Eric, I think you misunderstand the table (or perhaps the table words it inelegantly). When the table says "1 focal length extension tube" you are thinking the same focal length as the lens (50mm lens = 50mm extension tube). However, what it *means* is the amount of extension needed to go from 1:2 to 1:1 (effectively doubling the focal length/magnification). In the case of the Tamron 90mm f2.5 the extension tube is 45mm long (not 90mm long). Therefore the light loss is 1 stop (and the table is correct).
On Fri, Jul 13, 2012 at 5:01 PM, Eric Featherstone <eric.featherst...@gmail.com> wrote: > On 13 July 2012 22:40, Darren Addy <pixelsmi...@gmail.com> wrote: >> On Fri, Jul 13, 2012 at 2:42 PM, John Francis <jo...@panix.com> wrote: >>> The laws of physics suggest you are mistaken here; if you spread the >>> light energy out over a larger area it's not going to be as bright, >>> no matter how that spreading is done. You'll lose a little bit of >>> the light with an optical TC - nothing is 100% transparent - but to >>> a first approximation the two ways of getting to 1:1 are equivalent. >> >> Thanks again to John for forcing me to exercise my Google Fu on this subject. >> Found a great paper entitled "Supermacro Photography and Illuminence". >> The part we need for this discussion is the table here: >> http://www.antiqueauto.org/assets/LightLossWithVariousMacroMethods.png > > The table is wrong (for ext tubes), John is right. See, for example: > http://www.shutterbug.com/content/photographic-super-course-macro-photograqphy-page-2 > "...This means that, when the 50mm lens is set at f/8 and attached to > a 50mm extension tube, the effective f-stop of the combination if > f/16—two stops smaller than f/8." > > -- > Eric > > -- > PDML Pentax-Discuss Mail List > PDML@pdml.net > http://pdml.net/mailman/listinfo/pdml_pdml.net > to UNSUBSCRIBE from the PDML, please visit the link directly above and follow > the directions. -- PDML Pentax-Discuss Mail List PDML@pdml.net http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
