[ Pixel Size ] One of the more important camera specifications to optimize for imaging

The camera pixel size refers to the physical width and height of the pixel unit on the camera sensor, typically measured in micro-meters (μm). This is one of the more important camera specifications to optimize for your imaging application, as it influences both the sensitivity of the camera, and its ability to capture fine details.

While the sensor pixel size is important, it is in fact the resulting image pixel size that determines many of the imaging properties of the camera, after the magnification of the optical system is applied. The image pixel size is given by the sensor pixel size, divided by the total system magnification. There is therefore a significant difference between fixed focal plane optical systems such as microscope objectives, and focusable systems such as conventional camera lenses. In the latter case, magnification and hence effective imaging pixel size can be changed by moving closer to or further from the imaging subject, or using zoom lenses to alter magnification.

For optical systems using fixed focal plane microscope objectives, or lens-based systems where both zoom level and distance to the subject are fixed, larger pixels can collect more light, offering greater sensitivity. As an analogy, if you were aiming to collect rainwater, a bucket would be more effective than a cup. The pixel area is the important factor, meaning that when comparing cameras, a pixel that is twice the size in X and Y has a four times larger area, and therefore will be exposed to four times the number of photons. In low light imaging situations, increasing pixel size can then convey massive benefits to sensitivity, reducing the required exposure time or light level.

In these fixed optical systems, the downside of large pixels can come in the resolving of fine details within the image. The larger the pixel, the greater the degree of ‘pixilation’ of the image. If a pixel in your image is 1 μm across, it will not be able to display details smaller than around 2 μm without neighboring features blurring into one.

However, the camera pixel size is not the only limiting factor on resolving fine details. The optical system too will have a limit for how small details can be before they will be blurred. Each optical system then will have a corresponding minimum pixel size, below which little to no gain in detail resolution will be seen, but a penalty will still be paid in sensitivity. For microscope-objective-based systems, this limit is primarily determined by the objective’s Numerical Aperture (NA).

A camera with a pixel size of 6.5 μm is ideally matched to 60x high-NA microscope objectives. Cameras with 10 or 11 μm pixels are matched to 100x high-NA objectives. In each case, larger pixels can yield more sensitivity, but smaller pixels will not yield finer image detail resolution.