. Click 'Image,' 'Mode,' 'Grayscale.' The image should turn black and white.
Sometimes the image is not oriented to fit your application. You can rotate it. Click 'Image,' 'Rotate Canvas.' You will rotate the entire canvas, which is your image. You have options as to the degree of rotation. You also can specify the directions of rotation – CW stands for "clockwise" while CCW stands for "counter clockwise."
Changing the Background Color
Working With Filters
Correcting Scratches and Spots
Working With Layers
Working With Text
Text appears on your image in a second layer, Layer 2. Again, Photoshop created another layer, which contains only the text. Notice that the color of the text is pink, same as the background color. If you want another color, you have to go through the Color Picker.
Saving Layered Image
Flattening an Image
This has been a brief course in digital photography. We discussed the basic terms used with digital pictures, such as image size and resolution, image compression and file formats. We showed you step-by-step how to process digital images using popular software. The purpose was to enhance the original picture by adjusting brightness, contrast, color, sharpness, cropping and other image attributes. You must practice with the software to become comfortable with it and be able to process numerous images.
Mention of a commercial or proprietary product in this publication does not constitute a recommendation by the authors nor does it imply registration under FIFRA as amended.
Status and Revision History
Published on Oct 15, 2005
Published on Feb 27, 2009
Published with Full Review on Feb 13, 2012
Published with Full Review on Feb 17, 2015