Be able to resize an image as specified to maintain or adjust the aspect ratio of an image

Topic 12: Images – Resizing and Aspect Ratio

Understanding Aspect Ratio

The aspect ratio of an image is the relationship between its width and height. It is usually written as width:height (e.g., 4:3, 16:9). Keeping the aspect ratio the same when you resize an image prevents it from looking stretched or squashed.

Resizing While Maintaining Aspect Ratio

1. Determine the original width ($W_{orig}$) and height ($H_{orig}$).
2. Choose either a new width ($W_{new}$) or a new height ($H_{new}$) you want.
3. Calculate the missing dimension using the ratio:
• If you know $W_{new}$: $$H_{new} = H_{orig} \times \frac{W_{new}}{W_{orig}}$$
• If you know $H_{new}$: $$W_{new} = W_{orig} \times \frac{H_{new}}{H_{orig}}$$
4. Apply the new width and height in your image‑editing tool.

Changing the Aspect Ratio (Stretching or Cropping)

• Stretching: Set both width and height to values that do not preserve the original ratio. The image will look distorted.
• Cropping: Cut off parts of the image to fit a new ratio (e.g., turning a 4:3 photo into a 16:9 banner).
• Letterboxing/Pillarboxing: Add blank bars (usually black) to the sides or top/bottom to fit a new ratio without cutting content.

Practical Steps (Using Common Software)

1. Open the image in your editor (e.g., Paint, GIMP, Photoshop, or an online tool).
2. Locate the “Resize” or “Image Size” option.
3. Make sure the “Maintain aspect ratio” (or “Lock aspect ratio”) checkbox is ticked if you want to keep the ratio.
4. Enter either the new width or height; the other value will update automatically.
5. If you need a different ratio, untick the lock, set both dimensions, or use the crop tool.
6. Apply the changes and save the image (preferably as a new file to keep the original).

Common Mistakes to Avoid

• Forgetting to lock the aspect ratio when you only want to scale the image.
• Stretching an image to fit a container, making people look too tall or too wide.
• Saving over the original file; always keep a backup.
• Using very low resolution after resizing, which makes the image pixelated.

Quick Reference Table

💡 Tip: Always preview the resized image before saving to ensure it looks correct and that no important details have been lost.