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Forum: General Photography
06-02-2015, 06:18 AM
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Closing down the aperture does two things as I understand it. First, it blocks out errant light that may be coming into the lens from a non-perpendicular angle. Imagine a light beam striking the front element slightly less than parallel to the surface. That light will enter the lens through refraction and start bouncing around between the surfaces of the glass. It will take a longer path than the focused light will so it will be out of focus. Closing down the aperture may help prevent that light from passing to far. If this out of focus light passes through and strikes the sensor it will blur out details. You lose sharpness and contrast. So, aperture blades act like a filter. I think it helps make the light more tele-centric. Second, the aperture blades refract light. They bend them. The smaller the aperture then the greater the effect. Think of an extreme case like pin hole cameras. With a small enough hole you don't even need a lens. Glass refracts light too. I think you could say that the two refracting systems, the aperture and the glass, can work together to help each other achieve better focus and sharper images. This is why smaller apertures have more depth of field. The aperture refracted more of the light from in front of and behind the subject than the glass alone.
Larger aperture lenses needs to have a more complicated design, element profile, and use more exotic materials which typically gives the lens a better starting point that a stopped down aperture can further increase. However, don't take this a universal truth. There are plenty of large aperture lenses that aren't sharp to begin with. A lot more depends on the design than the aperture by itself. The DA 35mm and DA 50mm are f/2.4 and f/1.8. Are there faster lenses? Sure ... but these "not as fast as possible" lenses are also wickedly sharp!
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