Be able to crop an image

Topic 12 – Images: Cropping an Image

Learning Outcomes (Cambridge IGCSE ICT AO1‑AO3)

• AO1 – Knowledge & Understanding: Explain why cropping is used, how it influences resolution, aspect‑ratio, DPI/PPI and file size, and choose the most appropriate image format.
• AO2 – Application: Carry out cropping (and basic related edits) using common software tools, and record the steps accurately.
• AO3 – Evaluation: Critically assess a cropped image against the technical requirements of its intended medium (print, web, presentation) and suggest improvements.

Why Crop an Image?

• Remove unwanted background, borders or distractions.
• Change the aspect‑ratio to fit a specific layout (e.g. a newsletter column, PowerPoint slide or web banner).
• Direct the viewer’s attention to the most important part of the picture.
• Reduce the total number of pixels → smaller file size → faster loading on the web.

Key Concepts Before You Crop

Resolution & DPI/PPI

• Resolution = width × height in pixels.
• DPI / PPI (dots or pixels per inch) tells how many pixels are printed or displayed per inch.

  • Print: ≥ 300 dpi for high‑quality output.
  • Screen / web: 72 – 96 dpi** is usually sufficient.

• When you crop, the pixel count falls. Always verify that the remaining resolution still meets the target medium.

File‑size Reduction

• Crop → fewer pixels.
• Lower resolution (e.g. 300 dpi → 150 dpi for web).
• Reduce colour depth (24‑bit → 8‑bit) for simple graphics.
• Choose an appropriate compression format (JPEG – lossy, PNG – lossless).

Common Image Formats (exam‑relevant)

General Cropping Workflow (AO2)

1. Open the image in the chosen editor.
2. Select the Crop tool (icon usually looks like a square with a diagonal line).
3. Define the area to keep:

   • Drag corner or side handles.
   • Enter exact dimensions (width × height) if a specific size is required.
   • Lock the aspect ratio when the target medium has a fixed shape.

4. Fine‑tune the selection – move the whole box, use the rule‑of‑thirds overlay (if available), or enable “Snap to grid”.
5. Apply the crop – press Enter, click Apply/OK, or double‑click inside the frame.
6. Check resolution & file size – open the image properties or the “Image Size” dialog.
7. Save the edited file:

   • Use Save As to keep the original untouched.
   • Choose the format that matches the intended use (see table above).

Software‑specific shortcuts (quick reference)

Basic Image Edits Often Required With Cropping

Brightness & Contrast

• Photoshop: Image → Adjustments → Brightness/Contrast or the slider in the Properties panel.
• GIMP: Colors → Brightness‑Contrast.
• Paint: Adjust → Brightness/Contrast (simple slider).
• Tip: Make only modest adjustments (≤ 20 %) to avoid banding.

Rotate, Flip & Reflect

• Rotate 90° clockwise/anticlockwise or use a free‑rotate handle.
• Flip horizontally (mirror) or vertically (upside‑down).
• Common shortcuts:

  • Paint: Ctrl + R (rotate right).
  • Photoshop: Ctrl + T, right‑click → “Flip Horizontal”.

Grouping & Layering (where supported)

• Layers let you crop one element without affecting others – essential for composite graphics.
• Group several objects (e.g., a picture + a caption) so they move as a single unit.
• In Photoshop/GIMP: Ctrl + G to group; Ctrl + E to merge when you’re finished.

Tips for Effective Cropping (AO1)

• Maintain aspect ratio when the image must fit a predefined space – lock the ratio in the tool options.
• Leave a small margin around the main subject to avoid a cramped look.
• Apply the rule of thirds – imagine a 3 × 3 grid; place key elements on the intersecting points.
• After cropping, re‑check the resolution:

  • Print: ≥ 300 dpi.
  • Web: 72 – 96 dpi (or the exact pixel dimensions required).

• If file size is still large, try:

  • Saving as JPEG with 60‑80 % quality.
  • Reducing colour depth for simple graphics (e.g., 8‑bit PNG).

Common Mistakes & How to Avoid Them

• Cutting off important parts – use the preview pane and rule‑of‑thirds overlay before applying.
• Unintentionally changing aspect ratio – lock the ratio or enter the required width × height.
• Over‑compressing JPEG – keep quality ≥ 60 % to prevent visible artefacts.
• Saving over the original – always use “Save As” and give the new file a distinct name.
• Forgetting to re‑save after cropping – double‑check that the edited file appears in the folder.

Example Scenario (Practical Application)

Task: Emma is preparing the school newsletter. The original photograph is 4000 × 3000 px (≈ 12 MP). The column space requires an image of 800 × 600 px at 300 dpi.

1. Open the picture in Adobe Photoshop.
2. Press C to select the Crop tool. In the options bar set Width = 800 px, Height = 600 px and click the lock icon to keep the aspect ratio.
3. Drag the crop box so the students’ faces are centred; align the eyes with the top‑right rule‑of‑thirds intersection.
4. Press Enter to apply the crop.
5. Choose Image → Image Size. Verify that the resolution reads 300 dpi – suitable for print.
6. Save the result:

   • File → Save As → select JPEG.
   • Set quality to 80 % (gives a file around 150 KB).
   • Name the file newsletter‑event‑crop.jpg and keep the original event‑full.jpg unchanged.

7. Insert the saved image into the newsletter document (Section 17 – Word Processing) and check that it fits the column without distortion.

Cross‑Reference to Other ICT Units

• Section 17 – Word Processing: Inserting the cropped image, resizing within the document, and adding a caption.
• Section 19 – Presentations: Using a cropped graphic as a slide background or an accent image.
• Section 21 – Web Design: Optimising the cropped image for faster page loading (≤ 200 KB, correct dimensions).
• Section 8 – e‑Safety: “Never edit or publish images that contain personal data without permission.”

Evaluation Rubric (AO3)

Summary

Cropping is a core image‑handling skill that sharpens visual focus, adapts pictures to specific layouts, and can considerably reduce file size. Effective cropping requires an understanding of resolution, DPI, aspect‑ratio and suitable file formats, as well as the ability to apply basic edits (brightness, contrast, rotation). Always evaluate the final image against the technical specifications of its intended use and keep an un‑altered backup.

Self‑Check Questions

1. What are the main reasons for cropping an image, and how does cropping influence file size?
2. Outline the step‑by‑step process for cropping an image in Microsoft Paint, including how to check the final resolution.
3. Why is it sometimes essential to lock the original aspect ratio while cropping?
4. Identify two common mistakes made when cropping and describe how to avoid each.
5. Contrast JPEG and PNG in terms of compression type and support for transparency.
6. How would you decide whether a cropped image is suitable for a printed newsletter versus an online article?