Since not every aperture can achieve the optimum quality of a lens, any aperture will reach a different level of the lens quality. Each design, even each lens of the same design has an independent performance. It requires a special test to find the best aperture. For photographers ourselves, we need to understand our personal lens only by photographing. Any scientific methods would not be necessary. To avoid interference by the depth of field, it would be better to choose a bigger object, e.g. a building. By using tripod to secure the camera, focus and measure light, use difference aperture (with corresponding shutter speed) to take a few shots. It would be nice to take front lighting, side lighting and back lighting to have a better understanding. If we use slides, we can check the image quality by projection; if we use negatives, we may need 8x10 prints.

Resolution and chromatic aberration are the major part of lens quality, but these kinds of data would be boring for us. It will be better to use good, fair and bad by visual judgement. The judgement is subjective, as the use of lens is a personal choice. If it cannot meet one's expectation, how one can get a masterpiece.

The above images are for demonstration. It is not possible to have bigger size for all of them, so a strip from each is cropped and place together. We found that the image quality with aperture f/8 is the best. By using this type of test, we can find the best aperture of our lens. We can use this aperture whenever the best quality of that lens is needed.

In general, the aperture for best quality of a lens will usually be somewhere between f/5.6 and f8.