I didn't follow your math too closely, but I will give you the correct formulae for figuring the extension factor as applied to LF cameras. First, compute the image magnification (M) by dividing the lens to subject distance into the lens to film distance, measuring from the lens aperture position (I use the front of the aperture ring). The bellows (exposure factor) can then be calculated with the following formula: (M+1)�= the exposure factor.
You will find that you need to do the math once and keep a chart of exposure factors with the lens/ extension tube combinations you have on hand that you use. In the field, I carry a small collapsible ruler and a calculator for figuring bellows extension. William Robb ----- Original Message ----- From: "Brent Hutto" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Tuesday, November 27, 2001 7:10 PM Subject: Extension Tubes for Closeup Photography > 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 . - 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 .
