Response to IR focusing formula

Different lenses are corrected for different wavelengths. APO lenses are corrected for extra points along the wavelength to minimize aberration over wider ranges of wavelength. So it's hard to generalize here. If your lens came from a decent manufacturer, you might even be able to find technical data for your particular lens. Otherwise, I would make a test exposure with the lens focused at 5m target with visible light, and other targets sitting at various known distances. When I adjust rangefinder cameras or screen position of SLRs, I can use ground glass at the film plane but I usually make sure the focus is accurate using film with a procedure like this. Once you find where the lens was focused at with infrared light, you can formulate a correction factor to add to the bellows movement.

[ Previous | Next ]