Resolution appears on every quotation as a single number, and it is the number buyers compare. It is also the least useful number on the sheet, because 4 MP says how many pixels the file contains and nothing about how much light each of those pixels received.
What actually changes when the sensor size changes
Two sensors can both resolve 2560 x 1440 and behave completely differently at 22:00. The difference is pixel pitch. Squeeze 4 million pixels onto a 1/3" sensor and each photosite is small; spread the same count over 1/2.7" or 1/2" and each site collects more photons. Smaller sites mean more amplification, and amplification is what turns a dark scene into moving grey mush.
In daylit scenes the two sensors look almost identical in a screenshot. At night the small-sensor version needs longer exposure, and longer exposure means motion blur on anything that walks past. It also needs more aggressive noise reduction, and noise reduction smears exactly the detail you bought the pixels for: faces, plate characters, the edge of a package.
The bitrate consequence nobody prices
Noise is expensive to encode. A clean 4 MP stream at a moderate bitrate looks sharp; a noisy 4 MP stream at the same bitrate falls apart, because the encoder spends its budget describing grain. In practice a noisier camera needs a higher bitrate to look the same, which means more storage per day per unit, which is a running cost that outlives the purchase order.
This is why "storage planning" belongs in the RFQ and not in the installation manual. If a project quotes 32 GB microSD cards because the nominal resolution fit, and the actual night image needs 1.5x the bitrate to stay usable, the cards fill early and nobody knows why until a clip is missing.
What to ask the factory instead of asking for more pixels
Ask for three numbers together, never separately: sensor format, pixel size, and the minimum illumination figure with its conditions attached (lux, f-number, gain, whether IR is on). A lux number without those conditions is not comparable between suppliers.
Then ask what the firmware does when light drops: does it hold shutter speed and raise gain, or does it extend exposure and accept motion blur? Both are legitimate engineering choices; only one of them is right for a doorway where people move.
Matching sensor to scene rather than to budget line
A wide static car park and a narrow entrance corridor want different things. The car park benefits from resolution at distance, because the subject occupies few pixels and every one counts. The corridor is the opposite: subjects are close, so dynamic range matters more than pixel count, and a backlit doorway that blows out the face is a camera that failed regardless of its resolution.
Plan the scene first, then pick sensor and lens, then check the bitrate budget and storage. Doing it in the reverse order produces a specification that reads well and performs badly.
Compression is where detail dies
A camera produces a sensor image, and then the encoder decides what survives. At a fixed bitrate, an encoder facing noise spends bits describing grain and starves the edges. Two cameras with identical sensors can therefore produce very different recordings if their encoder settings differ, which is why buying by sensor alone still leaves a variable in the chain.
The practical consequence is that any comparison between suppliers should use the same scene, the same lighting and the same time of day, and should be judged on recorded clips rather than on live view. Live view is a preview stream and is tuned to look pleasant. The recording is the evidence.
A three-line specification that survives shipment
Write the sensor format and pixel size, the illumination figure with its conditions, and the main-stream bitrate ceiling for the recording mode the product will actually run. Three lines, all measurable, all comparable between suppliers.
Everything else on the camera page is a description. These three decide whether the archive is usable in six months.
XingLiao builds security cameras with sensor, lens and bitrate specified together against the scene they will be mounted in.
Related reading: camera module and OEM build options and the Wi-Fi, PoE and 4G camera range.
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