Infrared and backfocus
Backfocus (focusing behind the intended plane of focus) can lead to the invalid conclusion that infrared images are inherently “soft”. In fact, infrared images can be just as sharp as visible light images. Backfocus occurs not just when shooting in infrared, but also in visible light when a lens and/or camera is not performing to specifications.
Photographers whose work demands precision focus often vent their grievances in online message boards. With today’s high resolution digital sensors, a 10mm focus error at close range is a gross error (especially when photographing nose hair), blurring a 12-megapixel camera into a 1-megapixel equivalent. An impartial observer might reasonably conclude that backfocus is also the source of many unfounded claims of lenses being unsharp.
I’ve been hard at work on diglloyd GDI (Diglloyd Guide to Digital Infrared), spending an entire day today researching the backfocus issue with numerous lenses, with more work planned for tomorrow. The insights that I’ve painstakingly gleaned (and not just from today) will all be incorporated into DGDI, along with a wealth of other material which is more extensive and useful than anything I’ve yet come across (including all the books on infrared I’ve seen).
Backfocus in infrared (top) and near-optimal focus (bottom)
Canon 135m f/2L @ f/2, focus point on “Black” at left
