Alan -

First of all, thanks for the lucid explanation. If I may, let me ask
one, make that two, more questions to make sure I've got this
right...

1 - With TTL, If I'm photographing a bride in a white dress, and the
dress takes up a major proportion of the frame, I should increase
flash compensation to insure the dress looks white in the final print?

2 - If I place a diffuser on the flash, flash compensation is not
necessary because TTL will compensate (as long as I'm not pushing the
outer limit of the flash range)?

Dave

On Sat, 26 Jun 2004 11:11:13 -0400, Alan Browne
wrote:

Dave Yuhas wrote:

Hi -

If the camera (Maxxum 7) is set to f/8 in A-mode, does setting the
flash (5600 HS) compensation dial to +1 change the flash output to
f/5.6 or f/11? (My guess is the former). Flash is in TTL-mode.

Neither. When shooting TTL flash you are asking the TTL flash
meter to decide when to shut off the flash. By setting +1 you
are telling the meter to flash-expose 1 stop more than if the
flash were reflecting off of an 18% grey target.

If 18% grey, aperture f/8 and flash comp +1, then the flash
output would be as if for f/11.

If the scene were 18% grey, and your aperture were f/8, 0 flash
comp then the flash output would be f/8.

Without compensation, if the target was black, the flash would
tend to run about 2 stops too long; if white, about 2 stops too
short. Flash comp allows the photog to compensate for these
scene differences. It is of course harder when the scene is
composed of mixed elements.

[Note that when manually setting a flash using a flash incident
meter, you ignore the reflectivity of the scene and set the
flashes according to the aperture desired, with the lights, at,
below or above that setting according to the desired effect.]


Why does the flash range (as shown on the flash's display panel)
decrease when setting flash compensation to +1? If the flash is
putting out more light, shouldn't the range increase rather than
decrease? To put it another way, if the flash output is changed to
f/5.6, dividing the GN by 5.6 results in a bigger number (distance)
than does dividing by 8.

The max range displayed on the flash is based on the GN of the
flash and the selected aperture. the min range on the ability of
the flash to stop flashing when signalled to do so by the
TTL-flash meter.

The camera assumes that you are increasing/decreasing flash comp
in order to correct for the meter misreading the returned light.
The assumption is that the target is 18% grey.

If you are increasing the flash comp, it is because you believe
the scene is so light as to fool the flash-meter into
underexposure. So if you need to increase the compensation v.
the meter, the effective range of the flash is LESS for the
current scene, not more.

Remember that TTL-flash works on the reflectivity of the scene
during exposure to decide whan to stop the flash. This is
opposite to how you set studio lights with an incident meter.

Hope the above makes sense and helps.
Cheers,
Alan