Diffraction in Infrared Photography

When I started shooting in infrared, I noticed a lack of sharpness when using apertures that I had associated with sharpness in visible light photography. I started using lower f-stop values when shooting in infrared, but wanted to determine the cause. That cause was diffraction.

Diffraction is a property of light which can reduce the sharpness of a photographic image. It can impact visible light photos and can have an even greater impact on infrared images. It’s important to have an awareness of how diffraction will impact your images, especially when shooting with an infrared cut-off filter, such as 590, 720, 850 nm, or in full-spectrum.

Video

What is Diffraction?

Diffraction is the bending of a wave, such as light, as it passes around a corner or through an aperture, such as the aperture of a lens. While most of the light continues in a straight line when passing through an aperture, a small portion changes direction. Cameras are a “Diffraction-limited system”, since diffraction and optical imperfections limit the ultimate resolution a camera is able to capture.

Numerical approximation of diffraction pattern from a slit of width four wavelengths with an incident plane wave.
By Dicklyon, Wikipedia, Public Domain

A point of light creates a spot on the camera sensor called an Airy disk. Less diffraction results in a smaller Airy disk. Increased diffraction results in a larger Airy disk.

A computer-generated image of an Airy disk.
By Sakurambo, Wikipedia, Public Domain

What factors impact diffraction?

There are three factors which impact diffraction in digital photography.

  • aperture size or f-stop
  • wavelength of light
  • pixel size on the sensor

Aperture and Wavelength

Aperture and wavelength determine the size of the Airy disk. There is a formula for calculating the size of the Airy disk.

d/2 = 1.22 λ N

d is the resulting diameter of the Airy disk, λ (lambda) is the wavelength of light, and N is the f-stop. A photographer-friendly version of this formula could look like this.

airy disk diameter = 2.44 * wavelength * f-stop

Based on this formula, it’s easy to see that an increase in either wavelength or f-stop will increase the size of the Airy disk. Just like in visible light photography, larger f-stop numbers (smaller apertures) result in more diffraction. Of particular importance for infrared photography, longer wavelengths of light, such as near-infrared, produce more diffraction compared to visible light wavelengths.

For example, given the same f-stop, light at 850 nm (near-infrared) will produce an Airy disk that is double the size produced by light from 425nm (violet).

Pixel Size

Comparing the Airy disk size to the pixel size determines if the diffraction is visible in photographs. When the size of the Airy disk is smaller than the pixel size on your sensor, then diffraction will not be apparent in your image. However, when the size of the Airy disk exceeds the size of the pixel, that point of light is received by surrounding pixels and the results of diffraction are visible.

  • Airy disk size < pixel size = no apparent diffraction
  • Airy disk size > pixel size = diffraction visible

Image Tests

Here are a series of test images used to compare the amount of diffraction at various wavelengths of light. Visible light images were shot with the Fujifilm X-T2 and infrared images were shot with the Fujifilm X-T20, both cameras use the same sensor. The X-T20 was converted to 590 nm internally. 720 nm and 850 nm tests added an external filter. All images were shot with the same Fujinon XF 23mm f/2 lens, at ISO 200, using auto shutter speed, in direct sunlight. These are side-by-side comparisons at 400%. Screenshots are taken in 4K resolution.

Visible light

Visible light f/2 & f/2.8
f/2 & f/2.8: f/2.8 is substantially sharper then f/2 due to depth of field. Full size 4K

Visible light f/2.8 & f/4
f/2.8 & f/4: f/4 is sharper than f/2.8 due to depth of field. Full size 4K

Visible light f/4 & f/5.6
f/4 & f/5.6: f/5.6 is slightly sharper than f/4. Full size 4K

Visible light f/5.6 & f/8
f/5.6 & f/8: Similar sharpness. Full size 4K

Visible light f/8 & f/11
f/8 & f/11: f/8 sharper than f/11 due to diffraction. Full size 4K

Visible light f/11 & f/16
f/11 & f/16: f/11 substantially sharper than f/16 due to diffraction. Full size 4K

590 nm filter

590 nm f/2 & f/2.8
f/2 & f/2.8: f/2 is less sharp than f2/8 and contains chromatic aberration. Full size 4K

590 nm f/2.8 & f/4
f/2.8 & f/4: f/4 is substantially sharper than f/2.8. Full size 4K

