Diffraction Limit Calc

The Diffraction Limit Calc shows how small a point of light your optics can form before diffraction blurs it. In Lens / Telescope mode you enter the wavelength of light, the aperture diameter and the focal length, and it returns the angular resolution in arcseconds and radians, using 1.22 times the wavelength divided by the diameter, plus the Airy disk radius at the focal plane in micrometers. Microscope mode takes a numerical aperture instead and returns the Rayleigh limit, 0.61 times the wavelength over the NA, along with the Abbe limit.

The calculator has no camera or sensor setting, so you work from your lens. Leave the wavelength at 550 nm for green light, enter your focal length, and enter the aperture diameter, which is the focal length divided by the f-number: a 50 mm lens at f/8 has a diameter of 6.25 mm. Click Calculate Limit and note the Airy disk radius, about 5.4 µm in that example. Then work out your pixel pitch, the sensor width in millimeters divided by the image width in pixels, and compare the two.

Diffraction softening becomes visible when the blur spot grows large compared with the individual pixels on your sensor, and pixel size depends on the sensor's dimensions and megapixel count. A sensor with many megapixels packed into a small area has small pixels. When light passes through a narrow aperture it spreads into an Airy disk, and once that disk spans more than a couple of pixels, detail starts to blur across neighbors. Because the calculator reports the spot size rather than a camera-specific f-stop, you make this comparison with your own pixel pitch.

Use it whenever precise sharpness and deep depth of field both matter, which is typical of landscape, macro and architectural photography. In landscapes you often want everything from the foreground to the horizon in focus, which pushes you toward small apertures. Macro work usually needs heavy stopping down as well. By calculating the spot size at the apertures you plan to use and comparing it with your pixel pitch, you can judge the smallest aperture you can use before the physics of light begins to soften the fine details of your composition.

If your images remain soft at apertures where the Airy disk is still small compared with your pixels, other factors are likely at play. Camera shake is a common cause of blur, especially at slower shutter speeds without a sturdy tripod. Missed focus or a lens with front or back focusing issues also produces soft images. Every lens has a sweet spot, usually two to three stops down from its maximum aperture, where it performs best. Shooting wide open, optical aberrations such as spherical or chromatic aberration can reduce sharpness before diffraction becomes a factor.

Out-of-focus blur occurs when light from a point in the scene does not converge precisely on the sensor, usually because that part of the scene lies outside the depth of field. Some areas of the image are sharp while the foreground or background is blurry. Diffraction blur affects the whole image evenly. It is caused by light waves spreading as they pass through a small aperture opening, so even areas well within the depth of field look slightly soft and lose fine micro-contrast.

An Airy disk is the spot of light a perfect optical system with a circular aperture forms from a single point. Because light behaves as a wave, passing it through a small opening makes it diffract into a bright central spot surrounded by faint rings. The size of that spot grows with the f-number: as you stop down, the disk gets larger. The calculator reports the radius to the first dark ring, the Rayleigh criterion. When the disk grows larger than your sensor's pixels, light spreads across several of them and resolution drops.

If diffraction stops you from closing the aperture enough to get the whole scene in focus, the most effective alternative is focus stacking. You take a series of identical frames from a fixed position, usually on a tripod, while moving the focus point step by step from the nearest foreground object to the distant background. Because you are taking several shots, you can keep the aperture at the lens's sharpest setting. Afterward, software blends the sharpest parts of each frame into a single image with much deeper depth of field and full resolution.

The size of the diffraction spot does not change: at the same f-number and wavelength, the calculator gives the same Airy disk radius whatever camera is behind the lens. What changes is how that spot compares with your pixels. An APS-C sensor with the same megapixel count as a full-frame one has smaller pixels, so the same spot covers more of them and diffraction softening becomes visible at a wider f-stop. That matters if you shoot with bodies of different formats or are changing systems.

Diffraction depends on the f-number, which is the focal length divided by the aperture diameter, not on focal length alone. In this calculator the Airy disk radius works out to 1.22 times the wavelength times the f-number, so a 50 mm lens with a 3.125 mm aperture and a 200 mm lens with a 12.5 mm aperture, both f/16, give the same radius of about 10.7 µm at 550 nm. The angular resolution does differ, because it depends only on the aperture diameter: the larger opening of the longer lens resolves finer angles.