A Short Course Book
Sensors, Pixels and Image Sizes

Exploring Pixels and Colors

 
 
Click to open an Excel worksheet and use Part 4. Color Depth and File Sizes to follow the instructions in this section.
 
 
Click to open an Excel worksheet on scanning and follow the instructions in this section.
Part 3 on the Excel worksheet "Pixels & Images Calculator" calculates the size of print you can expect from a given file size and the dpi you choose to print at. The numbers in the descriptions that follow refer to row numbers on the worksheet.

1. Bits per color is where you enter the number of bits your image uses for each color—red, green, and blue.

2. Bits per pixel is calculated by a formula that multiplies bits per color (line 1) by 3 since three colors are used for each pixel.

3. Number of possible colors is calculated by raising the number 2 to number of bits per pixel (line 2).

4. Width of image (in pixels) is where you enter the image's width in pixels.

5. Height of image (in pixels) is where you enter the image's height in pixels.

6. Total number of pixels in image is calculated by multiplying the image's width by its height.

7. File size (uncompressed) is calculated by multiplying the total number of pixels in the image by the number of bits used to store each pixel. File sizes are shown in bits, bytes, kilobytes, and megabytes.

8. Table of color depths shows color depths used by various image types.

Exercises

Open the worksheet by clicking the Excel button in this section and enter numbers in the green cells to explore the questions that follow.

1. If an image assigns the following number of bits to each pixel, how many colors can be displayed?

  • 2 bits = __________ colors
  • 8 bits = __________ colors
  • 16 bits = __________ colors
  • 24 bits = __________ colors
  • 32 bits = __________ colors
  • 36 bits = __________ colors

2. If an image is 3000 x 2000 pixels and 32 bits of color, how large is the file in megabytes?

 
REVIEW: BITS AND BYTES

. When reading about digital systems, you frequently encounter the terms bit and byte.
. The bit is the smallest digital unit. It's basically a single element in the computer that like a light bulb has only two possible states, on (indicating 1) or off (indicating 0). The term bit is a contraction of the more descriptive phrase binary digit.
. Bytes are groups of 8-bits linked together for processing. Since each of the eight bits has two states (on or off), the total amount of information that can be conveyed is 28 (2 raised to the 8th power), or 256 possible combinations.

Scanning and Image Sizes

Color scanners work by creating separate red, green, and blue versions of the image, and then merging them together to create the final digital image. Some scan all of the colors in one pass while others take three passes, a slower but higher quality method. Which method is used depends on the scanner's image sensor. Most scanners use linear CCDs arranged in a row. Those that require three passes use a single row of photosites and pass different filters (red, green, or blue) in front of the sensor for each pass or use three different light sources. Others have 3 rows of photosites, each row with its own filter so they can capture all three colors on a single pass.

As the image is scanned, a light source travels down the photo (some print and document scanners instead move the document past the light source). The light source reflects off a print or passes through a transparency and is focused onto the image sensor by a mirror and lens system. Because of this mirror and lens system, the sensor does not have to be as wide as the area being scanned.

The horizontal optical resolution of the scanner is determined by the number of photosites on its sensor. However, the vertical resolution is determined by the distance the paper or light source advances between scans. For example, a scanner with a resolution of 600 x 1200 has 600 photosites on its sensor and moves 1/1200 of an inch between each scan.

Scanning and File Sizes

When scanning, your goal is to get a digital image file that contains all of the detail you need without the file being too large to work with. If you scan at too low a resolution, you'll lose detail. If you scan too high, you're file will be too large. When you scan an original image-either a slide or a print-the file size depends on a number of factors including the area being scanned, the resolution of the scanner, and the color depth, or number of bits assigned to each pixel. Let's take a look at this step-by-step as you'd calculate file sizes using the downloadable scanning calculator-Part 1.

Scanning and File Sizes

1. Enter the width in inches of the art to be scanned.

2. Enter the depth or height of the art to be scanned.

3. Enter the scanner's optical resolution or the resolution to intend to scan at.

4. The scanned pixels horizontally are calculated by multiplying the scanner's resolution (line 3) times the horizontal size of the original (line 1).

5. The scanned pixels vertically are calculated by multiplying the scanner's resolution (line 3) times the vertical size of the original (line 2).

