Tim Randles just posted:
I'm a bit confused about the extension tube thing, I sorta understand
the extension tube changes the focal length, so how does that allow you
to take close up images like this with a 50MM.
I gave a talk once on insect macrophotography, and preparing for it
made some aspects of magnification much clearer to me. Hope this helps:
Imagine a 50mm f1.8 lens, with an object out in front of it. The
lens takes the light rays coming from this object and refracts them as
they pass through the lens and then sends these light rays out of the
rear of the lens, basically in the form of an expanding cone. 42 mm (I
think) behind the rear mount of the lens is the OM film plane, right?
If the lens if focused on the object, the image of the object is also in
focus 42 mm behind the rear mount of the lens. Now, mentally remove the
camera. What one sees (or imagines) is that the cone of light emerging
from the rear of the lens keeps expanding as it gets farther from the
lens. And the image in this cone of light also keeps expanding the
farther one gets from the rear of the lens.
You can easily illustrate this by putting the 50/1.8 lens in the
front standard of a bellows, removing the flexible acordian part and the
rear standard of the bellows, and mounting just this front standard/lens
set-up on a tripod. Put a slide of a butterfly wing (that's what I
used) in a projector, set the projector up a couple feet in front of the
50/1.8, and project that butterfly wing image through the 50/1.8. Now
put a white paper or, better, a 2' square piece of white foam board
about 3" behind the rear of the lens, after you've marked out a 24 X 36
mm rectangle (this is the size of a single 35mm negative or slide) on
the surface of the foam board. Focus the image on the foam board and
note what portion of the image the 24 X 36 mm rectangle covers . Now
move the foam board 6" behind the rear of the lens, focus again, and
again note what the 24 X 36 mm rectangle covers. Do this at 12", 18",
24", and it's pretty clear to see how one achieves magnification using
extension. It's simple: while the cone emerging from the rear of the
lens gets larger and larger as it leaves the lens, the size of the film,
24 X 36 mm in our case, remains the same, so one is taking a smaller and
smaller portion of the image the farther the film is from the rear of
the lens. That is, you're getting magnification.
What happens to the light that comes through the lens but doesn't
hit the 24 X 36 mm rectangle? It's eaten by the bellows--it's lost--it
makes no contribution to forming the image. Which means, of course, that
at higher magnification, less and less of the light coming through the
lens hits the negative, so one either has to shoot wide open, use a long
exposure, use faster film, or, best yet, use flash to get an exposure.
I never understood the Inverse Square Law until I did this set-up.
Then it became very clear. But I can't put it simply into words right
now, sorry.
I don't think in terms of extension changing the focal length; I
think of extension as moving the 24 X 36 mm rectangle away from the rear
of the lens to where it covers a smaller and smaller proportion of the
cone of light coming from the rear of the lens.
Hope this helps.
Dean
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