Digital Images 101

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What is the best file format to use for my digital images?

The answer is - it depends. It depends on the type of graphic (e.g. photographic representation, illustration, flat-color image) and how you plan to use the image (e.g. web, print, presentation). There are basically two classes of graphic file formats: bitmap and vector. Both types come in a variety of formats, as seen in the chart below.

 

Graphic File Type Description Examples Pros & Cons
Bitmap (a.k.a. Raster)

This type of graphic is comprised of tiny dots called pixels. A color is assigned to each pixel to form a picture, like a puzzle. Because the image is stored as a collection of pixels, scaling the image (i.e. blowing it up) involves interpolation, a mathematical "guess" or calculation of new pixels based on known pixels. When you resize a bitmap image, you run the risk of compromising quality through loss of original image detail.

Bitmaps are generated by many software programs such as Adobe Photoshop, Paint Shop Pro, Paint, Painter, and many others.

GIF
JPEG/JPEG
TIFF
PNG
BMP
PSD

ProPros: photo quality.

ConCons: large file size; poor scaling.

*All Photoshare images are in the bitmap class: thumbnails in the online database are in JPEG format, and their corresponding high-resolution master files are archived as TIFFs.

Vector

This type of graphic uses lines and curves (not pixels) to make shapes. The image is stored as a geometric shape using a mathematical formula that provides undistorted scaling.

Vectors are generated by software programs such as Adobe Illustrator, Macromedia Freehand, Flash, Fireworks, and a few others.

AI
EPS
WMF

ProPros: small file size; easily scaled; best suited for illustrations, line art, and type.

ConCons: photo realism does not compare to bitmap; most Web browsers do not support vector images.

What is the difference between JPEG, TIFF, GIF, etc.?

Although digital photo users are primarily concerned with JPEG and TIFF formats, we also get questions about other bitmap file formats. The tables below compare several bitmap file formats.

GIF
(Graphic Interchange Format)

 

Best Use Properties Pros & Cons

Type: Low-resolution graphics with flat, solid areas of color (e.g. logos, illustrations, screen shots).

Purpose: Web, Powerpoint, CD-ROM, and other documents intended for viewing on a computer.

Colors: 8-bit (images can have up to 256 colors)

Encoding scheme: Lossless (however, you will lose quality with most color photographs due to the GIF's reduced color palette).*

ProThis format allows you to create transparent, interlaced, and animated graphics for the Web.**

ConThis format is not good for photographic images due to its reduced number of colors.

*The encoding scheme of a digital image is considered lossless if the image retains the original pixel-by-pixel data when it is saved and re-opened.
**All web graphics are rectangular. However with a GIF, you can create the illusion of an irregularly shaped image by making the background transparent. Interlaced images appear gradually before they are fully downloaded.

JPEG
(Joint Photographic Experts Group)

 

Best Use Properties Pros & Cons

Type: Low-resolution images of photographic quality (images containing many colors that blend and fade).

Purpose: Web, Powerpoint, CD-ROM, and other documents intended for viewing on a computer.

Colors: 8 bit grayscale (256 shades of gray); 24 bit color(16.7 million colors)

Encoding scheme: Lossy

ProCompared to a GIF, this format can better handle color gradations. On a computer monitor, JPEGs appear to retain almost complete image quality for most photographs.

ProSmall file size: this is the optimal format for photographic images on the web or any other document where minimum file size is more important than print quality (e.g. Powerpoint, CD-ROM, Email).

ConThis format does not work as well as a GIF for simple graphics. Also, the standard JPEG does not support transparencies, interlacing, or animation on the web.

ConUnlike a lossless format, the pixel quality of a JPEG is "fragile"(every time you save a change to a JPEG, you compress and further degrade the image). For this reason, you should never alter a JPEG.

TIFF
(Tagged Image File Format)

 

Best Use Properties Pros & Cons

Type: Low or high-resolution images of photographic quality (images containing many colors that blend and fade).

Purpose: Print or preservation.

Colors: 1, 2, 4, 8, 16 or 24-bit color; 1-8 bit grayscale.

Encoding scheme: Lossless; files can be compressed or uncompressed.

ProCompared to a JPEG, an uncompressed TIFF retains the best quality of a photographic image, making it ideal for digital masters (preservation) and print documents.

ConLarge file size

BMP/DIB
(Windows bitmap)

 

Best Use Properties Pros & Cons

Type: Relatively low-resolution images of photographic quality (images containing many colors that blend and fade).

Purpose: Background monitor display on the Microsoft Windows platform.

Colors: 1, 4, 8, or 24-bit color.

Encoding scheme: Lossless; bitmap files are generally not compressed.

ConLimited to RGB-images.

ConGenerally does not support effective image compression.

ConNot supported well across multiple platforms.

ConIt is best to avoid BMP files for pre-press application of photographic images because the TIFF is a more established and versatile format.

PSD
(Native Format of Adobe Photoshop)

 

Best Use Properties Pros & Cons

Type: Any custom-designed or in-process image with layers and complex or graded colors.

Purpose: Use this format to maintain maximum information and editability for pre-press images before exporting to TIFF.

Colors: 24-bit color.

Encoding scheme: Lossless

ProPSD can store a great deal of complex data (layers, effects, text, etc).

ProPSD is the only format that supports layers (other formats must be flattened on export).

ConPSD is an application-specific format.

