Re: Extension Tubes and M135/3.5
On Wed, 23 May 2001, Bill D. Casselberry wrote: Your 100mm macro most likely has ~50mm of extra (macro) extension built into it's barrel to give the more common 1:2 capability or ~100mm if it is a 1:1 capable one. The old 100mm BellowsTakumar had no helicoid at all - it was all done via the bellows. Ah, that makes sense. So my 100/4 macro (1:2) will go to 1:1 with 50mm of extension tubes, while the same tube will only take a regular 100mm lens to 1:2 (roughly). Thanks! chris - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org .
RE: Extension Tubes and M135/3.5
Chris Brogden writes: On Wed, 23 May 2001, Bill D. Casselberry wrote: 1:2 1:1 are easy calculations -- 1:1 is achieved w/ extension tubing ~= to the focal length Does the close-focusing ability of the lens affect this? That is, would I get the same magnification using a 50mm extension tube on a 100mm macro as I would on a regular 100mm? If not, then differences in close-focusing ability must affect the magnification ratio. As I understand it, yes and no. If the lens doesn't use any internal ofcusing elements, then the lens helicoid is mainly just extension, and then yes, it merely adds to the extension by tubes. If the lens uses has internal focusing elements, then its helicoid also affects the internal elements, which typically do their thing by effectively changing the focal length of the lens, and in that that case, it has an effect, but it isn't as easily figured since both extension and focal length are simultaneously changing. However, in practice, if you are trying to achieve an exact magnification on film, then just include a reference object, such as a ruler. That eliminates the need to control exactly how large the resultant print or projected slide is. For everything else, which is mostly what I do, knowing how to approximate what extension is needed is sufficient. In fact, I always set up my camera with enough extension to allow the lens to not be fully extended. That allows me to tweak the focus using the lens's focusing ring. hope that helps, patbob ([EMAIL PROTECTED] [EMAIL PROTECTED]) - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org .
Re: Extension Tubes and M135/3.5
Thanks for everyones replies, i think i'll aim for pentax ones, but if another brand comes up at a reasnoble price then i'll grab them. Thanks - Original Message - From: Provencher, Paul M. [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Wednesday, May 23, 2001 10:18 AM Subject: RE: Extension Tubes and M135/3.5 There are quality differences, mainly with how well the mount is affixed to the tube. If you are not going to use them frequently, this will not be a big issue. I have noticed that cheaper ones use cardboard as light baffles inside as opposed to metal or plastic. Again, not a big deal if you don't plan to use them a lot, or in harsh environments. Not being familiar with K-Mount tubes, I can't say what options are available in terms of meter and aperture coupling but you would probably want at a minimum to get meter and aperture coupling if possible - this makes everything easier and preserves TTL metering. Paul M. Provencher (ppro) -Original Message- From: Peifer, William [OCDUS] [mailto:[EMAIL PROTECTED]] Sent: Wednesday, May 23, 2001 12:48 PM To: '[EMAIL PROTECTED]' Subject: RE: Extension Tubes and M135/3.5 Paul Jones wrote: I am looking at buying some k-mount extension tubes. I am wondering if there is any difference in the performance of different brands? As if there like the screw mount ones i have seen then there is no glass in the tube. Are there any other cons of extension tubes? Also what sort of size entension tube would need to get an M135/3.5 down to say 1:2 or 1:1? if its possible. Hi Paul, You're right -- just like the screw-mount tubes, the K-mount extension tubes are just metal spacers with no optic elements inside. I recently bought a used set of three Vivitar K-mount auto extension tubes (the kind with the aperture linkage to allow open aperture metering) in like-new condition, and I paid $54 plus shipping. The equivalent Pentax tubes would have cost considerably more. I'm quite satisfied with the performance of the Vivitar set. As far as the length of extension necessary to get a particular magnification, you can use the simple formulas for thin lenses to get an approximation. The ratio of image size to object size is equal to the ratio of image distance to object distance. Thus, for 1:1 magnification, the distance from the center of a hypothetical thin lens to your focal plane will be equal to the distance from that same lens to your object. Also, the sum of the reciprocals of these two distances will equal the reciprocal of the focal length of this hypothetical thin lens. Thus, you'd need a lens-to-film distance of 270 mm, and a lens-to-object distance of 270 mm, to get 1:1 magnification with a thin 135 mm lens. Your SMC-M 135/3.5 is by no means thin -- it's a complex group of several lenses rather than a single thin lens. However, you can get a good approximation by focusing the lens to infinity, then measuring 135 from the focal plane to some location on the lens barrel. Assume that this point on the lens barrel is the location of a single thin 135 mm lens element, then do any further calculations by measuring from that point. Follow me so far? Hope this is reasonably clear, and that it helps a little. Bill Peifer Rochester, NY - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org . - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org . - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org .
Re: Extension Tubes and M135/3.5
Chris, on extension tubes, wrote: Does the close-focusing ability of the lens affect this? That is, would I get the same magnification using a 50mm extension tube on a 100mm macro as I would on a regular 100mm? If not, then differences in close-focusing ability must affect the magnification ratio. Yes, even w/ regular lenses, there is intrinsic extension when close-focusing. Notice that the barrel extends as you focus closer and infinity is no extension. Besides their specialized flat-field optics, macro lenses incorporate a built-in helicoid extension tube of sorts. There are moving group optical tricks on some of the more modern ones, as well. But these schemes alter the effective focal length, as well. The lens' own extensibility is why I used the approximately, any precise magnification calculations would combine lens and tube extension measurements. In most non-scientific imaging it is not necessary to target some precise magnification factor. Creating a consistent series of images of various sized objects would be the sort of thing that would benefit from calculations as opposed to trial error. Your 100mm macro most likely has ~50mm of extra (macro) extension built into it's barrel to give the more common 1:2 capability or ~100mm if it is a 1:1 capable one. The old 100mm BellowsTakumar had no helicoid at all - it was all done via the bellows. Bill - Bill D. Casselberry ; Photography on the Oregon Coast http://www.orednet.org/~bcasselb [EMAIL PROTECTED] - - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org .
