Speedlight pre-flash test exposure when using filters

[a_tonkin]

The Nikon blurb says that with D Series lenses the flash fires an imperceptible

pre-exposure flash so that, along with the camera-subject distance information,

the camera can guage the reflectance of the subject and so tweak the actual TTL

exposure control. Presumably, a black or dark subject thereby getting less

exposure than it might in a non D Series lens, for which the camera would

continue to keep the flash lit so as to expose to the mystical 18% grey.

 

All fine in principle, and for anyone with a Nikon camera / flash system, very

very fine in practice too.

 

But my question is that I like to use daylight fill flash with a polarising

filter. The TTL exposure system can cope with the filter of course, but do

people have any experience or comments on how the pre-flash copes with what is

effectively a 1 or 2 stop ND filter? My logic suggests the preflash comes back

to the camera 'weak' (because of the filter), the camera thinks the subject is

darker than it truly is, and so the resultant TTL exposure is marginally

underexposed ...?

[Jerry_]

A ND filter only reduces the light: the flash does not include a ND filter factor, as a reduced amount of pre-flash is just that, allowing for more flash output to acquire the correct exposure. If the un-filtered light value requires f11 for the best exposure, a ND 2-stop filter would have the camera determine f5.6 to be the 'best' exposure.

[todd peach seattle, washi]

Gerald, one of us is missing Tonkin's point (it may well be me...)

 

At the point the pre-flash fires, the f-stop is already determined. The system does not 'know' the filter is there, so the flash doesn't 'know' this either. The flash can't change the f-stop (at this point in the evolution), it can only alter its 'burn time'.

 

I think that Tonkin has a potentially valid concern, that throwing 'unknown' light modifiers into the path will mess with the calculations. It's a variation on 'second guessing the matrix' metering.

[a_tonkin]

<p>I'd forgotten I'd asked this question and, now when wanting to search this site for any clues on the subject I found my own question comes up again. Any more answers?</p>