SLR lenses
SLR lenses as a general rule are of better quality than the built in lenses in point and shoot cameras. They resolve more detail, can have a faster aperture, faster focus, less fringing, less distortion, etc and are interchangeable.
Because lenses are designed to be used in normal, visible light all of the coatings, glass elements and configuration are all optimized for visible light. When used in infrared light all bets are off. A lens that might be a top performer in visible light might fair very poorly in IR, the opposite it quite common as well – a cheap kit lens may turn out to produce excellent results in infrared light.
Keep reading to see some of the issues that can arise when normal lenses are used for infrared photography.
IR focusing
Near IR light has longer waves than visible light, so it focuses at a different point. Infrared photography requires a shift to achieve good focus, so it’s necessary to use IR focus marks or have the focus calibrated, which is explained in this chapter.
Infrared focus marks on zoom lenses. As can be seen it's not just one mark, instead a range which tracks with focal length.
With point and shoot cameras – which suffer from the same focus shifting problems – even if there were infrared focus marks (which there aren’t), the lens often can’t be manually controlled by the user (the focus is internal). Point and shoot cameras have to be calibrated so that they can focus using infrared light properly.
I know what you may be thinking: but P&S cameras focus through the imaging sensor kind of like the new “Live View” feature in DSLRs! That’s true, they do focus with the imaging sensor but the technique used is very different. The technique employed is something called predictive focus and is a faster method of focusing through the imaging sensor. If the calibration is off, although the camera thinks the subject is in focus – it won’t be in the captured image. Most DSLRs with a Live View focusing ability use a slower but much more accurate direct focusing technique – this is great news for IR photographers, as we explain earlier in this chapter.
