What is a UV/IR Cut Filter
Regular camera sensors are designed to detect a wide range of wavelengths, including UV and IR light, which can lead to unwanted color shifts and reduced image sharpness when shooting standard photos. A UV/IR cut filter blocks these non-visible wavelengths, ensuring that only visible light reaches the sensor.
For photographers who enjoy capturing images in ultraviolet (UV) and infrared (IR) spectrums but still want the versatility to take regular photos, UV/IR cut filters are a must-have accessory. This allows you to seamlessly switch between specialized UV/IR photography and normal photography without compromising image quality. By using a UV/IR cut filter, you can maintain the flexibility of your full spectrum camera while ensuring crisp, color-accurate images in everyday shooting scenarios.
The IRC 650nm filter
The Infrared Cut filter of 650nm, also known as the IRC 650nm filter, is a UV/IR Cut filter designed to block infrared light beyond 650nm. Given its ability to effectively cut off wavelengths above this threshold, the IRC650 should be well-suited for restoring a full spectrum camera back to the visible spectrum (ViS). By filtering out the IR light above 650nm, it ensures that the camera captures images within the visible light range, thereby maintaining accurate color reproduction and image clarity.
Most OEM camera filters cut wavelengths around 650nm to 700nm, effectively eliminating any unwanted red hue in the final image. This filter features a steep transition curve, dropping from around 98% light transmission to 0% at 650nm. Such a steep cutoff is generally suitable for filters used in astrophotography, as it allows H-alpha (which peaks at 656nm) to pass through fully. However, since this filter cuts off at 650nm, it may not be the best choice for astrophotography. This is why I was hesitant to test it as a UV/IR cut filter, as its cutoff point is similar to that of most DSLR and mirrorless cameras.
IRC650nm Test and Sample Images
A filter test was conducted using a still life scene that included a color chart and a gray card for accurate white balance (WB). The setup featured a plant with green leaves to check for any IR leakage on the vegetation. Initially, the WB was set to Auto, and then a custom WB was performed on the gray card during post-processing. The images below illustrate the color deviation when compared to those taken with a non-converted camera.
Results
The image produced in Auto White Balance (WB) mode is usable straight out of the camera, with no signs of IR light leaking through to the sensor. The overall color balance appears to be accurate.
The main drawback of this filter becomes evident when using a custom WB set with a grey card. The balance skews heavily towards green, resulting in a pronounced green cast across the entire image. Interestingly, I've found that using a white surface instead of a grey card yields a more balanced WB.
However, there is still a noticeable green cast around the edges of the image. This green tint, reminiscent of the vignetting often associated with lower-quality lenses or filters, is attributed to the specific properties of the IRC650nm filter. Although I've heard of this issue before, I hadn’t personally encountered it with any other filters or UV/IR Cut filters I've tested until now.
This colour vignetting is most prominent when using wide-angle lenses and lessens as the field of view narrows. In this particular case, a 14mm lens (20mm on APS-C) was used, making the green cast quite significant. The effect becomes even more apparent when the camera is aimed at a clear, homogeneous surface, as demonstrated in the following images.
Fuji X-T30 Full Spectrum Camera | Fujinon 14mm |IRC650nm | Auto WB
Fuji X-T30 Full Spectrum Camera | Fujinon 14mm |IRC650nm | Auto WB
As demonstrated in the images above, the vignetting is too pronounced to overlook. I would not recommend using this filter if your goal is to restore the camera's original UVIR cut filter properties.
As a landscape photographer, I can’t imagine working with such a heavily cast image. In previous posts, I've reviewed the BCF 700nm filter and the IRF from Tangsinuo's supplier—both of which are far better suited for this purpose. While the IRC650nm may be more affordable than the other two, I wouldn't suggest it as a viable alternative.
.jpg "IRC650 Vignetting 2 (2)")
