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It is an unfortunate fact that the point at which light is focused by a lens varies with wavelength (or colour in the visible spectrum). Even when working with normal visible light - daylight - the various wavelengths within the visible spectrum are brought to critical focus at different points. Lens manufacturers go to great trouble to minimize this variation. Designs that feature very small differences across the visible spectrum are known as achromatic lenses. However, lenses are not generally designed for use at infrared wavelengths, so the focus distance scale marked on the lens barrel is appropriate only for the visible light spectrum. In the days of film, many lenses consequently featured an infrared focusing mark against which the standard distance scale could be set when using infrared film.

The method used to achieve optimum focus is dependent upon the type of camera used. Compact camera cameras are auto-focused by contrast detection in the sensor plane. This system should function correctly with a R70 opaque filter attached to the lens provided there is sufficient light available. Given adequate light, the focus should lock at close to the correct distance for infrared wavelengths simply because only those wavelengths are present. DSLRs work rather differently, and use sensors positioned behind a system of mirrors but nevertheless at the same effective distance from the lens as the image sensor. This system should also arrive at something like the correct focus distance because both the image sensor and the auto-focus sensor see predominantly infrared wavelengths.However, some inaccuracy may result from the fact that the image sensor in a standard camera is shielded by an anti-infrared filter. Cameras with a live-view mode should work more or less as compact cameras.

In all the above instances, it is worth accepting that small focusing errors may exist. Use a reasonably small aperture to maximize depth of field and place the camera on a tripod to accommodate the use of very slow shutter speeds. Where a subject moves, or leaves blow in the wind, a degree of compromise may be necessary.It is also worth remembering that image resolution is effectively degraded at very small apertures. This effect is more apparent at infrared wavelengths because its magnitude is proportional to the ratio of absolute aperture diameter to the mean wavelength of the image-forming light.


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