The article about aperture has introduced how aperture affects image quality, with the key points summarized below:

1. The larger the aperture, the brighter the image. A larger aperture reduces the brightness requirement for light sources and allows shortening the camera exposure time to improve its response to moving targets.
2. The larger the aperture, the shallower the depth of field. When a large depth of field is required, for example, capturing targets at both near and far distances, the aperture needs to be set smaller to achieve good depth of field. Conversely, when a shallow depth of field is needed to minimize background interference, the aperture can be opened wider to isolate target features.
3. The larger the aperture, the more severe spherical aberration and coma become. Therefore, to achieve good image quality, the aperture should not be set to maximum. When sufficient brightness is available, a mid-range aperture setting delivers optimal results.
4. The larger the aperture, the more severe vignetting occurs, resulting in uneven illumination across the field of view. Such unevenness is unfavorable for some large-area particle analysis applications, so reducing the aperture is necessary.
5. Theoretically, a larger aperture yields higher resolution. In practice, however, the maximum resolution of a lens is achieved neither at its widest aperture nor at its smallest aperture, but at a middle aperture value, typically between F4 and F12. This must be verified through testing. It should be noted that the statement “larger aperture brings higher resolution” applies when all other conditions remain unchanged and the lens itself supports a wide aperture. For instance, an F1.4 lens delivers better resolution than an F2.8 lens. For a lens with a fixed maximum aperture, its resolution is not the highest at maximum aperture; instead, peak resolution occurs at mid-range aperture settings.

Point 1 is undisputed: a larger aperture definitely produces brighter images. Point 2 is also familiar to digital camera users: larger aperture reduces depth of field while smaller aperture increases depth of field. Points 3, 4 and 5 describe impacts on image quality. Increasing aperture raises light throughput, which benefits imaging in low-light environments. In machine vision, however, lighting can compensate for light quantity. As a result, the advantages of large apertures are less prominent. On the contrary, due to aperture’s influence on lens resolution, images captured at maximum aperture are less sharp than those taken at mid-range apertures such as F8. This can be validated by experiments. The following images demonstrate image quality at different aperture settings:

Figure 1: State 1 at F16 Figure 

2: State 1 at F8 Figure 

3: State 1 at F4