To use a 150mm lens on a MF camera to avoid getting on top of the image that is being photographed, what size extention tubes should be considered?

Donald, with your 21 mm extension that puts you out 18 inches out from the object with a 150 mm lens, am I correct in assuming that your focus would be set at infinity? And that the area of coverage would be about 14 sq. in. (therefore, assuming a square format, the image size would be approximatly 3 3/4 in. wide amd 3 3/4 in. high?

hortense, It depends on the close focus distance of your lens. You can use these two formulas: 1/f = 1/rd+1/od, and magn = f/rd-f; where f is focal length, rd is rear distance (to film plane) and od is the distance to the object. I'll use my own example. I have a (Bronica) 150mm lens that focuses to 1.5 meters (1:9 magnification). I can get the rear distance by rearranging the first formula as: rd = 1/(1/f-1/od), which with f = 150mm and od = 1500mm gives me 167mm rear distance. Now, if I want to determine the magnification an 18mm extension tube will get me, I can just add 18 to my rear distance (which is 150mm at infinity and 167mm at the closest distance) and calculate the magnification using the second formula. This gives me about 1:8.3 at infinity and 1:4.3 at the closest focus setting. Note that you can also recalculate the focus distances by manipulating the first equation.

The second equation should actually read magn = f/(rd-f) . I forgot the parentheses. sorry for any misunderstanding.

Well, a lot depends on which system you're using because THAT will determine what size tubes are available. For the 6X6 Bronica SQ series, there are only two, an 18mm and a 36mm. I don't know what's available for the Hassey or others. I bought both of the available tubes for the Bronica. The 18mm gets the most use.