Week 2 – What is a digital image?

What is a Digital Image Overview

What is a digital image?
A digital image is called a bitmap, which is a two dimensional mapping of pixels that are themselves made of bits. This is like a large mosaic of tile pieces. The tile pieces are the pixels and the more tiles you have the more resolution you have.

What is image resolution and megapixels?
Image resolution refers to how many dots-per-inch or dpi an image has. We can also describe this analogously as ppi or pixels per square inch. If you have a higher image resolution you will be able to print in a bigger size or format. Generally, a better quality digital camera, will be capable of delivering a higher image resolution. You will see this capability promoted as a camera’s megapixels.
A megapixel refers to a million pixels. If your camera is capable of taking a photograph 1200 pixels wide and 850 pixels tall, then 1200 x 850 = 1,000,000 pixels (approximately) or 1 megapixel.

How many megapixels do I need my camera to have?
It depends on how big you want to print your images. Technically, you can print your image as big as you want, however it will only have photographic quality and detail if their are 360 dots per inch worth of resolution. Some schools of thought think you can go down to 300 dpi or even 220 dpi and maintain photographic printed quality. My experience tells me 360 dpi is photo quality. So if that is the case how many megapixels would you need to produce a fantastic 8″ x 10″ print? An 8″ x 10″ photograph needs 8″ x 360 = 2880 pixels tall and 10″ x 360 = 3600 pixels wide. 2880 x 3600 = 10,368,000 pixels which mean you would need a 10.4 megapixel camera to produce a fantastic 8″ x 10″ image. My camera is 12.3 megapixels which means I can take 8″ x 12″ at 360 dpi or a 9.5″ x 14″ at 300 dpi.

What is a pixel?

Pixel stands for picture element and it is one tiny square, a combination of color and light, on your screen. It is analogous to one color dot… except that it is a square. The color of a pixel is determined by how many bits of memory have been allotted to it. That is called the ‘bit depth’.

For additional information about pixels: http://en.wikipedia.org/wiki/Megapixel#Megapixel

What is bit depth (color depth)?
How many bits of memory are being used to color the pixels of an image are its bit depth. You have probably heard that computers are just made up of ones and zeros and that is true. A bit or binary digit has only two values one and zero. So if your image’s bit depth is a 1 bit your picture can only be two colors black and white. For instance zeros would be used represent the black pixels and ones for the white. The point is that each pixel can only have two values 1 or 0, black or white. The binary number system functions just like the decimal except that instead of counting with 10 digits (0 through 9) binary is just 1 and 0. So for instance a grayscale image can have a bit depth of 8 bits meaning each pixel has 256 diferent combinations of 0 and 1, or 2x2x2x2x2x2x2x2 = 256 values of gray. A color image can have a bit depth of 24 bits. 8 bits for red, 8 bits for green, and 8 bits for blue (RGB) that means each pixel is made up of a possible 256 values for each channel of red, green, and blue. Total combinations of 1s and 0s is 2 to the 24th power or 16,777,216 different colors. The higher the bit depth the larger the file size and the more colors that are possible. It is important to know that there are different types of image files and each format may have a different bit depth and therefore a different file size.

For additional information about bit depth: http://en.wikipedia.org/wiki/Color_depth

What are the image file formats (saving images)?

You can save your images in different image formats which will result in different file sizes, bit depths, and other capabilities like transparency and saving layered information. Photoshop’s native file format is a .psd file which supports layer information. When saving photographs for posting to the web JPG and PNG are the most popular formats. PNG images can support transparent backgrounds whereas JPGs cannot, but if you do not need transparency JPG images are smaller files and quicker downloads. When saving an image as a JPG you can specify the amount of compression and therefore the quality of the output image. When saving an image as a PNG file you can specify whether to include transparency or not. Here is a table outlining some of the common file formats and their features:

File Format File Extension File Size Transparency Layers Colors/Bit Depth Compression Type
Joint Photographic Experts Group .jpg,
Small Files
No No 24 bit color Lossy
Portable Network Graphics .png Small Files
Large Files
Yes No 24 bit
8 bit
Graphics Interchange Format .gif Small Files
Yes No 8 bit
256 colors
Photoshop Document .psd Very Large
File Size
Yes Yes

32 bit

Tagged Image File Format .tiff
Large File Size No No

24 bit,
48 bit

Windows Bitmap File .bmp Very Large
File Size
Yes No

32 bit

Raw Image File Format
Multiple Formats (No industry standard)
.raw, .dng,
.crw, .cr2,
.nef, .3fr,
.dcr, .k25,
Very Large
File Size
No No




Learning At Lynda.com – What Videos to Watch

These are the videos I recommend watching this week at Lynda.com. Choose the Lynda title to watch based on your version of Photoshop or maybe check out both!:

  • Photoshop CS6 for Photographers – with Chris Orwig

    Introduction – Watch all of the videos.
    1. Strategies for Success – Watch all of the videos.
    3. Color Settings and Preferences – (Optional) If you are interested in configuring your photoshop color settings then check out these videos out.
    4. The Foundations of Color Management – (Optional) If you are interested in configuring your monitor settings then check some of these videos out.
    5. Getting Started with Photoshop – Great videos for learning the Photoshop interface. Watch as many as you want.
    6. Understanding Digital Images – Watch all of these videos!

  • Photoshop CC for Photographers: Foundations – with Chris Orwig

    Introduction – Watch all of the videos.
    1. Strategies for Learning Photoshop – Watch all of the videos.
    3. Settings Up Photoshop – Color and interface configuration settings.
    4. Getting Started with Camera Raw – (Optional) If you have a nice digital SLR camera that can shoot Raw photo format you may want to check out these videos.
    6. Getting Started with Photoshop – Videos for learning the Photoshop interface. Watch as many as you want.
    6. Understanding Digital Images and Resizing – Watch all of these videos!

Week 2 Assignment 1 – How to Save a Photograph in Different Sizes and File Formats Using Photoshop
Week 2 Assignment 2 – How to Crop and Straighten an Image


Author: Dan

Dan teaches computer networking and security classes at Central Oregon Community College.

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