> Is chromatic metering bias on bodies without RGB
> meters, like the N70, a relatively constant factor?
Modern sensors are fairly linear -- I've never had any
problems using colored filters with black & white.
> 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?
That's not a problem with the passive AF systems that
most SLR's (all Nikon SLRs) use. The camera focuses on
the same range of wavelengths that expose the film.
An active AF system (like on most rangefinders) might
have problems if the lens isn't well corrected for
chromatic aberration.
> A couple of times while shooting my daughter backlit,
> using an 85mm f1.8 wide open with a red filter, the AF
> sensor focused on the plane of her hair--specifically,
> the halo of backlit hairs--instead of on her face . . . .
> The AF sensor was probably picking up on the wealth of
> edge definition in the pattern of hairs . . . .
Right -- that's where the highest contrast was.
The AF system needs a lot of light to work with a dark red
filter. Adding a 3x filter factor to your f1.8 lens leaves
you with the same amount of light as an f5 aperture -- only
1/3 stop from the AF system's recommended limit of f5.6.
That's slow enough to make AF marginal in dim light.
> Does any top-end AF body allow you to dial in a focus
> compensation factor so you can shoot stuff like HIE
> using AF?
I don't think so.
If you're using a red filter and IR film, you can't just
focus on the IR index anyways. You have to keep everything
from visible red to IR in focus to get a sharp image, so you
have to stop down and set your focus so that the visible and
IR marks are both within your depth of field. It requires a
different amount of focus shift and a different aperture for
every different lens and focusing distance.
If you use an IR only filter, a passive AF system will focus
the IR image automagically.
-Don