6. The total scanned pixels is calculate by multiplying the horizontal scanned pixels (line 4) times the vertical scanned pixels (line 5).

7. The color depth is where you enter the number of bits assigned to each pixel. This would usually be 1 if the image is black & while, 8 if it's grayscale (like a black and white photography), or 24 if it's color.

8. The file size in bits is calculated by multiplying the number of pixels in the image (line 6) time the color depth (line 7).

9. The file size in bytes is calculated by dividing the file size in bits (line 8) by 8.

10. The file size in kilobytes is calculated by dividing the file size in bytes (line 9) by 1,000.

11. The file size in megabytes is calculated by dividing the file size in kilobytes (line 10) by 1,000.

Scanning an Image for Printing at a Specified Size

There are times when you know what size you want a print to be and need to calculate backwards to what size the image file should be. For most purposes, you can expect to get photorealistic quality with a print having about 300 dpi. This means, a 4 x 6 print needs an image file of 1200 x 1800, and an 8 x 10 print needs one 2400 x 3000.

You can stretch or shrink images you've imported into programs such as PageMaker and QuarkXPress. Because the number of pixels in the image don't change, the dots per inch have to. For example, let's say you placed an 800x600 image on a page and it was 2-inches wide. If you print it out at that size, it will print at 400 dpi (800 divided by 2"). If you now stretch the image to be 4 inches wide, the dpi drops to 200 (800 divided by 4"). If you want the image 4" wide AND 400 dpi, best scan it so it's 1600 pixels wide before you import it. However, if it's to be printed on a printing press, the rules change and become more complex. You should scan the image so its sots per inch are twice the lines per inch (lpi) at which it will be printed.

Scanning an Image For Printing at a Specified Size

1. Enter the width in inches of the art to be scanned.

2. Enter the depth or height of the art to be scanned.

3. Enter one of the the printer's output resolution in dots per inch (dpi).

4. The horizontal size of the original is calculated by multiplying the desired horizontal size of the output (line 1) by the desired output resolution (line 3).

5. The vertical size of the original is calculated by multiplying the desired vertical size of the output (line 2) by the desired output resolution (line 3).

6. Enter the color depth of the image. This would usually be 1 if the image is black & while, 8 if it's grayscale (like a black and white photography), or 24 if it's color.

7. The file size in bits is calculated by multiplying the horizontal by the vertical size of the original (lines 4 and 5) to calculate the total number of pixels in the image and then multiplying those by the color depth (line 6).

8. The file size in bytes is calculated by dividing the file size in bits (line 7) by 8.

9. The file size in kilobytes is calculated by dividing the file size in bytes (line 8) by 1,024.

10. The file size in megabytes is calculated by dividing the file size in kilobytes (line 9) by 1,000.

Scanning an Image for Screen Display

Scanning an image for the screen is the same as scanning one for printing except the output is usually specified in pixels, not inches. Although the actual number of pixels per inch on a monitor vary depending on its size and resolution, images are generally scanned at 72 ppi for screen display (although they are sometimes scanned up to 96 ppi). Making them any larger doesn't add any information to the image and just makes the files larger.

Scanning an Image For Screen Display

1. Enter the width in inches of the image to be scanned.

2. Enter the depth or height of the image be scanned.

3. Enter the screen's resolution in dots per inch (dpi). This is normally 72 dpi on average.

4. Enter the desired width of the image in pixels.

5. The vertical size of the image is calculated by dividing it's width on line 4 by the ratio of the original's width to height calculate by dividing line 1 by line 2.

6. Enter the color depth of the image. This would usually be 1 if the image is black & while, 8 if it's grayscale (like a black and white photography), or 24 if it's color.

7. The file size in bits is calculated by multiplying the horizontal by the vertical size of the original (lines 4 and 5) to calculate the total number of pixels in the image and then multiplying those by the color depth (line 6).

8. The file size in bytes is calculated by dividing the file size in bits (line 7) by 8.

9. The file size in kilobytes is calculated by dividing the file size in bytes (line 8) by 1,024.

10. The file size in megabytes is calculated by dividing the file size in kilobytes (line 9) by 1,000.


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