Infrared photography is done with a film or a digital image sensor, which is sensitive to infrared light. Infrared is a radiation of electromagnetic waves, which range from 0.74 to 300 micrometers. It is not visible to human eyes but this thermal radiation can be used to expose an image on film.
We require an infrared optical filter on the lens. This filter blocks most of the visible light and allows infrared light to pass through it.
The image formation depends on thermal radiation, so the result is totally different for the same subject, as compared to visible light. For example, sky will be recorded as a darker shade, whereas the skin will be recorded as a lighter shade.
Infrared wavelength is different from the visible spectrum. So the refractive index of this radiation is different, and the optics (lens), which is calibrated for the visible spectrum will not work with infrared radiation. There is a shift in focusing distance. Most of the manual lenses are marked with a red dot and "R" on the lens barrel. This red dot is the actual focusing, when we use an infrared film. Firstly, we have to focus the image, (without the infrared filter), in a normal way and then refocus it by turning the focusing ring to the red dot at the time of click.
As infrared photography requires refocusing of lens, the camera must not move and has to be supported on a sturdy tripod. An auto focus lens on a digital camera will not work for this setting so we will have to use the smallest available aperture. In such case, the shutter speed will be very slow and again, we will require a good, sturdy camera support.
Films for infrared photography are calibrated for different range of radiation. For each range, different type of infrared filter is required. So check the instruction manual of the film, and use the recommended filter. For each range, the focusing will defer because of change in refractive index. This is a reason why some lens manufacturer does not show red dot - "R" on the lens. We have to refocus the lens as per the type of the film and we may have to do some trial and error, or use catadioptric lenses (mirror lenses). Catadioptric lenses does not require this adjustment because mirrors does not suffer from chromatic aberration.
Digital image sensors are sensitive to infrared radiation. For normal photography, a sensor, which is sensitive to infrared will cause problems. It will interfere with exposure as well as auto-focusing of the lens. Secondly, a range of infrared radiation records the image by thermal radiation, which is coming from a body. This means that some colored clothes become invisible to the sensor. This is a social issue. So digital image sensors are fitted with an infrared blocker, which may be removable, depending on the make and the model of the camera.
So if we are planning an infrared photography with a digital camera, go for a type of camera, which has a removable infrared filter. Then we have to use an infrared filter on the lens and we can take infrared photographs.