10 minute read
The Next Steps with IR & UV Photography
Nate Rochefort continues his infrared and UV photography guide, covering lenses, filters, white balance, and more, following a once-in-a-lifetime expedition with FUJIFILM GFX100 IR
In the world of visual-spectrum photography, only a rare few outside forces strongly dictate one setting or another – thus is the power of today’s modern cameras. Aperture, white balance, use of lens filters and the like are almost exclusively creative choices. In infrared and UV image making, however, much clearer requirements emerge. Thankfully, with expert help, these essentials are not all too difficult to understand. Below, Nate shares his insight.
This specialist version of GFX100 is ordinarily unavailable to the general public – its capabilities reserved only for professional forensic, scientific, or conservation uses. However, for the purposes of this infrared photography series, Nate has been granted exclusive access to GFX100 IR with the intention of using its otherwise practical infrared and UV functionality for artistic pursuits.
Hotspots in IR Photography
In visible light, a camera’s sharpest aperture is usually a few stops down from wide open – typically around F8. However, using a small aperture for IR photography may not always be possible, due to a well-known issue in IR photography called a hotspot. A hotspot is a bright circle that can appear in the middle of an IR image that tends to increase in severity as the aperture is stopped down. Several factors can cause their occurrence and intensity, but the camera lens and aperture settings are the most significant.
Each lens is different, but as a general rule, the narrower the aperture, the more apparent the hotspot. Most GF Lenses are relatively free of hotspots when wide open, but will eventually develop a hotspot as you stop down. With the majority of GF Lenses, once they pass a threshold of roughly F11, there is a chance the hotspot will be nearly impossible to correct in post.
I’ve found the lens that performs best for IR photography with GFX100 IR is FUJINON GF32-64mmF4 R LM WR. It’s the optic I find myself gravitating towards whenever photographing in IR. It has excellent flexibility with its focal length range, is tack-sharp, and has the most significant buffer between a clean image and one ruined by a hotspot.
An infrared hotspot
Lens Transmission of UV and IR
Infrared light has no issue passing through a lens’s elements, making hotspots the most common problem when photographing in IR. Ultraviolet light is another matter. Because of UV’s relatively short wavelengths and the complex design of modern lenses, UV can struggle to reach the camera’s sensor.
Some specialty lenses are designed to allow as much UV as possible to pass through the optics, like lenses constructed from quartz. However, these can be prohibitively expensive and difficult to find.
Another option is a lens with a minimal number of elements. Because they often contain fewer elements, primes are the best for documenting UV images. While most GF primes are capable of UV photography, FUJINON GF50mmF3.5 R LM WR and FUJINON GF120mmF4 R LM OIS WR Macro stand out. They are my go-to optics.
Filters for UV and IR Photography
Infrared and ultraviolet photography require more than just a modified camera, they need filters. Filters are essential when using a full-spectrum camera, whether it’s used for UV, IR, or visible light photography. Filters block all the unwanted wavelengths, only allowing the target ones to pass into the camera.
There are three basic filter types commonly used for UV and IR photography:
- Longpass filters
- Shortpass filters
- Bandpass filters
What is a longpass filter?
Infrared photography typically utilizes longpass filters – these block all wavelengths up to a certain point. For example, a 720nm longpass IR filter will block all wavelengths below 720nm (UV and most visible light) and allow all wavelengths above 720nm to pass through. The point at which the filter begins to transmit light is called its cut-on wavelength. Many IR filters are referred to by their cut-on wavelength.
What is a shortpass filter?
Shortpass filters are the opposite of longpass filters – they transmit wavelengths up to a certain point and block all wavelengths beyond. The point at which a shortpass filter begins to block wavelengths is called its cut-off wavelength.
What is a bandpass filter?
Bandpass filters allow a range of wavelengths to pass through. They have both a cut-off and cut-on wavelength. One such option is a visible light filter. It blocks both the shorter UV and longer IR wavelengths. The visible light filter I use with GFX100 IR has a cut-on wavelength of 400nm and cut-off wavelength of 700nm.
An 830nm IR filter was photographed in visible light. When seen in the visible spectrum, the filter is entirely opaque. This is because it blocks all wavelengths of light below 830nm.
The Effects of IR Filters
Various IR filters are available and each can dramatically affect the final image. Some IR filters have a cut-on wavelength that begins in the visible spectrum, which allows some visible light to be documented alongside the IR, creating a false-color IR photograph. The following are examples of available IR filters.
Photo 2023 © Nate Rochefort | Visible Light | FUJIFILM GFX100 IR and FUJINON GF120mmF4 R LM OIS WR Macro, 1/250 sec at F9, ISO 100
As seen in the examples above, as you progress deeper in the IR spectrum, two things happen: exposure times become longer, and color leaves the image.
To the first point, exposure times are increasing because the filters block more light, and the camera’s sensor is becoming less sensitive to the longer wavelengths. All of the IR filters that allow some degree of visible light to pass (550-715nm) are relatively close in exposure timings. When moving from 715nm to 830nm, however, it takes an additional 2 1/3 stops. There is another significant jump of 3 1/3 stops from the 830nm filter to the 1000nm. The 1000nm filter is the deepest the camera can see into the IR region, and takes an additional six stops to create an image.
UV is another story entirely. Due the increased difficulty of UV reaching the camera sensor, it requires a long exposure time – even in bright light. The image below required an additional ten stops of exposure time compared to the visible light photograph.
Photo 2023 © Nate Rochefort | 365nm UV Filter (+2 exposure adjustment in post) | FUJIFILM GFX100 IR and FUJINON GF120mmF4 R LM OIS WR Macro, 1/2 sec at F4, ISO 100
Filters are necessary for making images with a full-spectrum camera, but they are just one piece of the puzzle. Knowing how to edit the images is of near-equal importance. While GFX100 IR does a wonderful job of metering and focusing, assigning a proper IR and UV white balance is something nearly all full-spectrum digital cameras need a little help with.
IR White Balance
If you were to photograph using a full-spectrum camera but no filter, the resulting image would have an intense magenta or red color cast, due to the camera sensor’s sensitivity to IR and red wavelengths. This makes it necessary to white balance your photographs before creating them, or in post.
In both scenarios, you would color correct as you would under any other circumstance – either by selecting a known neutral color in editing software or using an object with a neutral color to set the camera’s white balance. This begs the question: what is a neutral color outside the visible spectrum? For IR photography, the answer is relatively simple.
Many white balance targets reflect near-IR wavelengths similarly to visible light and can be used to set a custom white balance. However, there is another method I am partial to – using foliage or grass as the white balance target. Leaves and grass are solid reflectors of IR wavelengths, which is why they appear white in IR images. This well-established appearance in IR images makes them a consistent and accessible white balance target.
White balance for filters with a cut-on wavelength above 750nm is relatively consistent, and there might not be much need to adjust if switching from an 830nm filter to 950nm. But, when switching to a filter that allows some visible light wavelengths, it’s best to set a white balance for that specific filter. GFX100 IR allows three custom white balance settings to be saved, making it simple to set up white balances for the day, switching between them as needed.
Photo 2023 © Nate Rochefort | Visible light filter, auto white balance | FUJIFILM GFX100 IR and FUJINON GF32-64mmF4 R LM WR, 1/320 sec at F7.1, ISO 100
Join me in part three of this series, where we’ll continue with essential editing techniques.
GFX100 IR is a specialist camera designed for forensic, scientific, and cultural preservation applications, and is not available for general sale.
To learn more about GFX100 IR and its conditions for use, visit the dedicated page here. Alternatively, explore the rest of the GFX System and experience the power of ultra-high resolution photography.
