On 3 Dec 98 at 6:19, William Sommerwerck wrote:
> > I was considering getting ahold of a Vivitar 285 flash, mainly because it
> > seems a good choice for fill. Can I use the flash at less than full power
> > (1/2, 1/4 and so on) while in auto mode, and the flash will cut off power
> > at the fraction I choose? Or is variable power only available in manual?
>
> Variable-power flash is normally used only in manual mode. However...
>
> You can achieve the same effect in auto mode by changing the aperture
> setting on the flash WITHOUT changing the aperture on the camera. For
> example...
>
> Suppose you've set the camera and the flash to f/8 for auto-flash
> exposure. If you then move the aperture selector on the flash to f/5.6
> (but leave the camera at f/8), you will get half the autoflash exposure.
> The camera will automatically change its shutter speed, leaving the
> ambient exposure unchanged.
>
> Note that the sensor in the flash will add the ambient light to the
> flash. You might need to experiment to see which settings give the
> effects you want.
>
While this is basically correct, a few warnings are in order I think, based on
my experiences:
This won't work with any body/flash combo which lights the "ready" light in the
viewfinder. When this indicator is lit, it means the camera is setting 1/60
regardless of actual light measured OTF. (At least I think this is true for all
OMs using the flash ready light - please correct me if not).
In this case, you could end up with a 1 stop underexposure using the above
method, if the ambient light is quite low (ie: measured exposure is much less
than 1/60).
Personally, I have not encountered the above, since I use my QA310 mostly.
The other problem, which I have seen often enough to annoy me is this: If you
decide to provide a 1 or 2 stop under fill, by adjusting the flash to say f/5.6
or f/4 respectively, while leaving the camera at f/8 on auto, there is a risk
of ghost images when shooting handheld and/or with moving subjects. For
example, with the camera at f/8 on auto, and a flash combo *without* the ready
light coupling, the indicated shutter speed may be 1/15s. A -2 stop fill flash
will make very little difference to the recorded OTF exposure of the camera,
and it may very well expose for *longer* than 1/15 second, since the viewfinder
reading is only a guide, not the actual exposure. The result: A sharp outline
of your subject, surrounding by a ghost image created during the ambient light
exposure.
In theory shooting with normal auto flash and aperture matched with the camera
should result in close to 1/60 exposure, and -1 stop should be around 1/30,
etc., but I suspect this is often not the case. The OTF system finetunes the
exposure, closing the 2nd curtain when it sees enough light, and that may not
agree with the auto flash sensor.
========================
Shawn Wright
Computer Systems Manager
Shawnigan Lake School
250-743-6240
swright@xxxxxxxxx
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