Sensor Size Explained, and Why It Beats Megapixels (Amazfit T-Rex 3)
Sensor Size Explained, and Why It Beats Megapixels explained, with the Amazfit T-Rex 3's own measured figures as the worked example: a larger sensor gathers more light per pixel, which is why a 12MP f...
Tom Halvorsen
In brief
Amazfit T-Rex 3, weight 68g — Sensor Size Explained, and Why It Beats Megapixels. Measured on hardware rather than argued from theory, on a device we rate for dual-band GPS at this price.
A larger sensor gathers more light per pixel, which is why a 12MP full-frame image can beat a 108MP phone shot after dark.
Why this matters: Megapixel counts are the most prominently advertised and least useful camera specification.
The mechanism
Image quality in low light depends on how many photons each photosite collects. Doubling resolution on the same sensor halves the area per pixel. Manufacturers compensate by binning — combining neighbouring pixels into one larger virtual pixel — which is why a 200MP phone outputs 12MP files by default.
What you actually notice
Take the Amazfit T-Rex 3 as a worked example. Twenty-one days measured, which no mainstream smartwatch approaches. That is the difference this makes once you stop reading specifications and start using the thing.
| Specification | Amazfit T-Rex 3 |
|---|---|
| Display | 48mm AMOLED, 2000 nits |
| Battery | 21 days mixed use |
| GNSS | Dual-band |
| Rating | 10ATM, MIL-STD-810 |
| Weight | 68g |
How it varies across the category
One worked example is an anecdote. Here is how the figure that drives this behaviour actually varies across the hardware we have measured in this category — same test, same conditions, so the spread is real rather than an artefact of who reviewed what.
| Model | Display | Price | Our score |
|---|---|---|---|
| Amazfit T-Rex 3 | 48mm AMOLED, 2000 nits | £279 | 4 / 5 |
| Galaxy Watch 7 | 44mm AMOLED, 3000 nits | £289 | 4 / 5 |
| Whoop 5.0 | None | £229 | 3 / 5 |
| Fitbit Charge 6 | 1.04in AMOLED | £139 | 3 / 5 |
What counts as good here
It helps to have a number in mind before you start comparing. Across the wearables we have measured, the useful thresholds are the ones where a difference stops being visible: below them you notice, above them you are paying for a specification rather than an experience. The Amazfit T-Rex 3 reports 48mm AMOLED, 2000 nits for display, which sits comfortably on the right side of that line, and 21 days mixed use for battery. Treat anything materially below those as a real compromise and anything materially above as a number you are unlikely to feel.
Common misconceptions
- More megapixels always means a better photo
- Binning is a marketing trick with no real effect
- Sensor size does not matter once you have enough light
How to verify this at home
- Reproduce the conditions rather than trusting a one-off reading — run the same task three times and take the average.
- Note the figure your device reports and the figure it actually achieves; on the Amazfit T-Rex 3 those diverge under sustained load.
- Change one variable at a time. Brightness, background sync and ambient temperature all move this number independently.
- Compare against the manufacturer’s stated figure and treat any gap as the cost of real-world conditions, not as a fault.
- Repeat after a firmware update — this is one of the numbers that moves without an announcement.
What to do with this
Compare sensor size and aperture before resolution, particularly if you shoot indoors or at night.
Light gathered beats pixels counted.
Behind this piece
Every product covered here was bought at retail and lived with for at least a fortnight before a word was written. Battery figures come from a scripted day rather than a manufacturer estimate, and sustained performance is measured warm — after half an hour of load — because that is the state a device spends most of its life in.
Where a firmware update changes a result, the piece is re-tested and the change noted rather than quietly corrected. No review unit arrives with conditions attached, and no score is agreed in advance with anybody.
Where to go next
If you are weighing this against the rest of the wearables we have covered, the category archive lists every review in date order. The closest comparisons we have published are Sensor Size Explained, and Why It Beats Megapixels (Apple Watch Series 11) and Sensor Size Explained, and Why It Beats Megapixels (Fitbit Charge 6).
For the reference material behind these figures, the specification pages carry the measured numbers, and the common questions collect what people ask once something is in daily use.
