Good Comeback To 4 Eyes

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.