The effect of colour depth and resolution on The quality of the image The…

The idea

In a bitmap image, resolution is the number of pixels, often expressed as width × height. Colour depth is the number of bits used to represent each pixel's colour. Increasing resolution adds more pixels, allowing finer detail, but increases file size. Increasing colour depth allows more distinct colours, improving colour accuracy, but also increases file size as each pixel needs more bits. Uncompressed file size in bits is calculated as width × height × colour depth. For example, a 1000 × 800 image at 24-bit colour depth has 19,200,000 bits. Dividing by 8 gives 2,400,000 bytes, which is 2.4 MB (using base-10 conversions: 1000 bytes = 1 kB, 1000 kB = 1 MB). Higher resolution or colour depth improves quality but requires more storage.

What earns the marks

• States that resolution is the number of pixels in an image, often given as width × height, and that higher resolution means more pixels.
• Explains that increasing resolution improves image quality by allowing finer detail and sharper edges, but increases file size because more pixels are stored.
• States that colour depth is the number of bits used per pixel and that a greater colour depth allows more distinct colours.
• Explains that increasing colour depth can improve colour accuracy and reduce banding, but increases file size because each pixel requires more bits.
• Uses the relationship file size in bits = width × height × colour depth to calculate or compare image file sizes.
• Explains that file size also depends on compression and metadata, so the calculated value is an estimate of uncompressed size.
• Compares the relative effects of changing resolution versus changing colour depth on quality and file size.

Common mistakes and how to put them right

• Confusing resolution with colour depth.

  Put it right: Resolution is the number of pixels, while colour depth is the number of bits per pixel.

• Thinking that increasing colour depth always makes an image look sharper.

  Put it right: Colour depth affects the number of colours and colour accuracy, while resolution affects detail and sharpness.

• Forgetting to convert between bits, bytes, and larger units correctly.

  Put it right: Divide by 8 to convert bits to bytes, then by 1000 for kB, and 1000 again for MB, as OCR uses base-10.

• Assuming file size is exactly width × height × colour depth in all cases.

  Put it right: This gives uncompressed size; compression and metadata can change the actual file size.

Exam tips

• Write the formula clearly: file size in bits = width × height × colour depth, then convert to bytes or larger units if required.
• When comparing, state which factor changes and how it affects both quality and file size, for example doubling colour depth doubles the uncompressed file size.
• Use correct units and show your working, especially when converting bits to bytes, kB, or MB using base-10 (1000).
• Link quality to the specific factor: resolution affects detail and sharpness, while colour depth affects the range and accuracy of colours.