The resolution of readers taking one picture per well depends on the camera, especially their chip size which determines the average pixel size of the picture of one well of a 96 well plate. The most sophisticated analog cameras are able to reach a pixel size of 9 µm. Most readers use cameras offering a pixel size of 15 - 35 µm. Keeping in mind that 7 pixels are needed across the diameter of a spot for accurate evaluation, only spots larger than 7 x 9 µm = 63 µm are evaluated accurately. High end digital cameras are able to slightly decrease the pixel size. However, they increase the evaluation time and are very expensive. For Elispot evaluation purposes, pixel sizes reached with systems taking one picture per well should be referred to as low resolution systems. Objects (spots, artifacts) below 60 µm in diameter can't be analyzed precisely.