Iris diaphragm is a device in the lens to control the size of aperture. That is, a set of blades setting different sizes of opening same as the iris of our human eye. The maximum aperture is based on the lens, e.g. a lens of f/2.8, which means that the highest aperture is f/2.8. The smaller apertures in half a stop down are f/3.5, f/4, f/4.5, f/5.6, f/6.7, f/8, f/9.5, f/11, f/13, f/16, f/19, f/22 etc. respectively. As there is not enough space on the lens to list all these numbers, the lens will have f/4, f/5.6, f/8, f/11, f/16, f/22 etc. which are in one stop setting. Since the minimum diameter cannot be less than 2mm, different lens will have different smallest aperture.

Depth of field is the parts that can be considered sharp in a photo. In reality, it is the distance ranging closer and farther from the focusing point. For example, if a 50mm lens for 135 has the focus at 5 m., with f/8, all objects from 4 m. to 9.5 m. can be taken as sharp. According to Optics, any point on the focus distance will appear as a point on the film (focal plane), which is called pin sharp. For other distance, the point will diffuse into a hollow ring, which is called "circle of confusion". Viewing at 10 inches, when the diameter of this circle is greater than 0.01 inch, it will not be taken as sharp visually. The 0.01-inch diameter point is called circle of least confusion. Size of circle of confusion is affected by aperture, a higher aperture gives a greater circle, a smaller aperture gives a smaller circle, and this is why aperture can change the depth of field.

The shorter the focal length of a lens will have a greater depth of field.

Depth of field is part of the visual language. To get a subject outstanding, a shorter depth of field would make the foreground and background blur. For a great size subject, a longer depth of field would render an overall sharpness.

Since the small images can not show clearly, same part of f/2.8, f/6.7 and f/16 has been cropped and joined from top to bottem respectively. Checking the grass and wood plates, one can see the difference.