Thyristor doesn't see the same as the digital sensor? Pure balderdash
as far as I'm concerned. The digital sensor is seeing exactly the same
light that film would see and, when set on the proper ISO, responds
exactly the same. If the thyristor unit works properly with film it
will also work properly with digital.
Since you've done all this before with film you are most likely doing
things correctly but, just to be sure, there are a couple of things that
need to be verified.
1) Are the camera and flash both set to the same ISO?
2) I know you set the shutter speed but is the camera in full manual
mode and not shutter priority automatic mode?
3) Does the aperture on the camera match that set on the flash?
4) Does the angle of coverage of the flash match the lens focal length?
I don't have specs for the 2500 but the angle of coverage of the
Vivitar 2800 is only for 42mm in 35mm format and is probably optimistic
at that.
5) For underexposed shots: Is the area where you're trying to take the
shot unusually bright, eg lots of white or light colored walls?
Remember that the flash's auto exposure system is looking for medium
gray just like the camera would. If too bright the flash will quench
prematurely. Put the camera on a tripod in center-weighted averaging
mode at the same aperture and do an auto exposure. Does the camera get
it right? But watch out for the differences between what the flash
sensor sees and what the camera sees. The Vivitar 2800 has two
measuring angles; 10 degrees and 30 degrees. On the E-1 these would
equate to approximately what's seen by a 120mm lens and a 40mm lens.
What does the camera see at these focal lengths? Is it significantly
different background brightness than what's seen at the taking focal length?
6) For overexposed close-up shots: Are you sure you're not inside the
miniumum distance for the auto mode selected? As before, I don't have
specs for the 2500 but the 2800 may be similar. It has two auto modes.
One has a minimum distance of 3 feet at ISO 100 and the other has a
minimum distance of 6 feet. If you're too close to the subject the
electronics of the flash aren't fast enough to react to shut the flash
off and you'll get an overexposure. This is one area where the T-32 has
it over lesser flash units. It has speedy electronics and a relatively
short minimum distance.
Here's another diagnostic for you. Try manual mode and see if the flash
measures up. Likely findings: The guide number is very optimistic
especially when using NiCad or NiMH batteries. The guide number is
based on fresh alkaline batteries at 1.55 volts (and, in addition, is
probably overstated by at least 1/2 stop). Furthermore, although the
ready light might come on in 7-10 seconds, even a T-32 requires at least
20 seconds recharge time to reach maximum output. NiCad's and NiMH's
only have 1.2 volts when fully charged and won't deliver the full pop.
NiMH's also self-discharge very quickly. If you're using NiMH's that
haven't been fully charged within a couple of days you're not going the
max either.
This is an interesting problem. Let us know how it works out.
Chuck Norcutt
Peter Klein wrote:
> Is there some issue with old "auto-thyristor" flashes and digital cameras
> that would lead to inconsistent exposure? Somebody told me that when you
> combine auto-thyristor electronic flash with a digital sensor, the digital
> sensor may not "see" what the thyristor does. Anyone know more about this?
>
> I was fooling around with my E-1 and a generic automatic (non-TTL) flash--a
> Vivitar 2500. I hadn't used the flash for a while, so I used the E-1 to
> test it prior to shooting candids at a wedding with B&W film and an OT
> rangefinder camera. I found that the flash overexposed close up and
> underexposed *drastically* at medium to far distances (supposedly within
> its auto ranges). Almost as if the sensor wasn't working. I got the same
> general results with direct flash, bounce flash, and a Lumiquest "Pocket
> Bouncer." The little "exposure OK" light on the flash lit, but the camera
> did not see things that way.
>
> I tried this with an even older flash and got similar results. (Yes, the
> flashes' trigger voltages were within the E-1's tolerances, and I shot at
> 1/60--a little below the E-1's maximum speed for flash synch).
>
> The Vivitar 2500 worked fine when used with film--a little variation from
> near to far, but well within film latitude.
>
> What gives?
>
> --Peter
>
>
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