What you've discovered is the inverse square law. When you do bellows or
extension tube work, your exposure has to increase when you extend your
bellows or use of tubes. This is not in a linear fashion but increases
proportionately with the distance. If (and this is the important if) your
flash is attached to the front standard of the bellows or the filter ring of
your lens, whenever you extend your bellows the amount of light the flash
puts on the subject will increase also. But again it will not increase in a
linear amount depending on the extension but in proportion to the square of
the distance.
Both of these cancel out, as you have found out through experimentation.
Congratulations for empirically finding this out.
Warren
=Date: Thu, 26 Apr 2001 00:37:05 -0500
=From: Gary J Sibio <[EMAIL PROTECTED]>
=Subject: Re: Teleconvertor suggestion
=
=At 11:57 AM 4/25/01 +0300, you wrote:
=
=> Certainly not at life-size, when added extension inflicts
=> corresponding light loss. This stands true even with modern macro lenses
=> design, that still lose 1/2-1 stop of light despite IF design.
=
=Even at life size. I remember when the device first appeared that was one
=of the selling points. When I use it with a tiny manual flash, I have to
=close down the aperture more at 1:1 compared to 1:4 because the flash is
=closer to the subject. I even took some pics with it this morning just to
=see if I was wrong. There was no change in exposure as I moved in closer.
=
=I'm not a physicist so this is speculation but perhaps it has something to
=do with the fact that there are optics involved, not just extension. OTOH,
=I know that the effective aperature on a macro lens also changes as you
=move closer in. I don't know why there's no light loss but this has been my
=experience in the 15 years I've used the thing.
=
=
=Pax et bonum,
=Gary J. Sibio, SFO
-
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