3.3.6 Representing Images — AQA 8525

How Images Are Stored

A digital image is made up of thousands (or millions) of tiny squares called pixels (picture elements). Each pixel stores a colour as a binary number. The more bits used per pixel, the more colours are available.

Colour Depth (Bit Depth)

Colour depth is the number of bits used to store each pixel's colour. It determines how many different colours can be represented.

Colour depth	Number of colours	Common use
1-bit	2 (black & white only)	Simple icons, QR codes
4-bit	16 colours	Early computer graphics (CGA)
8-bit	256 colours	GIF images, web icons
16-bit	65,536 colours	High colour, older displays
24-bit	16,777,216 (True Colour)	Photos, modern screens

24-bit True Colour uses 8 bits for Red, 8 bits for Green, and 8 bits for Blue (RGB). Each channel can take 256 values (0–255), giving 256 × 256 × 256 = 16,777,216 possible colours.

Resolution

Resolution is the number of pixels in an image, usually given as width × height (e.g. 1920 × 1080).

A higher resolution means more pixels, so the image contains more detail — but also requires more storage space. Image resolution should not be confused with screen resolution (the number of pixels a display can show).

Calculating Image File Size

File size (bits) = width × height × colour depth

Then ÷ 8 for bytes, ÷ 1024 for KB, ÷ 1024 again for MB

-- Example: 800 × 600 image, 24-bit colour
File size = 800 × 600 × 24 bits
          = 11,520,000 bits
          = 11,520,000 ÷ 8 = 1,440,000 bytes
          = 1,440,000 ÷ 1024 ≈ 1406 KB
          = 1406 ÷ 1024 ≈ 1.37 MB

Metadata

Image files also contain metadata — data about the image stored in a header at the start of the file. Metadata includes:

Metadata item	Example value
Image width	800 pixels
Image height	600 pixels
Colour depth	24 bits
File format	BMP, PNG, JPEG
Date taken	2025-07-24

Metadata adds a small overhead to the file size, but is essential for software to know how to decode the image data.

Effect of Changing Resolution and Colour Depth

Change	Effect on quality	Effect on file size
Increase resolution	More detail, sharper image	Larger
Decrease resolution	Less detail, more pixelated	Smaller
Increase colour depth	More colours, smoother gradients	Larger
Decrease colour depth	Fewer colours, banding/dithering	Smaller

Exam tip: Always calculate in bits first (width × height × colour depth), then convert to bytes (÷8) and further if needed. Remember colour depth = bits per pixel, NOT number of colours.

⚠️ Common Mistakes

Using colour depth as the number of colours in the formula — the formula needs BITS per pixel, not the number of colours.
Forgetting to divide by 8 to convert bits to bytes.
Confusing 24-bit colour (16.7M colours) with 8-bit colour (256 colours).
Dividing by 1000 instead of 1024 when converting to KB/MB — always use 1024 in binary context.

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Key points

Images are made of pixels; each pixel's colour stored as a binary number
Colour depth = bits per pixel; 24-bit True Colour uses 8 bits each for R, G, B
Resolution = width × height in pixels; more pixels = more detail + larger file
File size (bits) = width × height × colour depth; ÷8 for bytes
Metadata (header) stores info about width, height, colour depth, format

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Worksheet — 3.3.6 Representing Images

8 questions · 20 marks

Q1What is a pixel?[1]

✅ Mark scheme

Mark scheme

The smallest unit/element of a digital image [1]; each pixel stores a colour value as a binary number.

Q2An image uses 8-bit colour depth. How many different colours can it represent?[2]

✅ Mark scheme

Mark scheme

2⁸ [1] = 256 colours [1].

Q3Explain what is meant by '24-bit True Colour'.[2]

✅ Mark scheme

Mark scheme

24 bits are used per pixel [1]; 8 bits each for Red, Green, and Blue channels, giving 2²⁴ = 16,777,216 possible colours [1].

Q4Calculate the file size in bytes of a 400 × 300 pixel image with a colour depth of 8 bits.[3]

✅ Mark scheme

Mark scheme

400 × 300 = 120,000 pixels [1]; × 8 = 960,000 bits [1]; ÷ 8 = 120,000 bytes [1].

Q5An image has a resolution of 1280 × 720 and uses 24-bit colour. Calculate its file size in MB. (1 KB = 1024 B, 1 MB = 1024 KB)[4]

✅ Mark scheme

Mark scheme

1280 × 720 × 24 = 22,118,400 bits [1]; ÷ 8 = 2,764,800 bytes [1]; ÷ 1024 = 2700 KB [1]; ÷ 1024 ≈ 2.64 MB [1].

Q6State two pieces of metadata that might be stored in an image file's header.[2]

✅ Mark scheme

Mark scheme

Any two: image width/height in pixels [1]; colour depth/bits per pixel [1]; file format [1]; date created/taken [1].

Q7Describe the effect on file size and image quality of reducing the colour depth from 24-bit to 8-bit.[2]

✅ Mark scheme

Mark scheme

File size decreases (fewer bits per pixel) [1]; image quality decreases — fewer colours means colour banding/less smooth gradients may appear [1].

Q8A website needs to load quickly on slow connections. A designer considers two options: a 1920×1080, 24-bit image, or a 640×480, 8-bit image. Calculate both file sizes in KB and justify which option is more suitable for the website.[4]

✅ Mark scheme

Mark scheme

Option 1: 1920×1080×24=49,766,400 bits ÷8÷1024≈6075 KB [1]; Option 2: 640×480×8=2,457,600 bits ÷8÷1024=300 KB [1]; 300 KB is much smaller [1]; suitable for slow connections as it downloads faster, though image quality is lower [1].

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How Images Are Stored

Colour Depth (Bit Depth)

Resolution

Calculating Image File Size

Metadata

Effect of Changing Resolution and Colour Depth

Key points

Worksheet — 3.3.6 Representing Images

Mini Test — 3.3.6 Representing Images