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Opening Aperture: f22

Opening Aperture: f45

In the previous article diffraction theory for circular targets, it was established that a separation between the centers of the Airy disks of:

d = 1.22 fλ

guaranteed, according to the Rayleigh criterion, two points of light very close to be resolved as different points in the image and do not constitute a single oval of light, something that obviously hurts their sharpness.

It is time to understand what are the practical implications of this statement.

If we take an APS-C sensor typical 23.6 mm. wide by 15.8 mm. high that contains within it a pattern of 3872 x 2952 fotocaptores of light, is quite simple to calculate what the estimated separation between the captors.

horizontal separation = 23.6 / 3872 = 0.0061 mm.

vertical separation = 15.8 / 2952 = 0.0054 mm.

As these results are merely estimates, we can say that the separation is about 0.006 mm.

If the incident light is predominantly bluish tint, which is most common in outdoor photography, its wavelength is about 0.0004 mm.

With these data we are able to estimate the number f that guarantee the Rayleigh criterion. If we use a value of f11, then:

d = 1.22 x 11 x 0.0004 = 0.0054 mm.

Therefore, with this value for the number f, the distance between the centers of the Airy disk is less than fotocaptores existing between two points ahead and the image is properly solved.

In the heading of the article are two pictures taken at exactly the same conditions using a tripod, remote shutter release and the Tamron 90 mm macro lens. f2.8.

The only difference is that in the first of them has used an f value of 22 and the second a 45. It is clear that, despite being a value significantly steeper than the threshold corresponding to f11, the former is not observable a noticeable loss of sharpness, while the second does. The f values \u200b\u200bfor these numbers are:

d22 = 1.22 x 22 x 0.0004 = 0.011

, D45 = 1.22 x 45 x 0.0004 = 0.022

In the first case the distance is about twice that which exists between two consecutive fotocaptores. The second is four times larger.

Just keep in mind that the above calculations are merely estimates and their only intention is to give a qualitative approach to the problem, without trying to be absolutely accurate.

Finally, it should be noted that the title of the article is related to the fact that these so closed diaphragms are only used in practice photomacrographs.

Mobil Free Beautiful Agony

photomacrographs Diffraction and Diffraction in photography tripods

When light shines on the corners of an object or through an opening in it, is diffracted, which means that it shows an interference pattern consisting of alternating light areas and shade. In the event that the opening is approximately circular, so as forming the diaphragm, these zones are concentric rings. Yes a size very very small. In our everyday experience diffraction is the cause of that area intemedia in brightness between the parties strongly illuminated by the sun and the darker areas, which sometimes occurs in the corners of buildings.

The figure accompanying this article, we assess the situation in which the infinite light from passing through the lens exposed through an aperture of diameter D. Under these conditions, the camera sensor must be located at a distance F, corresponding to the lens focal length, if we get a sharp image. When light passes
creates inteferencia pattern above, which is a first illuminated circle (called Airy disc) and a series of concentric rings alternating dark and bright concentric rings. The brightness of these rings fell sharply as we go away from the center. Is what the wave shown in the diagram. The theory of Fraunhofer diffraction (Optica. Hecht-Zajac, American Educational Fund, pp.372-375) states that, for circular cracks, the angle θ formed by the lens optical axis and the starting position of the first ring Dark is calculated using the formula: λ

Θ = 1.22 ---

D

where λ is the wavelength of incident light .
If we have two very close light rays, the diffraction patterns will overlap. If they are really close, the two respective Airy Physicians will overlap and two points of light will be indistinguishable in the sensor. Form a continuous oval. The Rayleigh criterion states that for both points of light can be distinguished, the centers of the respective Airy discs should be separate at least by the radius of any of them. (We must remember, as shown in the figure, even within the Airy ring, the luminosity drops sharply as we move away from downtown).
Therefore, the separation d - as also seen in the figure - is expressed by the following formula (Bearing in mind that for very small angles senθ is approximately equal to θ) d = F



Substituting θ in above formula, we find that:

λ F d = 1.22

D ---

But it appears that the ratio F / f D is the number for that opening.
From which we have the exprexión:

d = 1.22 λ f

which allows us to calculate the distance between the centers of the Airy disk, for two rays of light coming so they can be resolved as two distinct and not be mistaken as a single point of light.
In the next article we will the practical consequences of this formula.