Contrast is the difference in brightness between the lightest and darkest parts of an image. Increasing contrast makes the light areas lighter and the dark areas darker, giving a more vivid appearance. Decreasing contrast reduces this difference, producing a flatter, softer look.
Why adjust contrast?
Improve readability of scanned text.
Enhance visual impact for presentations or reports.
Correct images that appear washed‑out or overly dark.
Prepare images for printing where tonal range is important.
Key concepts
The contrast adjustment can be expressed mathematically. For each pixel value \$p\$ (where \$0 \le p \le 255\$ for an 8‑bit image) the new value \$p'\$ after applying a contrast factor \$c\$ is:
\$p' = 128 + c \times (p - 128)\$
where:
\$c > 1\$ increases contrast.
\$c = 1\$ leaves contrast unchanged.
\$0 < c < 1\$ decreases contrast.
Values are clipped to the range \$0\$–\$255\$ after calculation.
Steps to adjust contrast using typical ICT software
Open the image in the editing program (e.g., Paint, Photoshop, GIMP).
Locate the “Contrast” control – often found under Image → Adjustments → Contrast or a similar menu.
Choose the method:
Slider method: Drag the contrast slider left (decrease) or right (increase) while watching the preview.
Numeric entry: Enter a contrast factor or percentage directly if the program allows.
Observe the effect on the image preview. Ensure that important details are not lost in shadows or highlights.
Confirm the adjustment by clicking “OK” or “Apply”.
Save the edited image using File → Save As to keep the original unchanged.
Common pitfalls and how to avoid them
Clipping: Excessive contrast can push pixel values to 0 or 255, causing loss of detail. Use the preview and adjust gradually.
Colour shift: Some programs apply contrast uniformly to all colour channels, which may alter hue. If colour fidelity is critical, adjust contrast on the luminance channel only.
File format: Save the final image in a format that supports the required colour depth (e.g., PNG for lossless, JPEG for web use).
Contrast adjustment comparison table
Contrast setting
Effect on image
Typical use
Decrease (c < 1)
Shadows become lighter, highlights become darker; overall flatter look.
Shadows become darker, highlights become brighter; more vivid.
Emphasising details, creating dramatic effect.
Suggested workflow diagram
Suggested diagram: Workflow for adjusting image contrast, showing steps from opening the file to saving the final version.
Assessment checklist
Can you locate the contrast adjustment tool in the software you are using?
Can you explain how the contrast factor \$c\$ influences pixel values?
Can you adjust contrast without losing important image details?
Can you save the edited image in an appropriate format while preserving the original file?
By mastering contrast adjustment, you will be able to improve the visual quality of images for a wide range of ICT tasks, from creating presentations to preparing digital portfolios.