Hi David,
> Within the Z-1p I believe the TTL flash sensor is centre-weighted.
This is the case with all Pentax bodies with TTL flash.
> So if you're trying to flash a small object close to the lens, where
> the background is comparatively much further away, the flash ends up
> overpowering the subject to try and achieve an acceptable average over
> both foreground and background, within the coverage of the sensor.
This is a common problem with center-weighted metering, not just with TTL
flash with a center-weighted sensor. Common solutions are (I am sure not
new to you) matrix metering and manualy setting an exposure compensation
value.
> I figure that a better way of doing this would be to control the flash by
> making the lens communicate its focus distance back to the body, and the
> flash communicate its GN. The body can then figure out exactly how much
> flash power to apply for a correct exposure at that distance, as a fraction
> of what the flash can actually provide. For consistent results the flash
> power would have to be controlled by the flash itself, perhaps using a
> sensor behind the tube used in a similar fashion to the old auto-flash
> sensors (I wouldn't want a totally open-loop system).
What you just described is "modern" auto-flash operation. The only
difference is that by "body" you mean "camera body," and by "body" I mean
"the photographer's body."
In this setup you completely lose the advantages of TTL flash: correct
metering with filters, tilting and swiveling flash, etc.
> Has anyone actually implemented such a system? I'd totally love it for my
> macro work, and any portraiture where the subject doesn't totally dominate
> the frame.
I guess that what you really trying to describe is a TTL flash with a
matrix meter controlling the flash burst. Of course, for such a thing one
needs to compose the photo, then fire the flash once without opening the
shutter, determine the matrix of constants to use for each metering
segment, and then change the shot (flash once again and shutter also).
There are two problems that I see with this, and both are bad but not
enough to spoil the idea completely. First, your subject needs to sit
still between the pre-flash and the actual flash. Second, you need a very
powerful flash in order to be able to fire it twice within a very short
time.
One simplification can be done, however: use a TTL-flash matrix meter and
couple it to the active AF point. Then you will at least have the subject
that is in focus exposed the same as 18% gray. In this case there will be
no need for a pre-flash.
Just my "random" thoughts... Cheers,
Boz
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