• Dr. Jitendra Katre

Lenses for infrared photography

Can you see that slightly brighter spot with diffused edges in the center of above image? It is called 'Hot spot'. This is found with many lenses, even some expensive lenses are known to cause this. In above picture this spot lies over the sky, so maybe easy to remove while post-processing. However, it may not be so easy to remove it, if it lies over some details in the picture... And this is just one of the common problems found with lenses used for infrared photography.

1) Hot spots - The lenses we use are not designed to be used for infrared photography. Hot spots are supposed to be caused by internal reflection of infrared radiation within the lens barrel. Not all lenses cause this, and there is no way of knowing if any particular lens causes it or not, because all copies of any particular lens don't behave similarly. For example, some copies of Canon EF 24 105 F4IS are very good, but some copies do cause some hot spot. There are some well compiled lists of lenses on the sites like 'kolari vision", which can be very useful to select the lens. There are some very good lenses like Canon 16 35 f/2.8 which are known to cause hot spot, while at the same time some cheap lenses like Canon 18-55 IS2 are literally free from this problem.

Smaller apertures are more prone to develop hot spot, many lenses known to have this problem can give relatively good image when used wide open.

It is also found that some lenses which are known to cause hot spot on digital sensor, do not show this problem when used on infrared film. Therefore, it is possible that it could be a result of reflections from digital sensor surface. It is more likely that hot spot is a combination of these two factors.

2) Color fringing - When infrared light is mixed with some visible light, all wavelengths of the visible light spectrum are not bent equally by lens optics combined with infrared-pass filter. This causes fringing. This can be severe with some lenses and it is prominently seen near edges of the frame. Color fringing can be very difficult to remove during post-processing.

3) Infrared wavelengths focus at different point compared to the visible light. All cameras converted for infrared work also need some focus calibration. Some lenses are marked with infrared focus point to be used while doing infrared photography with unconverted cameras. Focus is supposed to be shifted manually to this spot after achieving auto-focus.

4) Images taken with extreme wide angle lens are likely to have very soft edges. This is particularly true if lens is not calibrated for ultra-wide focal lengths.

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