A trade-off answer has four parts: the change, the effect on file size, the effect on quality, and why that suits the purpose. Exam questions often say “explain” or “discuss” after a calculation, and these four parts are where the marks sit.
This lesson builds on image size, audio size and compression.
What can you change, and what happens?
| Change | Effect on size | Effect on quality |
|---|---|---|
| Higher image resolution (more pixels) | Larger | Sharper, more detail |
| Higher colour depth | Larger | More colours, smoother shades |
| Higher sample rate | Larger | Sound wave captured more closely |
| Higher sound bit depth | Larger | Finer difference between loud and quiet |
| Lossy compression | Smaller | Some detail permanently lost |
Every row has a cost on one side and a benefit on the other. The skill is to say both.
How do you calculate the effect?
Because size is a product of quantities, you can compare two settings by seeing which factors changed. Halving one factor halves the size. Halving two factors quarters it.
Worked example
A school wants a photo of 1 200 × 800 pixels at 24-bit colour depth for its website. A student suggests reducing it to 600 × 400 pixels for a mobile page.
Original size: 1 200 × 800 = 960 000 pixels. 960 000 × 24 = 23 040 000 bits = 2 880 000 bytes = 2 812.5 KiB (1 KiB = 1 024 bytes).
Reduced size: 600 × 400 = 240 000 pixels. 240 000 × 24 = 5 760 000 bits = 720 000 bytes = 703.125 KiB.
Comparison: 2 880 000 ÷ 720 000 = 4, so the reduced image is one quarter of the size.
Written answer: Halving the width and height gives a quarter of the pixels, so the file is a quarter of the size and loads faster on a mobile connection. However, there is less detail, so the picture would look blurry if shown full-screen on a large display. For a small mobile page the lower quality is acceptable.
The mistake to watch for
A common slip is to give a one-sided answer.
Mistaken answer: “Using a higher sample rate is better because it improves the sound.”
The student states a benefit but no cost, and does not mention file size at all.
The correction: “A higher sample rate takes more samples each second, so the recording follows the sound wave more closely and sounds closer to the original. However, the file is larger, so it uses more storage and takes longer to transfer.”
Check yourself
Try these, then open each answer.
1. The same image is saved at 24-bit and then at 8-bit colour depth, with the same resolution. How many times smaller is the 8-bit file?
Show answer
Only the depth changes, from 24 to 8, so the size is divided by 24 ÷ 8 = 3. It is one third the size, but with far fewer colours (256 instead of more than 16 million), so shading may look banded.
2. Explain one trade-off of recording a podcast at a low sample rate.
Show answer
A low sample rate takes fewer samples per second, so the file is smaller and quicker to upload. However, the wave is captured less closely, so the sound may seem less clear. For speech this loss can be acceptable.
3. A sports club stores match video for coaches to review in detail. Should it choose maximum compression? Justify your answer.
Show answer
Heavy lossy compression would save space but remove detail coaches need, such as small movements. The club should accept larger files, or use light compression, because detail matters more than storage here.
Where this leads next
All of these answers use units, so finish the module with converting storage units, then test everything in the practice set. The Python reasoning sandbox lets you compare two settings quickly as code.
One-sided trade-off answers lose marks quickly, and a teacher can mark yours line by line with you in online one-to-one Computer Science tuition.