I use extension tubes all the time - a few basics:
1. A lens's focal length is that distance at which that lens will focus at infinity. Multiple element lenses complicate this a bit, but in the most simple terms a 100mm lens, for example, will focus at infinity 100mm from the film plane. That's why it is called a 100mm lens. :-)
2. As you move the lens further from this point, you move the plane of focus closer. A lot of lenses focus by just moving the a bit further out from the film. This moves the lens off infinity focus, and lets you focus closer.
3. The point at which the extension of the lens equals the lens's focal length results in a magnification of 1:1 - i.e. the image on the film is the same size as the object itself. So - with a theoretically simple 100mm, that is focused at infinity when it is 100mm from the film, if you add 100mm of extension, the lens will now focus very close, and the image on the film will be the same size as the object photographed.
4. When you focus closer, you magnify the image. Understanding the optical effects of magnification is they key to understanding macro and close up photography. Everything else is just derivative from the impact of magnification.
5. Every lens has aberrations and distortions. When you add extension and magnify the image, those aberrations and distortions are also magnified. In theory, macro lenses have been designed to overcome these problems. I have yet to see a situation where a non-macro lens matched a macro lens in performance at high magnification (greater than 1:1). That said, some lenses - like the Pentax 50mm f1.7 M, A, F, or FA, (all the same optical formula) do work very well with extension.
6. There's no problem using tubes with Digital SLRs. I use Vivitar tubes that have the full contacts, and with A or later lenses I loose only autofocus. I also use tubes and bellows for snowcrystal photos - lots of tubes. In that case I rely on the TTL flash to control the exposure. Pentax does not make tubes with contacts needed for auto exposure / aperture control, but third parties do. The thing with tubes - you can buy just plain tubes with no contacts and no mechanical connector to stop down the lens, you can get them with just the mechanical connectors to control the aperture but no electronic contacts, or you can get them with both mechanical and electronic contacts. The latter will result in an A or better lens working fine with a digital SLR, except that you will have to manually focus.
7. A less noted use of tubes is to allow you focus more closely with telephotos, where the minimum focusing distance may not be close enough.
I'd see buying a set of extension tubes as a good first step into macro / close up photography. You can always use them later, even if you buy a dedicated macro lens. But a _good_ macro is worlds better than a regular lens on tubes, especially as you move away from the center of the image. I have used an M 200 f4 on tubes, and an A* 200 f4 macro without tubes. Guess what? The $1000 macro outperforms the $75 regular lens! Similarly, a $200 100mm macro will outperform a zoom or even a regular 100mm lens on tubes.
If you have only zoom lenses, I'd recommend getting close up filters for macro work - but there again, good ones are not cheap.
HTH -
- MCC
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Mark Cassino Photography
Kalamazoo, MI
www.markcassino.com
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----- Original Message ----- From: "Village Idiot" <[EMAIL PROTECTED]>
To: <[email protected]>
Sent: Tuesday, April 05, 2005 1:39 PM
Subject: Macro Extension Tubes
I have questions about macro extension tubes. Does anyone use extension tubes? On what on what lenses do you usually use them? What is the difference between using extension tubes and using a macro lens? Also, why do extension tubes not work on DSLRs?
I have always been curious about extension tubes and was thinking of purchasing some (maybe on eekbay). I thought I might try them out on flowers and bugs.
Thanks in advance for your help,
Derek