590 nm f/4 & f/5.6
f/4 & f/5.6: f/5.6 is sharper than f/4. Full size 4K

590 nm f/5.6 & f/8
f/5.6 & f/8: f/8 is slightly sharper than f5/6. Full size 4K

590 nm f/8 & f/11
f/8 & f/11: f/8 is slightly sharper than f/11 due to diffraction. Full size 4K

590 nm f/11 & f/16
f/11 & f/16: f/11 is sharper than f/16 due to diffraction. Full size 4K

720 nm filter

720 nm f/2 & f/2.8
f/2 & f/2.8: f/2.8 is substantially sharper than f/2 due to depth of field. Full size 4K

720 nm f/2.8 & f/4
f/2.8 & f/4: f/4 is sharper than f/2. Full size 4K

720 nm f/4 & f/5.6
f/4 & f/5.6: Similar sharpness. Full size 4K

720 nm f/5.6 & f/8
f/5.6 & f/8: f/5.6 is sharper than f/8 due to diffraction. This is where diffraction has an impact at f/8 for 720 nm, making f/5.6 sharper. Full size 4K

720 nm f/8 & f/11
f/8 & f/11: f/8 is substantially sharper than f/11 due to diffraction. Full size 4K

720 nm f/11 & f/16
f/11 & f/16: f/11 is substantially sharper than f/16 due to diffraction. Full size 4K

850 nm filter

850 nm f/2 & f/2.8
f/2 & f/2.8: f/2.8 is substantially sharper than f/2 due to depth of field. Full size 4K

850 nm f/2.8 & f/4
f/2.8 & f/4: f/4 is substantially sharper than f/2.8 due to depth of field. Full size 4K

850 nm f/4 & f/5.6
f/4 & f/5.6: f/4 is slightly sharper than f/5.6. This is where diffraction has an impact at f/5.6 for 850 nm, making f/4 sharper. Full size 4K

850 nm f/5.6 & f/8
f/5.6 & f/8: f5.6 is slightly sharper than f/8 due to diffraction. Full size 4K

850 nm f/8 & f/11
f/8 & f/11: f/8 is sharper than f/11 due to diffraction. Full size 4K

850 nm f/11 & f/16
f/11 & f/16: f/11 is substantially sharper then f/16 due to diffraction. Full size 4K

Image Test summary

Filter Sharpest apertures
Visible Light f/5.6 & f/8
590 nm f/8*
720 nm f/4 & f/5.6
850 nm f/4

* Based on prior experience and the trends here, I expected the sharpest 590 nm aperture to be f/5.6. This result may be due to a flaw in the test or my methodology.

Summary

  • Larger f-stop numbers increase diffraction.
  • Longer wavelengths increase diffraction. As a result, the impact of diffraction will be apparent at lower f-stop numbers for near-infrared light compared to visible light.

Recommendations

To avoid diffraction in infrared photography, you will need to use a lower f-stop number than you would need for visible light photography.

I consider the “optimal” aperture for landscape photography to be the highest numbered f-stop, which provides the most depth of field, before diffraction sets in. In my experience with visible light photography, this is typically the third-highest f-stop number for the lens. These recommendations are based on selecting an aperture which is a number of f-stops lower than this “optimal” aperture.

These recommendations are based on an APS-C sensor size. Results may vary with larger or smaller sensors.

590 nm (red, orange, and near-infrared)

Use 1/3 to 1-stop less than optimal for visible light. For example, if your highest f-stop before diffraction with visible light is f/11, then for infrared, you would use f/8, f/9 or f/10.

720 nm (some red, near-infrared)

Use 1-stop less than optimal for visible light. For example, if your highest f-stop before diffraction with visible light is f/8, then for infrared, you would use f/5.6.

850 nm (near-infrared only)

Use 2-stops less than optimal for visible light. For example, if your highest f-stop before diffraction with visible light is f/8, then for infrared, you would use f/4.

Full Spectrum (near-ultraviolet, all visible light, near-infrared)

Full spectrum has the broadest range of wavelengths and therefore the broadest range of diffraction. Use 2-stops less than optimal for visible light. For example, if your highest f-stop before diffraction with visible light is f/11, then for infrared, you would use f/5.6.

Sources

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