Why You Should – and How to – Avoid Diffraction in Infrared Photography

1.Diffraction in infrared photography is probably worse than you thought

Recently I have completed testing a few Olympus lenses in infrared light, and I have noticed very strong diffraction effects at smaller apertures, much stronger than in visible light. Actually, this is expected since diffraction depends on both the aperture and wavelength of the light. I just did not believe it would be so visible. The picture at the top of this post is such an example.

If you believe diffraction is not important for your infrared pictures, here is a visual comparison of how diffraction impacts sharpness at various apertures. Please note that the crop is 2× enlarged, so even at f/2.8, the picture will not appear perfectly sharp. This is a totally non-scientific example, but it should get the point across:

• Camera: Olympus OM-D E-M5, converted to 720 nm Infrared
• Lens: Olympus ED 12-40mm F2.8 PRO at 25 mm
• Non-sharpened 256 x 256-pixel crops from the picture center, 2× enlarged for better visibility
  

You can see that starting with f/8, the picture gets more and more blurry. But how can we calculate the maximum aperture we can use without impacting the sharpness of our pictures?

1a.Calculating infrared diffraction limits

There are a lot of sites out there where it is possible to calculate the diffraction limits for your camera, but – to my knowledge – none of them deals with infrared light. This is where I decided to create my own diffraction calculator, specifically for infrared photography. And since I am a visual person, I also wanted to include a nice chart for a graphical representation of the result. Finally, I wanted to compare the levels of diffraction for your chosen infrared filter and visible light, to give you an idea about the difference.

All you need to do is to enter the required values in the below fields and click Calculate. You will be taken to the resulting diagram and further explanations.

2.Diffraction Calculator

2a.Chart Explanation

Please note that all values stated below are specific to your selected combination of camera and infrared filter. For a different combination, just modify the input fields and re-calculate the result.

: The diameter of the infrared airy disc. The size depends on infrared wavelength and aperture. For the calculation, the arithmetic average of the IR filter cutoff frequency and 1100 nm is used, in this case .

: The diameter of the visible light airy disc. The human eye and Bayer pattern type sensors are most sensitive to green light, with the peak sensitivity around 555 nm. This is also the wavelength used for this model.

: This is based on the resolving ability of the human eye. It is based on what is known as the Zeiss formula. The Circle of Confusion (CoC) is calculated considering a normal viewing distance, based on the angle of view (typically 60°). Diffraction above this level will likely affect the perceived sharpness of an image viewed at normal viewing distances. In the table below, it is being referred to as visual diffraction.

: This is the resolving ability of the sensor, based on pixel pitch. It is measured in µm or line pairs per mm [lp/mm]. Diffraction above this level is likely visible at the pixel level, but not necessarily at normal viewing distances. In the table below, it is being referred to as technical diffraction.

2b.What does this diffraction chart mean for your infrared pictures?

This depends on your approach to photography:

If you apply the above recommendations, you will be able to avoid diffraction in your pictures. Just be aware that sometimes you will have to stop down beyond the limits shown here, for example when you need large depth-of-field. As always, it is about finding the right balance.

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