NATURE PHOTOGRAPHY Isaac Szabo INFRARED CONVERSIONS

 

*This page is a work in progress. Please bear with me as I complete it.

Camera Choice
Nearly every digital camera can make a good IR camera, but some cameras are a little better suited for it. In general, the biggest advantage that some cameras have is liveview, which is helpful for IR for more accurate focusing, exposure, and composition. Mirrorless cameras and compact cameras use liveview exclusively, and nearly all recent DSLRs offer a liveview mode. Older DSLRs without a liveview mode can still make good IR cameras – they just have a few limitations compared to cameras with liveview.

Mirrorless Cameras and DSLRs in Liveview Mode:
•Accurate focus with all lenses (if calibrated properly)
•Accurate exposure preview and autoexposure modes
•Compose using the actual IR image

DSLRs without Liveview:
•Focus is calibrated to a specific lens and may not be accurate with other lenses (especially of significantly-different focal lengths)
•Exposure meter reads visible light and is not always accurate for IR light (may have to take test shots to determine correct exposure)
•Compose using the visible light image (to see what a scene looks like in IR, one must take a photo and review it on the LCD)

Compact Cameras:
•Accurate focus
•Accurate exposure preview and autoexposure modes
•Compose using the actual IR image
•Cannot change lenses (problematic if the lens happens to have a bad IR hotspot)
•Some models do not provide the option to set a custom WB (IR images are too red)
•Some models do not provide the option to shoot raw (no option to correct WB in post)

Lenses and IR Hotspots
When considering what lenses to use for IR, the main potential issue to be aware of is that some lenses have an issue called a hotspot when used for IR. An IR hotspot is a circular area of lighter tone, lower contrast, and often differing color temperature in the center of the frame. It is somewhat similar to sun flare, though it is caused by reflections inside the lens. It is impossible to know for sure whether or not a specific lens will have an IR hotspot without trying it. For that reason, online lists of lenses that have been tested for IR hotspot issues can be very helpful when trying to decide what lenses to use for IR. Also, it is important to note that hotspots increase in severity the more the aperture is stopped down, so many lenses that have mild or moderate hotspots can be successfully used for IR by shooting wide open or close to it.

White Balance
IR images require a custom WB (white balance), otherwise they will be too red. Most cameras provide an option to set a custom WB (some cameras call it manual WB or preset WB). It is best to set the custom WB in the same lighting conditions under which one will be shooting. The correct object to set the custom WB on depends on which IR filter is being used. With the 590nm filter, the custom WB should be set on a neutral object like a gray card or white piece of paper. When set correctly, vegetation will be blue and the sky will be reddish-brown. With the 720nm filter, the custom WB should be set on green vegetation. When set correctly, vegetation will be neutral (white/gray) and the sky will be brownish. 830nm is a monochrome filter, so the custom WB can be set on just about anything. When set correctly, the image will be uniformly B&W with no color cast.

However, some camera manufacturers arbitrarily limit the amount of adjustment available with the custom WB function, and with these camera models the neutral WB for IR is beyond the range of adjustment, making it impossible to set a neutral IR custom WB. All recent Nikon DSLRs have this issue when using the color IR filters (590nm and 720nm), as do Fuji mirrorless cameras and various other camera models. A neutral WB is required in order to achieve good results with many of the color IR post processing techniques, and the only way to achieve that with the cameras that cannot set a neutral IR WB in-camera is to shoot RAW and correct the WB in post during the raw conversion. So, as long as one is comfortable shooting and processing RAW files, this issue is really not all that significant. Also, this issue does not affect B&W IR, since those images can simply be desaturated to B&W even if they do not have neutral color balance.

RAW Processing
Processing IR raw files can be a little tricky, mainly because the most popular software (Adobe) does not handle IR raw files very well. The biggest issue is that the neutral WB for IR is beyond the range of adjustment of Adobe’s WB sliders, forcing the IR images to be too red. However, there is a work-around by using a custom camera profile created with Adobe’s DNG Profile Editor that shifts the WB into the range of the Adobe WB sliders. There are a number of articles and videos explaining how to do this on the web. However, what is not widely known that using a custom camera profile that only shifts the WB into the range of Adobe’s sliders will result images that have reduced contrast and poor color compared to the in-camera jpg or the raw image processed through the camera manufacturer’s software. So, in order to get comparable results with Adobe’s raw converter, it is necessary to make contrast and color adjustments to the custom camera profile in addition to the WB shift. Explaining how to accomplish this is beyond the scope of this webpage, but I will happily perform this service for my conversion customers (or for others for a small fee).

Channel Swapping
The most popular color IR post processing technique is to change the sky to blue. The most commonly used method to achieve this is to perform a red/blue channel swap, though it can also be done in other ways including a ~180° hue adjustment. Before performing the channel swap, the image should have a neutral IR WB and be in sRGB color mode, otherwise the effect will not work well (the colors will not look quite right). To perform the channel swap in Photoshop, add a channel mixer adjustment layer. Then, on the red output channel, change the red slider to 0% and the blue slider to 100%. Next, select the blue output channel and change the red slider to 100% and the blue slider to 0%. The sky should now be blue and the vegetation should be white (720nm) or yellow (590nm). The look of the colors can be further refined using a hue/saturation adjustment layer.

Channel Swap Action
Click here to download a Photoshop action file that will apply the channel swap in one easy step (including converting the image to sRGB). If you have never used Photoshop actions before, this short video explains how to install and use an action.


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