Aperture Effects: Explaining Brightness & Dimming

[Robert Burdge]

If a distant object is viewed through a small ciruclar aperture that is placed close to the eye, there is an apparent "shadowing" of the center of the viewed image, while the inner perimeter of the aperture is bright.

The small aperture enhances the depth of field for good focus, but the dimming of the center of the image makes viewing more difficult. The aperture is defocused because of its proximity to the eye, and because the point of focus is intentionally at a distance. If a pair of short slits in the form of a cross (with width equal to the original circular aperture) is used instead of the circular aperture, the center of the aperture image is bright.

The aperture size (~ 1.0mm) is large compared to the wavelength of light which would suggest that diffraction is not the cause of this phenomena. Defocus can account for the general dimming of the image, but not the brightness around the perimeter. The cross-slot aperture appears to solve the problem of dimming at the expense of depth of field. Can anyone explain what causes these effects?

 

[James Jackson]

Sounds very much like a Fresnel Zone Plate effect. Search for that on Google and you'll find all you need!

P.S. A pinhole camera focuses the image by being, essentially, the central circle of a Fresnel zone plate.

P.P.S. It is diffraction. What do you think makes shadows 'blurry' the further they are from the object blocking the light? That's diffraction too!

 

[Robert Burdge]

James,

I thought about that, but the effect seems to be fairly indifferent to the distance of the aperture from the eye. If I understand the Fresnel Zone effect it should pass through several different patterns in short order as the observation point is moved relative to the aperture, with the ring pattern and number of rings changing noticeably.

The far-field/near-field boundary for light through a 1.0 mm aperture is about 1.8 m . With the eye about 3 cm from the aperture this would definitely qualify as Fresnel near-field. I have also read, though, that the focusing effect of the eye will tend to produce Fraunhofer diffraction even at close distances. Lenses are used in this way in experiements to produce far-field effects at finite distances.

Robert