I just got my Pentax Extension Tube Set B from KEH and have been playing around with them on my 50mm f/2 SMC-A and 135mm f/3.5 SMC-M lenses. I can get to around 1/2 life size with the 135mm lens and have over a foot of working room between the front of the lens and the subject. I can get 1:1 life size by stacking the 26mm and 19mm tubes on the 50mm lens. That gives me 3 inches between front element and subject which is about the minimum I'm comfortable with without worrying about nicking the lens. Of course, I can put the 12mm tube on there too and to about 1.2:1 as my absolute maximum magnification.
These tubes don't couple the aperture information into the camera. In fact, the camera can't stop them down so you just adjust the aperture ring like a purely manual setup. I want to use my (manual) flash bouncing off a white board to light my closeup subject. Is it pretty accurate to figure the effect f-stop as follows: Original focal length: 50mm Original f-ratio: 2.0 Original aperture: 50 / 2.0 = 25mm Focal length with tubes: 50 + 25 + 19 = 94mm F-ratio with tubes: 94 / 25 = f/3.8 and just round off to f/4 for figuring the amount of light I need? - 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 .
