Excellent explaination Dean! I don't see how it could be made any easier to
understand than that.
Darin
----- Original Message -----
From: "Dean Hansen" <hanse112@xxxxxxxxxx>
To: <olympus@xxxxxxxxxx>
Sent: Sunday, August 06, 2006 4:31 PM
Subject: [OM] picture a cone
> 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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