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Marco Raugei | profile | all galleries >> Technique >> Lens Sharpness tree view | thumbnails | slideshow

Lens Sharpness

The following is a short discussion of the issue of lens sharpness, and what consequences it has in real-world photography.

Generally speaking, all lenses exhibit relatively inferior performance in terms of sharpness and contrast at their widest setting, while they perform best at middle aperture settings.

In particular, most prime lenses and fast (f/2.8) zooms reach peak performance at around f/5.6.

A few specialized, super-fast primes (e.g. 50mm f/1.2, 85mm f/1.4, 135mm f/2, 300mm f/2.8) may be optimized for wide apertures and attain maximum performance earlier, at just one stop down from maximum aperture (e.g. f/2.8 for an f/2 lens); such maximum sharpness is then however generally maintained until around f/5.6.

Slower consumer zooms (e.g. f/4-5.6) may on the other hand reach their (relative) peak performance later, at around f/8.

In all cases, when stopping down further, lens resolution invariably starts to decline again, at first gently, and then more rapidly. This latter behaviour is primarily caused by a physical phenomenon known as DIFFRACTION.

Diffraction happens when the the wavelength of incident light is of the same order of magnitude as the dimensions of the object with which it interacts.

The sharpness-reducing effects of diffraction are more visible in digital photography than they ever were with film: this is because of the nature of the sensor in digital cameras (CCD or CMOS), which is composed of a number of discrete photosites, each of which has a definite (and very tiny) dimension.

For example, for a 12MP reduced-frame (DX) digital SLR cameras, diffraction starts to take its toll past f/11 (approximately).
Full-frame cameras of the same resolution (e.g. 12MP) have a bit of extra leeway (say, till f/16), by virtue of the fact that for the same resolution, each photosite is approximately 1.5 times larger (this is almost exactly the same relative size difference between adjacent f/stops, which is 1.4).
If, however, one moves on to a higher-resolution full-frame camera, this modest advantage is readily eroded away. A 24MP full-frame camera, for example, has double the resolution of a 12MP DX camera, i.e. 1.4 times its linear resolution, hence the photosites of these two cameras share approximately the same dimensions, and they will behave in the same way with respect to diffraction!

The following table summarizes my quick-and-dirty recommendations on how to choose the "best" possible aperture settings, taking into consideration the intended effect, as well as what has been said above in terms of optimal sharpness.

Aperture (F/stop) Comment Recommendations
1.4 Very wide Reduced contrast. Use only when strictly necessary (e.g. hand held in low light)
2 Very wide Reduced contrast. Use only when strictly necessary (e.g. hand held in low light)
2.8 Wide Use this for shallow DOF effect (especially with tele lenses)
4 Medium-wide Middle-of-the-road setting
5.6 Medium Usually the BEST setting to maximize sharpness. Use this when DOF is irrelevant
8 Medium-narrow Middle-of-the-road setting
11 Narrow Use this for large DOF effect (especially with wide-angle lenses)
16 Very narrow Reduced sharpness because of diffraction. Use only when strictly necessary for maximum DOF
22 Very narrow Reduced sharpness because of diffraction. Best avoided