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RE: [OM] OM: Extension tubes and focusing distance

Subject: RE: [OM] OM: Extension tubes and focusing distance
From: "Ron Spolarich" <caesar2@xxxxxxxxxxxx>
Date: Sun, 21 Nov 1999 09:39:00 -0500
Goin' to school! <g>  Thanks for the great explanation Chuck. Though I've
not had the desire to shoot at such close ranges, I've often wondered how
extension tubes work.

Seeing you're from the same neck of the woods as I, though I'm an Ohio
transplant, (I live in Methuen, work in Needham)and there's another guy in
the list whose name escapes me ("Expose" yourself again to me!)who lives
nearby, we should get together sometime, if schedule permits. Ron

Ron & Mary Spolarich
caesar2@xxxxxxxxxxxx
www.mscollectibles.com

-----Original Message-----
From: owner-olympus@xxxxxxxxxxxxxxx
[mailto:owner-olympus@xxxxxxxxxxxxxxx]On Behalf Of Chuck Norcutt
Sent: Sunday, November 21, 1999 7:13 AM
To: siddim01@xxxxxxxxxxxxxxx; Olympus mail list
Subject: [OM] OM: Extension tubes and focusing distance


Acer asks:
"With ext. tubes, one can't focus to the distance?"

Right.  Watch your lens as you focus from infinity to closest focus.  At
closest focus the lens is fully extended in its helicoid.  At infinity
it's pulled all the way back in and is as near to the film plane as it
will go.

The total focus travel of a Zuiko 50mm f/1.8 is roughly 7.5 mm. Now
consider sticking a 7mm extension tube between the body and the lens.
Despite the fact that the focusing ring might be indicating infinity,
what you've just done is push the lens away from the body by a distance
roughly equal to the closest focusing position of the lens without the
extension tube.  Using the focusing ring now you can focus even closer
but you can't focus any farther.

According to the tables in Shipman's book, putting a 7mm tube on the
50mm f/1.8 gives you a closest focus of 7.9 inches or 20 cm.  It also
limits you to a farthest focus of 15.4 inches or 39.1 cm.  Note that the
now furthest focus of 39.1 cm. is not far from the (nominal) 45 cm.
closest focusing distance of the 50mm f/1.8 without ext. tube.  That's
because the 7mm extension of the tube is nearly the same as the full
extension of the lens helicoid.  If the lens helicoid extended exactly
7mm then, adding the 7mm tube would make the farthest focus with the
tube exactly equal to the closest focus without the tube.

When any lens is focused at infinity the optical center of the lens is
exactly the focal length away from the film plane.  As you move in
closer than infinity the lens has to move away from the film plane to a
distance greater than its focal length.  Finally, when the object is
brought so close to the lens that it's only the focal length away you
can't focus at all since the film plane would have to be an infinite
distance away.

Chuck Norcutt
Woburn, Massachusetts, USA

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