Pentax 67II - Manual (Page 76)

ADVANCED OPERATION
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The Effect of Aperture
Aperture refers to adjusting the amount of light that hits the film, by widening or narrowing the beam, or “thickness” of light
that passes through the lens.
If the aperture is opened, thus widening the beam of light, the objects in front of and behind the subject in focus are blurred.
In other words, the focus range becomes smaller. On the other hand, by narrowing the aperture and thus narrowing the beam
of light, the focus range becomes larger.
For example, when taking a picture of a subject against a background, if you widen the aperture and you take a picture, the
areas other than the subject focused on become blurrier, making the subject appear to be in the foreground. On the other
hand, by narrowing the aperture, it is possible to keep the background areas in focus.
Narrowed Aperture Opened Aperture