Diffraction impacts infrared photography more than in visible light. Calculate the aperture limits for any IR filter, based on your personal requirements.
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
2b.What does this diffraction chart mean for your infrared pictures?
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I found this interesting. However, am curious why diffraction is not desirable for infrared photography. Back in the film days, it was one of the desired qualities of deep IR. Since digital IR has taken over it seems that there is an expectation that IR photography will meet the standards of sharpness as standard visible light spectrum photography.
I myself am guilty of this, I think in an attempt for my IR photography to be acceptable and understood while I was still learning. But as an IR community, I question why we don’t leverage more off of these IR quirks.
Curious about your thoughts on this.
Well, you can take my guide and use it to produce the exact opposite result with a lot of diffraction – that’s completely up to you. Just be aware that most of today’s IR photographers – myself included – likely never had any experience with IR film and therefore won’t know about the quirks you mentioned.
I think it is good to have a choice – you can process your digital IR pictures the way you prefer. As a creative photographer, for me there are no ‘standards’, I consider them as ‘proposals’. If your style is to mimic IR film, that’s wonderful and I am sure a lot of people will like the result.
Thanks Robert for creating this calculator.
It is a neat tool with all different camera presets.
The calculator is very nice for illustrating the increased problem with diffraction for IR photography.
I think that calculating using the arithmetic average is not quite correct as then photons from 1100nm is weighted equally as from the lower filter cutoff wavelength.
The sensor has a much lower sensitivity at longer wavelengths.
If biasing the weight towards the cutoff wavelength you would get a more realistic result from the actual diffraction effects on an image.
When doing pixel peeping there is another phenomenon that can cause problems for most lenses. They are not optically corrected into the IR range. If using a filter with a shorter cutoff wavelength like 590nm all wavelengths might not be in focus at the same time.
That effect will be even worse if you want to simulate the iconic Kodak Aerochrome film that in the recommended setup used a blue blocking filter Tiffen 12 with a cutoff of around 530nm, (that is not covered by your calculator).
I agree with pretty much everything you say. If you are aware of formulas taking these effects into consideration, I would be happy to integrate them here. The Aerochrome filter is not a simple highpass filter like the other conventional IR filters, with unknown (to me) optical characteristics , therefore it is not covered here. The point I am trying to make with this calculatur is to make paople aware of the rapidly increasing diffraction effects when using higher resolution sensors and longer wavelength IR filters.
I was not referring to the IR Chrome filter from Kolari Vision, but the real Aerochrome films from Kodak. The filter for the film is a simple high pass filter!
Here is a link to more Aerochrome film information:https://www.robwalwyn.com/aerochrome
The IR Chrome filter is a stacked filter with unknown filter glass components, It is likely that one of the optical glass components is an IR attenuating filter glass like the KG3 filter glass from Schott.
The IR Chrome do give foliage a red colour but it fails to emulate two aspects that images from the Aerochrome film have:
1: A higher contrast and deeper blue sky
2: Red objects is rendered as yellow with the film and skin tones are often getting a yellowish tint.
In the original instructions for the film there is really a recommendation of a simple blue blocking filter Kodak Wratten 12.
If searching for “aerochrome film instructions” the instructions can be found.
As IR-Chrome from Kolari is lacking all these features I consider it as a visual filter using some tiny bit of IR to change the tint of foliage to red.
It is very clear that it is just a small part of the IR spectra that is used as there are none of the coloured fringe effects normally seen with real IR Pass filters for non IR-corrected lenses.
If you just want red foliage (and correctly yellow rendered reds) there are LUTs that work well with simple visual images and videos. See the Infrared LUT package:
https://www.mathieustern.com/luts
I have bought and successfully tried that LUT. I use the LUT when I do not want to go through with the Tiffen12 action with 550nm-filtered IR images. :
Both give yellow rendered reds and a higher contrast and more dark blue skies.
I actually prefer the LUT as it do not give the fringe problems.
Thank you Ulf for sharing all these details about the Aerochrome filter and the comparison to the Kolari IR Chrome filter. I am sure this will be useful for people interested in the IR Chrome filter.
Robert I just read this article with great interest and ran the numbers for the EOS 5D Mk IV and was wondering what I have to do to get a comparison with my converted EOS 5D Mk II?
Martin, you will need to enter the required data (sensor size, total megapixels) manually. Your camera manual should provide you with all the details.
Great stuff Robert and thanks. Your DNG Editor solution for a B&W profile for my 850nm images works a treat. Maybe I can move on from here. Thanks again.
Thank you for the feedback Martin, I am glad you found it useful!
Robert, you raise some interesting issues. I note that in the table you offer only quite new Panasonic cameras. I use older models with larger pixels and wonder if this gives me more flexibility. I rarely experience diffraction, and am about to invest in an f2.8 zoom with IR very much in mind. I hope I am not wasting my money!
Maggie, you are definitely not wasting your money. The combination of a low-megapixel sensor with wide-aperture lenses will give you a lot of flexibility, without running into diffraction issues. I am sure you have already calculated the results for your combination.
I had to restrict the number of models for the calculator, otherwise the list becomes unmanageable. This is why you see only the newer models.
Thank you, Robert! I’ve just placed my order and I’m really looking forward to using it with either of my converted cameras. Best wishes.
Thanks for letting me know Maggie! Have fun with your converted camera!
Robert, thanks for aggregating this info! I had not considered the impact of diffraction in IR. I tend to shoot at f/8 on my Fuji X-T20 (same sensor as X-H1), but I will try lower f-stops to improve sharpness. This could certainly explain the softness of some of my early IR shoots at f/8 with a Canon SL1 (similar sensor as 80D), which appears to be more sensitive to diffraction.
Thank you Rob, I certainly appreciate your feedback! Glad you found this page useful. Now you can in advance determine the optimal aperture setting for a given camera / IR filter combination. It definitely helped my IR photography a lot :-)
You have some very nice IR shots on your website – I would guess some are taken with the Kolari IR Chrome filter?