Aperture-Induced Image Softening

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The trade-off in aperture choice

A lens usually performs best at a middle aperture. Wide open, aberrations such as spherical aberration and coma are strongest, because light passes through the outer parts of the lens. Stopping down reduces these aberrations, because only the central, better-corrected part of the lens is used.

Stopping down too far brings diffraction into play. As the aperture gets smaller, the Airy disk grows, and the image gets softer for a different reason. The best aperture is the point where these two effects balance.

When diffraction becomes visible

Diffraction becomes noticeable when the Airy disk is comparable in size to the spacing between pixels on the sensor. A disk much smaller than a pixel does not limit sharpness much. A disk several pixels across produces visible softening, because neighbouring detail blends together.

The Airy disk diameter can be calculated from the wavelength and f-number, as described in the Airy disk article. Comparing it with the pixel pitch gives a practical idea of when stopping down starts to cost sharpness.

What to test

Photograph a fine, high-contrast target at several apertures, from the widest to the smallest you use. Compare sharpness at the centre and at the edges. Note the aperture where the centre is sharpest, and where the edges are sharpest. The two often differ, and the chosen aperture depends on which part of the frame matters most.

Limits of the rule

Diffraction depends on the wavelength, the f-number and the aperture shape. It does not depend on the focal length directly. Lens design, sensor size and processing all change how much softening is visible, so a single aperture value cannot be recommended for every camera.

Common mistakes

Assuming the smallest aperture always gives the greatest sharpness. Assuming sharpness at one aperture predicts sharpness at another. Comparing lenses at different apertures without stating which aperture was used.

Why the widest aperture is often soft

Wide open, a lens may show soft corners and a soft centre at the same time. The centre can be softened by spherical aberration, and the corners by field curvature and coma. Stopping down removes the outer zones of the lens, where most aberrations are strongest. This is why many lenses look noticeably sharper one or two stops down from their widest setting.

Why the smallest aperture is also soft

At the smallest apertures, the Airy disk can grow to several pixels across. The image then looks soft even though the lens is free of aberrations at that setting. This is the diffraction limit at work, and no lens design can remove it. The softness is the same kind of blur that a perfect lens would produce at that aperture.

Using the balance in practice

A practical approach is to identify the aperture range where the lens performs best across the frame, then use the smallest aperture in that range that gives the depth of field the scene needs. Confirming the choice with a test shot of a flat, detailed subject is more reliable than relying on general advice, because each lens and sensor combination behaves differently.

The role of focus accuracy

Focus error can look like diffraction softening, and the two are easy to confuse. A lens that is slightly out of focus shows blur that is not due to aperture, and it may appear to sharpen when the aperture is stopped down because depth of field increases. Before testing aperture softening, confirm focus with live view or a focus target, using the same method for every aperture.

Testing lens and sensor together

Test charts reveal the combined effect of lens and sensor. Use a flat, evenly lit chart with fine but resolvable detail. Photograph it at each aperture with the same focus, shutter speed and white balance. Crop identical areas of the centre and the edge, and compare them at the same magnification. Record the aperture at which each area is sharpest.

Stopping down for depth of field

A common reason to stop down is depth of field rather than sharpness. The depth of field increases as the aperture gets smaller, but the diffraction softening also increases. The decision is a trade-off: a small loss of sharpness at the plane of focus may be acceptable in exchange for keeping more of the scene in focus. The Airy disk calculation shows the size of that loss in concrete terms.

Relevant standards

ISO 12233 describes methods for measuring resolution and spatial frequency response of digital still cameras. Using a standard method means that results can be compared across tests and labs, which is important when reporting how aperture affects sharpness.

A short checklist for an aperture test

Use a tripod and a remote release to avoid camera shake. Keep the focus at the same point for every aperture. Shoot in raw format with the same white balance and exposure compensation. Store the frames with their aperture recorded in the file name. Compare the centre and corners at the same magnification, and note the aperture of the sharpest result for each area.

Interpreting the results

A sharp centre and a soft corner at the same aperture point to field curvature or focus misalignment rather than diffraction. A soft centre and soft corners at the smallest aperture point to diffraction. A soft centre at the widest aperture that sharpens when stopped down points to spherical aberration.

Keeping a record

Record the lens, focal length, aperture, focus distance, sensor and test chart for each test. Results from different days or light conditions cannot be compared directly, so repeatable conditions matter more than the number of frames.

Going further

Soft results at the extremes are normal and expected. The useful question is where the sharpest result lies for your subject, and whether that aperture gives enough depth of field for the picture you want to make.

Summary

The key point of this article is the one stated at its start. Check the formula, use the stated assumptions, and confirm any result with a test before relying on it in practice.

Sources

Eugene Hecht, Optics, Pearson (any current edition).
Max Born and Emil Wolf, Principles of Optics, Cambridge University Press (any current edition).
ISO 12233: Photography, Electronic still picture imaging, Resolution and spatial frequency responses.

Next step

Return to the pillar guide or the Diffraction summary to read the related articles.

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General information only. This page explains optical principles and does not replace lens design work, lens testing or advice from a qualified optical engineer.