Stephanie, First thing I try to estimate is the desired magnification. Magnification is size of object depicted on *film* compared to actual size of the object, and not its size in a print or when projected as a slide.Approximate dimensions of a 35mm film frame is 1 x 1.5 inches. I measure the subject size and typically add a little to that, then compare that to how I want to fit it into the film frame. If I want to fit something that's 2 x 3 inches into a 35mm film frame, then the highest magnification I can use is 1:2, or 0.5X. Next step is to approximate the standoff I will have with a particular focal length. The equation for this is: S = f + (f / M), or S = f * [(M + 1) / M] S = distance from subject to front lens node f = lens focal length M = magnification The front lens node is *not* the physical front of the lens. Be cautious when working at very high magnifications. The location of a front lens node can actually be inside the lens. This is only an approximation and the actual distance from subject to the lens filter ring is often somewhat less. If I want 1:2 magnification, the standoff will be slightly less than 3X the lens focal length. At 1:1.5, or 2/3 magnification (subject size of 1.5 x 2.25 inches), it will be slightly less than 2.5X lens focal length. Next is how much extension tube is required. Many textbooks will show distance from rear lens node to film plane "v" with the equation: v = f * (M + 1) This works OK with finding total bellows length for view cameras where the lens board approximates the rear lens node, and it can often be collapsed very nearly to the film plane. However, it requires modification for practical use of extension tubes with 35mm format and most medium format cameras. The rear lens node is already one focal length from the film plane when it's focused at infinity. For these cameras, the additional extension required (by extension tubes), then subtracts one focal length from total extension by using: x = f * M x = additional extension required from infinity focus. In practical use, the lens focusing helical is used to fine tune exact focus and it's desirable to add something less than the additional extension required from infinity focus. How much less? Most prime lenses focus rings (nonmacro type) will extend them approximately 1/7th of their focal length. Thus, a 90mm lens focus ring will extend it about 13mm at closest focus without any tubes. A 135mm lens focus ring will extend it about 19mm. This provides a workable range for the additional tube length required. To facilitate focusing, tube lenght added should be somewhere near the middle of this range so that critical focus can be acheived with the lens focus ring near its midpoint of the extension it can provide. At 1:2, or 1/2 magnification, additional extension required from infinity focus for a 90mm lens is 45mm. Subtract off what the lens focusing helical can provide and tube length added should be in the range of 32mm to 45mm with greatest lens focusing flexibility at about 39mm of extension tube. A 135mm lens would require about 67mm of tube setting the lens at infinity focus, would have a tube length range of 48mm to 67mm, and greatest focusing flexibility using the lens focus helical with about 57mm of extension tube. At 1:1.5, or 2/3, magnification additional extension from infinity focus for a 90mm lens is 60mm. Workable tube length range would be 47mm to 60mm and optimal for focusing using the lens focus helical would be 54mm. For a 135mm lens, additional extension from infinity focus is 90mm with a workable tube length range of 71mm to 90mm and an optimal tube length of about 80mm focusing using the lens focus helical. Choose the combination of tube lengths to use that puts you closest to the middle of workable tube length range. Hope this helps you out in planning your macros. Do the math in detail for a while. With some experience doing that, you'll gain a very good feel for how much standoff you'll have and how much tube length you'll need for a particular lens by simply estimating the magnification you want.  John
12/2/2002 11:02:52 PM
