BetterPhoto Q&A
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Aperture & Sharpness Question

I ran a test for the sharpness of the lens as impacted by aperture setting by doing the following: I mounted my camera and lens on a tripod and used a remote shutter release. I set up a dummy head at a distance of six feet and then, keeping all factors constant with the exception of shutter speed and aperture, I made six equivalent exposures as follows: f1.8 @ 1/50sec; f2.0 @ 1/50sec; f2.8 @ 1/25sec; f4.0 @ 1/13sec; f5.6 @ 1/6sec and f8.0 @ 1/3sec. Although every exposure is in focus, there is a distinct increase in sharpness at f8.0 @ 1/3sec as compared to f1.8 @ 1/50sec.

Is the difference in sharpness due to: a) the laws of physics, i.e. all else being equal, exposures at apertures as big as F1.8 have an inherently shallow depth of field and thus an inherently soft focus; b) my Canon lens model, due to its design, performs sharpness much better at F8.0 than at F1.8; c) my specific copy of this lens is out of adjustment and needs repair.

Note – If aperture impacts sharpness, this is news to me because I’ve always understood that shutter speed affects sharpness whereas aperture affects depth-of-field.

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November 26, 2006

Allow me to explain.
The lens acts like a wave guide, bending (actually refracting) light waves. Think of a line of target shooters on the 50 yard line. Imagine shooters standing shoulder to shoulder all with target rifles, all aiming at a single bull’s-eye located in the center of the end-zone. The shooters near the center of the field have the shortest gun-to-target distance and are square with the target. The shooters nearer the sidelines have a further gun-to-target distance and they are not square with the target. The more central shooters have the advantage. All are highly skilled, but when we inspect the target, after a volley, we find not one clean hole dead center but a scalloped hole and some misses. This pattern is exactly the shape made by a lens. In the case of the lens we have a tiny scalloped spot of light called the airy disk, otherwise known as the circle of confusion.

We want the airy disk to be so tiny that it is immeasurable. We find that as we open the lens aperture the rays from the lens periphery are disadvantaged. As we test, as you did, from largest opening to smallest, we generally find that the lens functions best about two stops down from the largest opening. As we continue to stop down further, the airy disk becomes less defined and the resulting images produced are compromised as to sharpness. This phenomenon is due to diffraction.

Diffraction is a tendency of the light rays to spread around the hole that is the aperture. As we adjust our iris diaphragm (stopping down) the hole (aperture) becomes smaller and the percentage of rays striking the blades become greater. As light rays pass close by an obstacle they cause interference and become misdirected. Thus tiny apertures are not as sharp as the middle-range.

In short a lens functions best about two stops down from the maximum. It is a myth that small apertures like f/16 – f/22 – f/32 – f/64 are the sharpest -- they are not. However they do yield greater depth of field.

Hope this helps.

Alan Marcus
ammarcus@earthlink.net

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November 26, 2006

Yes indeed. Your response is very helpful and I feel grateful for your time and attention. Thank you for sharing these details pertaining to the physics of light in relation to lenses.

Just as you say, my lens starts rendering sharper images at f4.0, 2.33 stops down from the maximum aperture of f1.8.

My thanks also to Jim Miotke for starting this great photography Q&A forum!

Uriah Carr

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November 27, 2006