I am assuming you are working with reflected light, i.e the lighting is on
the subject, not shining through it. A couple of thoughts come to mind.
One is to filter the modelling light to daylight balance. This would allow
you to leave it on 1000f the time and still get reasonably accurate color
reproduction. Another is to boost your flash output with either a stronger
macro flash (twin t-28's) or a seperate, off camera, flash, say a t-20.
Have you tried a wider aperture to verify the t-10 has enough power to
properly expose at the bellows extensions you're using? Or perhaps a higher
speed film (you don't mention what you are using) I get the feeling you are
right at the limit. At full extension, with the kind of magnifications you
are getting, it takes a lot of light.
The formula for manually calculating the bellows extension factor is:
(bellows extension)^2
compensation factor = ------------------------------
(focal length)^2
So with the 20mm lens, and the bellows at max extension of 231mm, you need a
compensation factor for the bellows extension of 133, or about 7 stops, if I
have my math right. With a guide number of 10 (meters) with ISO 100 film,
and the lens to subject distance of 10mm, or 0.01m, then f/1000 would be
needed, excluding the bellows extension. Adding 7 stops to f/1000 gets you
very close to f/8, again if I've done my math right.
So, a few things to try. Set the T-power control to manual and run a series
of full power, manual exposures through the whole range of f/stops in full
stops with the OM-2 set to 1/30 sec on manual. Another is to take aluminium
foil and loosely wrap it around the flash and subject area. This will
effectively form a light tent, allowing any "stray" light from the flash to
bounce around and add to the exposure instead of being lost to fall off.
John P
______________________________________
there is no "never" - just long periods of "not yet".
there is no "always" - just long periods of "so far"
Richard Dale <Richard_Dale@xxxxxxxxxxxxx> wrote:
>I'm using the VST stand with bellows, zuiko 20/38 macro lenses and T10
>flash for macro work. The stand has a tungsten modelling bulb which I have
>left on during the exposure, sometimes, depending on the length of the
>exposure and perhaps proximity of the lens to the subject, the pictures are
>tinted with yellow tungsten light.
>
>If I switch off the modelling light sometimes the OM2n decides to expose
>for an inordinate length of time, obviously after the short flash emission
>there is only ambient light to collect. I think this happened when the
>bellows were fully extended. The TTL flash system has presumably decided
>that there is not enough light for the exposure even with the flash.
>
>Surely all thats needed, even with bellows fully extended, is an exposure
>time of say 1/30 to 1/2 sec or perhaps 1 sec at the most. Lenses are
>normally set at F8. The lens to subject distance is measured in
>millimeters. My current course does require me to record the exposure
>details, including the shutter speed, though I may give a guestimate (they
>can't dispute it).
>
>Could there be a problem with the camera, one answer may be to obtain
>daylight bulbs for the VST and leave the modelling light on, can you get
>them in the US?
>
>Is there any way of using the flash manually then bracketing, adjusting for
>light loss through the bellows.
>
>Richard
>
>
>
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