Steve Kompier
(K=4629) - Comment Date 9/2/2002
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Russel...
Extension tubes are used to "extend" you lens from the camera body, thus allowing you to do macro shots. The further the lens is from the camera body, the greater the magnification.
But..everything has a price. You lose light getting to the film so you have to compensate, but the cameras built in meter does it for you, so no biggy.
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Russell Love
(K=7006) - Comment Date 9/2/2002
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Steve,
Thanks for the info!
Russ
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Adam E. J. Squier
(K=9803) - Comment Date 9/3/2002
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Here's the way I think about extension tubes. Take a regular, everyday, lens -- like a 50mm 1.8. Focus on infinity. Then slowly turn the focusing rail and watch the lens grow (or "extend") until it reaches the closest focusing distance.
Now, if you want to focus closer than that, what should the lens do? Get further away from the film. And how do you do that? With extension tubes! Or a bellows unit, or a cardboard tube and duct tape. ;-)
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Paul Auen
(K=186) - Comment Date 9/4/2002
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Cardboard tube and duct tape? Now that's a high tech!
Is there a way to figure exposure lose from an extension tube? Or can I still use the regualr aperture setting for flash pictures? My camera is completely manual with built in meter.
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David Goldfarb
(K=7611) - Comment Date 9/4/2002
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To figure out the exposure factor manually, there are a few choices. You can do it based on focal length and total lens extension or you can do it based on magnification. Any book on macro photography will have the formulas and tables for determining exposure factor by different methods.
I usually determine the magnification by comparing a ruler at the focal plane to the width of the frame, so if I'm using a 35mm camera, I know that the frame is 24x36mm, so if the ruler measures 36mm in the viewfinder along the long end, the magnification is 1:1 (width of the field of view:width of the frame on film), and exposure factor is two stops. I have a table of factors in 1/3 stop increments and magnification to look these up quickly, rather than computing the factor the formula each time. On my large format cameras, where one needs these factors fairly often (an ordinary headshot might be 1:2 or 1:3!), I tape the table to the back of the camera.
There is also a simple device available for free online called the "QuickDisk" designed for large format, but you could scale it down to work with smaller formats:
http://www.salzgeber.at/disc/index.html
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Richard Ries
(K=614) - Comment Date 9/6/2002
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If the metering is through the lens, you just go with the meter. (I once had an ancient Pentax where the meter would sit on top of the pentaprism housing!)
Probably the low-tech way of testing light-loss would be to meter a nice, smooth, evenly-lit area without the extension, and then see what you get with the extension in place.
Also, if you were really serious about the cardboard tubes, see if you can get some smaller tin cans - they'll last longer. Just watch out for the duct tape - it'll leave gunk all over the camera!
--Rich
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