Extension Tubes

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As shown, tubes are hollow, without elements and fit between lens and body. Since there are no additional optics, they maintain the full quality of your lens. Note, focusing distance decreases, and exposure changes when using extension. (See included chart)

Canon EF Extension Tube:

A normal (non macro) lens reaches life size reproduction on film, or sensor plane when corrosponding millimeter of tubes are used as millimeter of lens:
 A 50mm lens + 50mm of extension = 1:1 
 100mm lens + 100mm of extension = 1:1
 Accordingly:

50mm lens + 100mm of extension = 2:1 or double life size.       150mm lens + 50mm of extension = 1:3 or one third life size.

Asahi Pentax Extension Tube Set:

While manual tubes were once desirable due to price, in recent times after-market brands such as ProOptic Auto Tubes are inexpensive, and better suited for most needs. 

An alternative to stackable tubes are Borg M42 Helical Focusers.  These offer a screw-in/out design which allows various tube lengths with just a twist, and available in a couple of sizes, but are only manual.

50mm macro lens at life size. 

Magnification increase with  tubes:

.

Magnification, and exposure compensation amounts with added tubes for 50 & 70mm Lenses. *NOTE: Exposure compensation not needed when using TTL Metering (through the lens) since metering is done at sensor or film plane.

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