Wow. Thank you for the information Mark, it helps a lot. I am definitely going to get myself of extension tubes now.
Thanks again for the help and the enablement, Derek > 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 > > - - - - - - - - - - - - - - - - - - - - > Mark Cassino Photography > Kalamazoo, MI > www.markcassino.com > - - - - - - - - - - - - - - - - - - - - > ----- Original Message ----- > From: "Village Idiot" <[EMAIL PROTECTED]> > To: <pentax-discuss@pdml.net> > 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 > > > > > W