ConLarge file size.

What is a pixel?

A pixel, short for "picture element," is the smallest unit in a digital image. Every digital photo (bitmap format) is comprised of pixels, almost like a puzzle in which all the pieces are square. As you zoom into an image (as seen below), you will see that your digital photo is comprised of hundreds and thousands of pixels.

Comparison

This image is 150 x 113 pixels (height x width) viewed at 100% (left) and zoomed in at 500% (right).

Let's continue with our puzzle analogy...imagine a puzzle with 4 million pieces! That's how many pixels it would take to compose a 5.7 x 7.6 inch print at 300dpi. Because pixels are the building blocks of an image, it is important to make sure that your digital camera and the settings you choose are capable of generating enough pixels to meet your needs. For example, if you want the digital equivalent of a 4 x 5.3 inch print, you must use a pixel resolution of 1600 x 1200 or greater (calculations are explained below). RULE OF THUMB: the greater the number of pixels, the better the print quality of the photo.

What is the difference between camera resolution, pixel resolution, and print resolution?

Camera resolution is commonly expressed in "megapixels," or millions of pixels. This simply refers to the maximum pixel dimension, or effective pixel resolution, that your camera is capable of generating. For example, if 2048 x 1536 is the highest pixel resolution on your camera, you are working with a 3.1 megapixel camera (2048 x 1536 = 3,145,728 pixels).

Print resolution is commonly expressed as dpi (dots per inch) or ppi (pixels per inch). A standard print resolution is 300dpi (keep in mind that standards may vary among printers and organizations). Having an adequate pixel resolution is more important than having a particular dpi, because the dpi of any image can easily be changed in an image editing program without altering the original pixels. However, you cannot change the number of pixels (i.e. blow up an image) without potentially degrading the quality of the image. Keep in mind that since dpi is related to output (print) size, a change made to the dpi will proportionately adjust the available output (print) size and vice versa.

Confused? The interplay between pixel resolution, print resolution, and output size is a very difficult concept to grasp. Here is an example that illustrates the flexibility and limitations of digital images.

Example: Print Quality Problem
Rosa took a 640 x 480 pixel photo with her digital camera. She wants a 4 x 5 inch copy of this photo to appear in her organization's newsletter. Her graphic designer tells her that the resolution of the image is too low. Rosa doesn't understand because the image appeared large on her computer monitor. She opens the image in Adobe Photoshop and sees that the print size is 6.7 x 8.9 inches.

Figure 1a

Figure 1a

What is the problem?

Figure 1a shows that Rosa is looking at a print size of 6.7 x 8.9 inches at 72dpi. Her graphic designer tells her that this is not an acceptable resolution. Rosa asks, "well, why don't you just change the dpi to 300?"

Her graphic designer shows her that when the dpi is adjusted to 300dpi (preserving the original 640 x 480 pixels), the print size decreases to 1.6x2.1 inches.

Figure 1b

Figure 1b

Rosa's graphic designer explains that a 640 x 480 pixel image is only capable of generating a 1.6 x 2.1 inch print at 300 dpi (see Figure 1b). Rosa realizes that she cannot judge print size or print quality according to how the image appears on her monitor. She also begins to see the interplay between print resolution (dpi) and output size (print size in inches).

Rosa has a bright idea. She says, "why don't we just increase the pixels? Can't we blow up the image?"

Rosa's graphic designer explains that image quality generally suffers when an image is blown up (when the pixel dimension is increased). A minimal increase may not harm the image noticeably. But it is unlikely that this particular image will look good at 4 x 5 inches. Rosa learns that if they increase the pixel dimension and print size, they are essentially asking the computer to stretch the image out and fill in the gaps with new pixels (interpolate). The computer must "guess" at how to best invent this new color data. Unfortunately, Rosa is very disappointed with the computer's interpolation guess (see Figure 1c below).

Figure 1c

Figure 1c

The blown up image in Figure 1c appears "pixelated" and blurry along contours. Rosa now understands how important it is to capture an adequate number of pixels in the first place. Knowing that she may want to print any photo she takes in the field, she resolves to purchase a 256 MB memory card for the digital camera and use the highest pixel resolution on her camera.

Before you scan an image or start clicking away on your digital camera, consider your desired final output and choose a setting accordingly. Use the chart below as a guide.

Pixel Dimensions Related to Print Size

 

Pixels Megapixels Max Print Size at 300dpi (Inches)
640 x 480 .3 1.6 x 2.1 (good for web, email, and powerpoint only)
1024 x 768 .8 2.6 x 3.4
1280 x 960 1.2 3.2 x 4.3
*1600 x 1200 1.9 ~ 2 4 x 5.3
2048 x 1536 3.1 5 x 6.8
2272 x 1704 3.9 ~ 4 5.7 x 7.6
2304 x 1728 3.9 ~ 4 5.8 x 7.7
2560 x 1920 4.9 ~ 5 6.4 x 8.5
2592 x 1944 5.0 6.5 x 8.6
3072 x 2048 6.3 6.8 x 10.2

*We recommend that you shoot photos at 1600 x 1200 or larger.

Still not sure what your camera is capable of? Not sure how to choose the proper setting for your camera or scanner? Send us an email telling us the make and model of your equipment. Also, send a sample image (unmodified), and we'll give you some individual feedback about the camera or scanner you are using.