----- Original Message -----
From: Ayash Kanto Mukherjee
Subject: Twin flash for Macro: Some thoughts
Hi Ayash:
You had best use both flash units in manual. As you know, in
auto, they will quench very quickly when used close to the
subject. Don't worry about point source or whatever, as that
isn't germaine to light measurement anyway.
Here is what I would do:
If you can keep the flash to subject distance constant from
setup to setup, then the calculations become simpler.
I would use the 285 as my main light, as it is the most powerful
of the two.
First, calculate the aperture needed, either via the guide
number or the calculator dial, based only on the flash to
subject distance. Next, calculate the number of stops you need
to open up the aperture.
You can do it the hard way, based on the amount of lens
extension.
The formula for that is :
Extension factor = (extension from infinity focus)squared,
divided bt (focal length)squared.
If you are using a macro lens, the extension factor is often
engraved on the lens barrel.
Or, you can do it the easy way, using the TTL meter of your
camera. Meter a gray card at infinity, then meter it again at
the lens extension that will be used. The difference is the
number of stops you lose to lens extension.
If you are using the second flash as a backlight, then you can
ignore it for the base exposure, but you will need to calculate
it's effect on the image, again using the guide number, or
exposure scale. Figure you want it a stop or so less bright than
the main light.
If you are using the second flash as a fill, then you will want
to calculate the distance from the subject you want it to be, to
give an appropriate fill ratio. You will find that having the
fill somewhat less bright than the main will give the most
pleasing effect.
Be sure to make careful notes, especially making careful
measurements of the flash to subject distance, as that is the
most important measurement for calculating the aperture. You
will find that you probably won't have to do a lot of testing
until you get it right, providing you follow a fairly scientific
approach.
William Robb
> Hallo all!
>
> I have two flash units. I want to use both the flashes
simultaneously for
> frontal lighting, sidelighting and back lighting macro shots
of flowers
> etc. I have a synchronization cord, eye and cord extensions (2
m) to fire
> the flashes simultaneously. The problem is automodes of the
flash won't
> help me while doing macro macrophotography because the
f/stop-film
> speed-distance chart at the back of the flash is made on the
assumption
> that the source is a point source (Am I right?). When the
flash is near (1
> m) to the subject, it is no more a point source but an
extended source.
> The only way that I
> can control the light is by adjusting the separation between
the subject
> (flower) and the flash or adjusting the output levels of the
flash.
> Has anybody got some experience in using twin non-TTL flash
for macro? I
> can not understand what aperture shall I set. Do I have to
calculate
> Bruhlich factor and prepare the nomograms? If yes, it becomes
tedious.
>
> The other thing that is not clear to me, is using the non-TTL
flash as a
> fill flash for macro shots. Is it possible to use the
automodes here?
> Or shall I adjust the output level? If the second is true,
what aperture
> shall I choose for a particular distance (1-1.5 m) for so and
so guide
> number.
>
> I use Vivitar 2800 (G.N. 22 m) and Vivitar 285HV (G.N. 36 m at
Normal
> setting of the zoom head).
>
> Thanks for reading.
>
> With regards,
> Ayash.
-
This message is from the Pentax-Discuss Mail List. To unsubscribe,
go to http://www.pdml.net and follow the directions. Don't forget to
visit the Pentax Users' Gallery at http://pug.komkon.org .
