Why infrared images are not always sharp?
Here is an image that I shot from a moving vehicle. I made it sure to keep the shutter speed as high as possible to get the shot sharp. But did I get it sharp?
Maybe at this smaller dimensions it looks sharp, but when printed at large size, it is not very sharp. Problem lies in the lens. The lenses are not designed to be used with infrared cameras, they are made to handle visible light and not infrared light. Infrared light behaves differently, it is bent at different angles than that of normal light. If only infrared light is used to make the picture, as in the case of 850nm filter, the image will look sharp. However, if some part of visible light is also used to make the picture, as in the picture seen above using 590nm filter, visible light and infrared part of light is bent differently. This results in gross fringing and image doesn't look as sharp.
Below is a crop from the picture above.
This problem is much more pronounced near the edges of the picture. Cheaper kit lenses tend to be more soft than better made, expensive lenses. This chromatic aberration can be handled to some extent during post-processing, but it is very difficult.
This image is also shot using 590nm filter, however, this time it was Canon 16-35 f4 lens, a much better made lens than a kit lens. As you can see, image is much sharper and there is hardly any chromatic aberration.