An interesting correction to my own post ... (clipped a bit) >> Does a deep red filter (I've used both a Nikon R60 and Tiffen >> #25) throw off the N70 AF sensor due to a similar chromatic >> bias? > I am not convinced that this is always true in all situations. > There is plenty of evidence that the AF system will work with the > extreme red filters often used for IR films (with an autofocus > assistance light). But most film is not responsive to these > wavelengths. If this is the case (and not due to another effect), > then in some unusual situations you might get a spectrally induced > bias. BUT this will not be due to your filtering (using normal > photographic filters), but rather to an unusual scene spectral > distribution such as molten steel in which case I would be > bracketing in any case. Ilford warns (in their product data guide 1999) for their DELTA 3200 film that in camera exposure meters may get the exposure wrong by up to 1.5 stops underexposure if you use a deep red or orange filter! Looking at the spectral responsivity data for this film it is fairly uniform with wavelength and responds from ~400nm to ~700nm. Whereas "ordinary" FP4+ only responds from ~400nm to ~650nm and the response at ~600nm is nearly twice that at ~400nm. Andy Shaw
