Megapixels count how many wells sit on the chip. Pixel size says how large each well is. People buy the first number and ignore the second. That is how you get a 12 MP night picture that is just a noisier crop of a 2 MP picture.
In one sentence. Resolution is how many buckets. Pixel size is how much rain each bucket can hold. Format is the size of the field they sit in. You need all three.
The same 8 MP is not the same part
8 MP on 1/2.8" is a small pixel. 8 MP on 1/1.7" is a larger one. The first will need more light, a sharper lens, and more careful CRA. The second is easier to live with and harder to pack into a tiny M12 module.
On Camemaker, filter Resolution (MP) and Pixel Size together. If you only move the megapixel slider, you are sorting marketing, not physics.
When more pixels help
- You will crop. Digital zoom, a region of interest, a barcode in a corner.
- The lens can actually resolve the pixel. MTF at that pitch still has contrast.
- You have light, or you have HDR, or you have a large enough pixel that the well is not starving.
When they do not
- Night, or a dark machine cabinet.
- A cheap M12 that was sharp on 2.9 µm and is mush on 1.4 µm.
- An interface that cannot get the extra pixels off the chip at the frame rate you need.
The newspaper test
A broadsheet printed with very fine type looks detailed in the sun. In a dim kitchen the same page is grey soup. You did not lose the letters because there were too few of them. You lost them because each one was too small to gather the light.
That is a 1.0 µm pixel in a warehouse. The megapixel count is still printed on the box.
How to read the library
In the sensor library, set the format you can cover, then look at pixel size before you look at megapixels. If two parts share a format and a shutter, the larger pixel is usually the one that still works at 17:00 in November.
Then ask the lens whether it can resolve that pitch. A 2 µm pixel with a soft image circle is wasted silicon.