Lens quality and image sharpness

Lens technology has come a long way over the past two decades and many photographers are prepared to invest in high-performance lenses.

This article identifies the key factors that can affect lens performance, and  shows you how to get the most from your lenses.

Lens sharpness

A popular rule of thumb says most lenses are sharpest two stops down from their maximum aperture. While it may be true for centre sharpness with some lenses, it’s seldom the case with edge and corner sharpness.

Lens performance can vary, as we’ve found with our Imatest testing, some of the results of which are reproduced below. Performance also varies, depending on the camera used for testing because Imatest assesses camera+lens performance. Edge and corner sharpness are rarely as good as centre sharpness, although they should improve as the lens is stopped down ““ until diffraction starts to take effect.

A comparison of two prime lenses, both with the same focal length and covering the same range of apertures. The top graph shows a lens in which the edge sharpness is closer to the centre sharpness than the lower lens, which would have noticeable edge and corner softening. The top lens also offers fairly consistent resolution from wide open to about f/8, while the lower lens is sharpest at around f/5.6 and then declines rapidly thereafter.

These two graphs compare two zoom lenses that cover the same focal length range. The top lens has a constant f/1.8 maximum aperture and shows only slight edge softening. Resolution peaks at around f/4, which is 2.3 stops down from maximum aperture. The maximum aperture of the lower lens varies from f/3.5 to f/4.5 but the highest resolution is much the same for all focal lengths and remains at the same level from almost wide open to two stops down.

Factors affecting sharpness

All lenses are affected by aberrations. Most appear when points of light in the image aren’t translated into single points with the same relationship to each other after passing through the lens. The table below lists the main aberrations that affect lenses and whether they can be corrected.

Aberration	Effect on image	Corrected by stopping down	In-camera correction	Post-capture correction
Distortion	The image near the edge of the frame bows outwards (barrel distortion) or inwards (pincushion distortion).	No	Automatic for JPEG files in most digital cameras.	Possible in raw converters and image editors.
Vignetting	Edge and corner darkening in the image frame.	Yes	Automatic for JPEG files in most digital cameras.	Possible in raw converters and image editors.
Lateral chromatic aberration	Mostly seen as coloured fringing along high-contrast edges.	No	Automatic for JPEG files in most digital cameras.	Possible in raw converters and image editors.
Axial/Longitudinal chromatic aberration	Different colours focus at different distances from the image plane.	Yes	Corrected with ED glass in the lens.	Rarely required with modern equipment.
Spherical aberration	In-coming light rays focus at different points on the image plane.	Yes	Corrected with aspherical lens elements.	Rarely required with modern equipment.
Curvature of field	The lens focuses sharply at only one point. Most common in wide-angle lenses.	Yes (unless severe)	Related to the optical design of the lens.	None
Coma & astigmatism	Off-axis points are blurred, stretched or haloed.	Yes	Corrected with aspherical lens elements.	Rarely required with modern equipment.
Flare and ghosting	Bright spots or haze covering the frame.	No	Reduced with lens coatings and use of lens hoods.	Difficult or impossible to correct.

Different lenses have different combinations of aberrations and that can produce different end results, particularly when one or more of the aberrations can’t be corrected, either within the design of the lens itself or through image processing algorithms (in the camera or in editing software). Additionally, some corrections that work well at the centre of the frame may reduce resolution around the edges.

And then there’s the effects produced by diffraction, which is a separate issue.

Diffraction

Diffraction affects all waveforms in the electromagnetic spectrum as well as waves moving through water or air (in the form of sound). When a wave comes across an obstacle, it will bend around it to get past. This creates ‘drag’ where part of the wave is held back through contact with the obstacle while the rest passes by unimpaired.

In the case of a circular opening ““ like that created by the iris diaphragm in the lens ““ the portion of the wave near the centre of the aperture passes through largely unchanged, while the areas close to the edges of the aperture are bent off at different angles. Larger apertures allow more of the wave to pass through unimpeded, while smaller apertures diffract a greater portion of the wave.

This diagram shows how rays of light are bent (diffracted) as they pass through a circular aperture to create an Airy disk.

Light passing through a circular aperture creates a series of concentric soft-edged circles at the smallest point to which the beam of light can be focused. This pattern is known as an Airy disk and its diameter depends upon the wavelength of the illuminating light and the size of the circular aperture.

Optically, a smaller aperture will introduce more blurring through diffraction than a larger one so as a lens is stopped down two things happen:

1. A point is reached where diffraction causes some loss of resolution.

2. Lens resolution increases as the effects of aberrations are reduced.

When you begin stopping down, the lens resolution increases until it reaches maximum sharpness. Beyond this point, diffraction softening starts to take effect, setting an upper limit on the resolving power of the lens. Between these points is a ‘sweet spot’ where the effects of diffraction and optical aberrations are as low as possible and the lens will be delivering its optimal quality.

That point may have been the best quality you could get in the days of film. But that’s not the case today. Because the effects of diffraction are predictable, we are able to correct diffraction blurring through post-capture processing on a computer.

Simple unsharp masking (which is provided in most image editors) can counteract some of the softening caused by diffraction. Better results could be achieved with more advanced tools, such as the Smart Sharpen tool in Photoshop, or third party tools from Nik Software such as Sharpener and Dfine. Some experimentation may be required to match the sharpening to specific camera/lens combinations.

Don’t expect to be able to make images taken at f/22 look as sharp as those taken at f/5.6 or f/8 (or whatever the sweet spot is for your particular camera and lens). But you should be able to see a definite gain in sharpness.

Diffraction Limited Aperture

Some review sites report on ‘DLA’ (Diffraction Limited Aperture) values, based on the notion that this marks the point at which diffraction kicks in as a result of the performance of the lens. While it’s true that the DLA refers to the lens aperture where diffraction begins to affect image quality, it has little to do with the lens and much more to do with the camera’s image sensor and the size of its photosites (pixels).

At the DLA, the size of a resolved point of light is as large as a pixel. This means sensors with smaller photosites should theoretically become diffraction limited at wider apertures than those with larger photosites.

Interestingly, smallest achievable Airy disk diameter ““ which is dictated by the capabilities of the lens design ““ can be larger than the pixels in an image sensor. This can make it difficult to achieve the full resolution capacities of a sensor with any usable level of contrast.

When you photograph subjects containing lots of detail, assuming you use a lens with high resolution, a sensor with smaller photosites will usually be better equipped to record that detail than one with larger photosites. The reason lies in how well the Airy disk diameters produced by light coming from the image details correspond with the size of the photosites. When they are a close match (as you’d find with small photosites) or the Airy disks are slightly larger than the photosite, details are resolved.  

This diagram, based upon a single wavelength of light produced by a laser, shows what happens when imaging light produces Airy disks that are larger and smaller than the sensor’s photosites. The same point source of light produces the same disk size, which fills the smaller photosite but occupies only about a quarter of the larger photosite. Provided the image is not noise-affected, both sensors should be able to resolve at least as much detail at diffraction-limited apertures.

In actuality, all images are made up from multiple points of light with overlapping Airy disks that tend to soften the image. But even when the resolved detail is softened by diffraction, provided noise is not an issue, smaller pixels will still be resolving as much or more detail than larger pixels at diffraction-limited apertures.

Article by Margaret Brown

Excerpt from  Photo Review Issue 68    

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