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1.7 Diffraction



In Photography there is a conventional wisdom is that one can achieve a sharper image by stopping down to a smaller aperture, but this misses the mark in two fundamental ways:

 

1. The image will always be sharp at the point of focus. What a smaller aperture gives you is an apparent sense of greater sharpness by extending depth of field over a wider range of distances in front of and behind that focus point.

2. A phenomenon known as diffraction can cause you to actually get progressively less sharp images beyond a certain aperture, even at your focus distance

 

At a sufficiently large aperture, light rays can pass through unobstructed on their way to the film or sensor. But as the aperture gets smaller, less light can get through. This is well known in photography and is the reason why you have to use slower shutter speeds to achieve an equivalent exposure. But at really small apertures light rays can barely squeeze through at all and are actually bent slightly on their way to your film or sensor and begin to interfere with one another.



You can actually see this effect in everyday life. If an object is placed in front of a light source, it will cast a shadow on whatever is behind it. Light rays passing beyond the edges of the object still continue unimpeded, but those that strike the object get blocked by it, resulting in a shadow. But if you look closely, no shadow ever has a perfectly sharp edge. The light rays that pass closest to the object without actually being blocked get diffracted or bent slightly, blurring the edge of the shadow. This same effect occurs as light passes near the edges of the aperture opening in your lens. The effect is negligible for large apertures, but it becomes increasingly more significant as the aperture gets smaller.

Diffraction bent light rays will have to travel further than those not being diffracted. This means they will also take longer to reach the sensor than rays able to pass straight through the aperture unaffected. As they become increasingly out of phase with their neighbors, light waves will begin to interfere with each other, lessening sharpness.

The interference pattern thus produced includes a central spot called the Airy disk, named after Sir George Airy and a number of surrounding diffraction rings. The size of the Airy disk depends only on the wavelength of light and the aperture, but as it approaches the diameter of the Circle of Confusion for the film or sensor format being used, it limits the resolution possible.


Airy disk surrounded by diffraction rings
Let's take a look at what all this translates to in actual practice. Shown here is a 200% view at various apertures of a lens test chart taped to a wall on the other side of the room from a Nikon D2x camera. Up through around f/16, each row looks reasonably the same as the one before it. From there on down though, successive apertures yielded progressively worse resolution. The f/40 shot (the limit of the lens used) is downright horrible on the right-hand end of the chart where the lines are closest together.

Note: Diffraction always occurs, even at wider apertures. What makes it a problem is when the percentage of light being diffracted gets to be high enough in relation to the non-diffracted light. At sufficiently large apertures, most of the light getting through passes far enough away from the edge of the hole to be unaffected and totally drowns out any loss of sharpness from the light that is being diffracted. In terms of impact on the overall exposure, the diffracted light just doesn't even register. As you stop down and the opening gets smaller though, the ratio of diffracted light to non-diffracted light goes up. As it does, there gets to be enough diffracted light contributing to the exposure that it causes a noticeable decrease in the sharpness of the resulting image.

Copyright 2006 Bob Johnson, Earthbound Light.

Conclusion: Someone who has a more facial-masking-regions, that blocks a lot of in-falling light (see topic: 1.1 The Facial Masking & Alignment Region), can have a larger pupil-opening, giving him less diffraction and a sharper-sight.
 
 
 

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