Ripolini
Offline
• • • •
Upload & Sell: Off
|
Modern lenses are designed in order to minimize residual optical aberrations (field curvature, coma, spherical and chromatic aberrations, etc.).
Optical aberrations tend to diminish as the aperture is closed, except for lateral chromatic aberration (LaCA), which is scarcely affected by the aperture setting.
Best (and most expensive) lenses are designed to correct aberrations effectively at the wider apertures. MTF data provide information on the optical performance of lenses.
As you probably know, diffraction is NOT an optical aberration; it's due to the wavy nature of light (Lumen propagatur seu diffunditur non solum Directč, Refractč, ac Reflexč, sed etiam alio quodam quarto modo, DIFFRACTE'; F.M. Grimaldi, 1618-1663).
Consequently, the concept of diffraction-limited-aperture (DLA) must be introduced:
DLA = photosite size/(1.22 ∙ λ ),
where λ is 560 nm (0.56 µm), i.e., the average wavelength of visible light.
This is the aperture at which the diameter of the central circle of the Airy disk exceeds that of a single pixel on the sensor, and image quality begins to deteriorate due to diffraction.
With the increase in resolution of modern digital cameras, the diffraction limit is moving, for each format (APS-C, FF, MF), towards wider apertures. For example, with an 8-megapixel APS-C camera (with pixels 6.4 µm in size and an area of 41 µm˛), by using the above equation we find that diffraction will begin to appear at f/9. With a 61-megapixel full-frame camera (with pixel pitch = 3.8 µm; photosite area ≈ 14 µm˛), diffraction appears as early as f/5.6!
However, let me stress that diffraction at DLA is barely noticeable when viewing full-resolution files on a monitor (100%) or in a print. Apertures corresponding to the DLA, and even smaller, can certainly be used, as the loss of sharpness begins at DLA and gradually worsens as the diaphragm is further closed. In fact, the transition from “sharpness” to “softness” induced by diffraction, fortunately, is not abrupt.
Although higher-resolution sensors, with smaller photosites, tend to be more susceptible to diffraction, they are still capable of providing a greater level of detail, even at apertures smaller than the diffraction limit: a 24-megapixel sensor has twice the resolution of a 6-megapixel sensor; if we shoot at f/11 with the former, we will still obtain a more detailed image than one taken with the 6-megapixel sensor at the same aperture. Diffraction will reduce the maximum resolution achievable by the optical system (lens + sensor), but certainly won't halve it!
If you're interested in taking high-quality landscape pictures, I recommend that you use focus stacking (at intermediate apertures, say f/4-f/5.6) rather than very close apertures.
|
