Digital Photography Workshop

Image of flower

Toward a Theology of Tools, or What in the World Is This Thing For?

Workshop Objectives

  1. Adjust a digital camera's settings to optimize image quality.
  2. Transfer images from a digital camera to a computer.
  3. Describe the makeup of a digital image.
  4. Describe the most common digital image formats.
  5. Determine the appropriate size for an image that will be used:
  6. Use image editing software to perform tasks such as:
    1. Crop and resize images.
    2. Adjust the brightness and contrast of improperly exposed images.
    3. Perform color corrections.
    4. Change undesirable portions of a picture (power lines, litter, etc.).
    5. Add text, lines, arrows, and boxes to an image.
    6. Create and edit multi-layer images.
    7. Use filters to modify images.

Adjusting a Digital Camera's Settings

Transferring Images From Camera to Computer

Image Transfer Demonstration

What Is a Digital Image?

A digital image consists of a large number of colored rectangles called pixels. Your eye combines the pixels into a picture.

Each pixel is a single color. The colors are defined numerically, using a specific color model. The most common color model for computer displays is RGB (Red Green Blue). This is an additive color model in which the three primary colors of light--red, green, and blue--are combined to create millions of other colors. The most common way to specify colors is by using a 24-bit additive color model. Each of the primary colors can range in intensity from 0 (no color at all) to 255. This provides 256 shades of each primary color. (256 = 28 bits.) Colors can be specified in decimal (0-255) or hexadecimal values (0-ff, where ff = 255).

Photography of tabby cat.

Digital Color Exercise

Use an image editing program to perform the following tasks:

  1. Open an image of your choice and use its color sampling tools to measure the color of various pixels.
  2. What color is ffffff?
  3. What color is 000000?
  4. What color is ff0000?
  5. Discuss with your neighbors: How would you identify and specify spot colors for a publication, presentation, or Web page that complement an image you are planning to use?

Digital Image File Formats

Digital images may be stored in a variety of file formats. A file format is a particular way of organizing and storing the pixels that make up a digital image. File formats may be lossless, meaning that no pixel information is lost when the image is stored, or lossy, meaning that the file format uses a compression algorithm that results in the loss of pixel information. JPEG (Joint Photographic Experts Group) is an example of a lossy format. TIFF and BMP are examples of lossless file formats.

File Format Exercise

  1. Save the same image in the following formats: JPEG, TIFF, BMP, GIF. How do the file sizes of each format compare?
  2. If your image editing program supports variable JPEG compression, save an image at the lowest compression setting and at the highest compression setting. How noticeable is the difference in image quality?
  3. Discuss with your neighbors: What do you think will happen to the quality of a JPEG image as it is repeatedly edited and saved?

Sizing Digital Images

Digital images are made up of rectangular chips of color (pixels). When a digital image is viewed on a computer display, each pixel in the image will normally occupy one pixel on the screen; that is, an image that is 800 pixels wide by 600 pixels high will occupy the entire screen area of a 800x600 display. Certain exceptions:

Web Pages

There are no hard and fast rules for sizing images for Web pages, because you don't know how big your visitors' displays are. The World Wide Web Consortium maintains browser statistics based on its visitor logs. These statistics will provide you with an idea of what a fairly large sample of Web users are using; keep in mind that w3.org visitors are a more technically advanced lot than the general population. I use a very conservative approach on my own pages: no image wider than 600 pixels or higher than 400 pixels. That ensures that no one will ever have to scroll to see an image in its entirety, even if they are using an obsolete 640x480 display. Here is the code to insert an image in a Web page:

<img src="samplepic.jpg" width="600" height="640" alt="some text to describe the image for vision-impaired visitors or others who can't see the image" />

Images for PowerPoint

Powerpoint is the easiest program to use when it comes to sizing digital images. Size the images so that they are no larger than the resolution of your output device. For projectors, that will most likely be SVGA (800 pixels by 600 pixels), or XGA (1,024 pixels by 768 pixels). If the images are in portrait mode or don't fit the 4x3 aspect ratio of the projector, make sure that their largest dimension is less than the corresponding dimension on the display. For example, if you have a vertical image and an XGA display, the image should be no taller than 768 pixels. Following this methodology ensures that you can size your images in PowerPoint to the maximum size of the screen with no quality loss. If you use the images at a smaller size, PowerPoint will scale them down accordingly. PowerPoint does a fairly good job of scaling images up, but the loss in quality may be noticeable if you stretch an image too far.

Images for Print

Resolution is a factor in sizing images for print. Resolution is usually measured in pixels per inch or ppi. For printing on low-end inkjets or color laser printers, a resolution of 120 to 200 ppi is usually adequate. For professional printing, you may need to go as high as 300 ppi. A couple of examples will familiarize you with the math:

Consumer-Quality Inkjet Example
You want to print an image six inches wide and four inches high. 150 ppi should be adequate, so use your image editing program to size the image to 900 pixels wide by 600 pixels high, with a resolution of 150 pixels per inch. If you have a photo printer, consult your documentation to see if higher resolutions are recommended.
Professionally Printed Materials
You are creating images for your school's yearbook. The yearbook company wants TIFF images at 300 ppi. You want to print a 5x7 image in portrait mode. Your image should be 1500 pixels wide and 2,100 pixels high, with a resolution of 300 ppi.

Emailing Images

In a ideal world, you would email digital images to your friends and colleagues at the images' original (largest) size, so your recipients could do with them as they see fit. Due to bandwidth limitations, you shouldn't do that. Some email servers limit attachments to 1 megabyte, which is still pretty large for a JPEG image. Please note that compressing a JPEG image using a zip utility won't really make the image much smaller, because it is already compressed. If you have a lot of images you would like to share with friends, consider a photo sharing website.

Sizing Exercise

  1. Open an image of your choice and determine its width and height in pixels.
  2. What is the resolution of the image, measured in pixels per inch (ppi)?
  3. Can you change the ppi resolution without changing the size of the image?
  4. What happens if you decrease the number of pixels in the image?
  5. What happens if you increase the number of pixels in the image?
  6. Size an image to display as a thumbnail on a Web page. Copy this HTML code into Notepad, modify it to point to your thumbnail image. Save the file as test.htm and view the file in a Web browser.
  7. Modify the HTML code in the previous step so that you can click the thumbnail and display a larger version of your picture.
  8. What happens if you modify the width and height attributes of your thumbnail image so that they no longer match the thumbnail's actual dimensions?
  9. Determine the resolution of your computer's display. Size an image appropriately for full-screen usage in a PowerPoint presentation. Create a PowerPoint presentation and insert the image, and size it to full-screen.
  10. Duplicate the PowerPoint page you just created, then resize the image to approximately quarter-screen size. View the presentation. Is there any loss in quality from the full-screen to the quarter-screen page?

Terry Hawthorne