Camera resolution

Cameras have been moving away from TVL (television lines, the analogue measure) towards digital megapixels for years, and for a long while both conventions were printed on boxes side by side. Converting between them is awkward, and adding aspect ratios makes it worse. These are the conversion tables Handykam kept for its own use and published for customers. They have not dated, because the arithmetic has not changed.

Why the two measures are not comparable

TVL counts the number of distinguishable vertical lines a camera can resolve across a width equal to the picture height. It is a measure of resolving power under test conditions, not a count of anything. A 700 TVL analogue camera — the sensor used in most of the wired nest box systems — is genuinely sharp within the limits of the PAL signal it produces, but that signal caps out around 976 × 582 pixels no matter how good the sensor is.

Megapixels simply multiply the width and height of the sensor's pixel grid. That makes them easy to compare with each other and useless for comparing with TVL. And a megapixel figure says nothing about lens quality, low-light sensitivity or compression — which is why a 5MP camera behind a poor lens can look worse than a 2MP one behind a good one.

Megapixels to pixel dimensions

Resolution names

Note the ambiguity that used to catch people out: "HD" was applied to both 720p and 1080p, and "4K" to two different widths. When a camera specification says HD without a number, it is usually the lower one.

Aspect ratios — typical sizes

4:3 — the old television and analogue camera shape

600×800, 768×1024, 864×1152, 960×1280, 1024×1280, 1050×1400, 1080×1440, 1200×1600, 1260×1680, 1440×1920, 1536×2048, 1920×2560, 2048×2560, 2100×2800, 2400×3200, 3000×3750, 3072×4096

16:10 — monitors and some projectors

960×600, 1152×720, 1280×800, 1440×900, 1680×1050, 1920×1200, 2560×1600, 2880×1800, 3840×2400, 5120×3200

16:9 — modern video and network cameras

960×540, 1024×576, 1280×720, 1366×768, 1600×900, 1920×1080, 2048×1152, 2400×1350, 2560×1440, 2880×1620, 3554×1999, 3840×2160

If you know one dimension and not the other, you can work it out with a calculator or use one of the many screen-ratio calculators online.

What resolution actually buys you in a nest box

Inside a bird box the subject is around 150mm from the lens and the whole field of view is a hand's width across. Even a modest sensor puts plenty of detail on a blue tit at that distance. What matters far more is light — a nest box is dark, and the sensor's low-light performance and the infra-red illumination determine whether you see a brood or a grey smear. The second thing that matters is compression: a high-resolution stream squeezed hard for a phone can look worse than a lower-resolution one at a generous bitrate. Almost everything in this field encodes to H.264 or its successors, and the bitrate you allow it is as important as the pixel count you feed it.

Higher resolution does cost you elsewhere. Recording space grows with it — the calculation is worked through in the DVR and NVR FAQ — and so does the bandwidth needed to get the picture out of the garden.

Related pages: CCTV camera lenses and board lens tips · Digital IP versus analogue · Wildlife camera glossary