Infrared testing with the Fuji GF 45-100mm f/4 and GF 100-200mm f/5.6

Captured in 590nm with the LifePixel Super Color Infrared filter at a 45mm focal length, the edge performance of the Fuji G 45-100mm f/4 is disappointing

Created by poor farming practices in the 1800s, Providence Canyon is home to steeply eroded cliff walls as high as 150ft. On this trip to the canyon I wanted to do some testing and experiment with the viability of stitching infrared panoramas with the Fuji GF 45-100mm. The above image has had its blue and red channels swapped to produce the blue sky and orange foliage.

The edge performance of the GF 45-100mm f/4 is disappointing. In visible light the lens performs well at the edges, and is generally quite sharp, but it can’t properly focus infrared light which leads to aberration and fringing, and to a sloppy panorama.

The 45-100mm also presents a hotspot, most visible and defined at 45mm and at smaller apertures. This is apparent in the panorama, primarily in shadows. The hotspot is more subtle at the 590nm range, and increases in visibility the deeper into infrared one goes. The variation in color in the above white walled cliffs are only partially caused by the hotspot, as there is some natural variation in color which is mostly, but not completely, neutralized in infrared.

In short, the 45-100mm is usable in infrared only when tightly cropped at the center at its wider focal lengths, assuming the hotspot isn’t overpowering the image. It is more usable with longer focal lengths, but the hotspot issue remains. However, the Fuji GF 100-200mm performs far better in infrared at all focal lengths, so this limits the use of the 45-100mm for imagery with optimal image quality.

Providence Canyon. 4 image panorama captured with the GF 100-200mm and Full Spectrum GFX 100. Shot with Kolarivision’s IR Chrome filter meant to mimic Kodak Aerochrome film.

The GF 100-200mm f/5.6 is another story. This lens handles infrared far better, as is expected in longer focal lengths. If other G series lenses behave similarly to the 45mm in IR, I imagine the best quality wide angle of view IR images coming off the GFX would be stitched panoramas captured with longer focal lengths. This is a significant increase in effort, but may not be necessary if other wide angle lenses perform better than the 45-100mm.

Wide angle lenses are known for having greater issues with longer wavelengths of light, particularly at the corners. Lens designers make glass that performs well in visible light, which results in poorer IR performance due to the different location at which IR light focuses. This offset is magnified by the complex design of wide angle lenses, and further underlined by the medium format GFX 100 sensor, as wide angles must retain image quality across a wider image area than would typically be required in a smaller sensor camera. Using the GFX in 35mm mode would likely make the 45-100mm more usable, again assuming the hotspot isn’t overpowering the image.

One might say that using 35mm mode on the GFX misses the point of a medium format body converted to full spectrum, but it’s important to remember that other lenses at longer focal lengths perform far better across the full image area of the GFX. The ability to use the full 43.9mm x 32.9mm sensor in full spectrum, and then to have the flexibility to, when desired, swap to a smaller sensor size to accommodate the vast library of lenses available to the GFX either natively or by adaptation, is huge.