AQA · GCSE · Computer Science
Course version: 8525 · Version from Sept 2020
Students should be able to describe how higher numbers of pixels and…
Full specification wording
Representing images: Students should be able to describe how higher numbers of pixels and higher colour depths can affect file size, and should also be able to use examples.
A bitmap stores a colour code for every pixel, so file size grows with both the number of pixels and the colour depth.
Checked against the AQA 8525 specification (opens in a new tab) on .
- Point 10 of 18
Study notes
From the checked teaching notes for this specification point.
The idea
A bitmap stores a colour code for every pixel, so file size grows with both the number of pixels and the colour depth. More pixels (a larger width × height grid) means more colour codes to store, so file size increases. Higher colour depth means more bits per pixel, so each pixel takes more storage and file size increases. For example, 100 × 100 pixels at 8-bit colour depth is 10,000 × 8 = 80,000 bits = 10,000 bytes; doubling the colour depth to 16-bit doubles the file size to 20,000 bytes. A multiple-choice question may ask which change increases file size, or which image is larger. Read each option carefully and check whether it changes pixel count, colour depth, or neither.
What earns the marks
- Higher numbers of pixels increase file size because more pixels each need a stored colour code.
- Higher colour depth increases file size because each pixel needs more bits.
- Total bits = number of pixels × colour depth, and bytes = bits ÷ 8.
- A worked example shows the effect, e.g. 100 × 100 at 8-bit = 80,000 bits = 10,000 bytes, and doubling colour depth doubles the size.
- Doubling both width and height quadruples the pixel count and so quadruples file size at the same colour depth.
Common mistakes and how to put them right
Thinking higher colour depth reduces file size.
Put it right: More bits per pixel increases file size.
Confusing number of pixels with colour depth.
Put it right: Pixels are the count of grid squares and colour depth is bits per pixel.
Ignoring the bits-to-bytes conversion.
Put it right: Divide total bits by 8 for bytes.
Assuming a larger image is always higher quality regardless of file size.
Put it right: Link file size to pixels × colour depth specifically.
Exam tips
- Identify which factor each option changes — pixel count, colour depth, or neither.
- Use the formula total bits = pixels × colour depth to check numerical options.
- Eliminate options that claim a higher colour depth or more pixels reduces file size.
Try the lesson
The start of the Representing images: Students should be able to describe how higher numbers of pixels and higher colour depths can affect file size, and should also be able to use examples. lesson, made from checked teaching notes for this specification point.
Have a go first
A photograph is stored as a bitmap. If the same photograph is stored with more pixels but the colour depth stays the same, what happens to the file size?
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