BetterPhoto Q&A
Category: New Answers

What is a Teleconverter?

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August 23, 2002

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August 24, 2002

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August 25, 2002

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October 04, 2002

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October 08, 2002

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October 11, 2002

Best results with extension tubes are almost always when used on a prime lens (fixed focal length) instead of a zoom. Reason? Nearly all primes are simpler lens designs and focusing is done by moving the entire lens cell using a helical. The extension tube adds to the distance the lens focus helical can move the lens away from the film plane. Zooms are much more complex and nearly all of them are "internal" focusing performed by moving elements inside the lens. An extension tube can be used with a zoom, but getting a specific magnification and focusing is trickier, often with more adjustment of camera position, focal length (zoom) and focusing.

I now use extension tubes exclusively for macro photographs. The reason for several tube lengths and having them is ability to cover a wide range of magnification. Indeed, I have two different sets with three tubes each. The one with shorter tubes works best with a 50mm and 85mm lens. The one with the longer tubes works best with a 135mm and 200mm lens. I choose focal length based on the "standoff" distance desired from lens front to subject. The amount of extension for a given magnification depends on focal length. To achieve life-size (on film) requires total extension equal to focal length. Thus, a 100mm lens allows twice as much standoff distance for the same magnification, but it also requires twice as much extension too. I don't recommend using more than three tubes stacked together. For this reason I may not choose a longer lens such as the 200mm for very high magnification, but a shorter one such as the 85mm or perhaps the 135mm and sort out how to work with less standoff distance. Sometimes equipment limitations require tradeoffs between focal length and total length of extension used. My two favorites for standard macros in the field (outdoors) are the 85mm and 135mm lenses. If I also had a 100mm I would likely be using it too. Work with your tubes for a while with different lenses and you will get a "feel" for which lens and which tubes to pull out of the bag for a particular task (subject size, standoff desired and magnification desired).

-- John

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December 01, 2002

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December 02, 2002

Tonight I'll provide a little more on optics with its associated math (it's not that difficult) and a couple examples showing how I plan a macro shot based on desired magnification. Understanding the rudiments of focal length, lens extension and magnification helped me do macro work much more efficiently. I could quit guessing at focal lengths, tube lengths and having to move the tripod numerous times to get what I wanted.

-- John

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December 02, 2002

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 (non-macro 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 mid-point 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

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December 02, 2002

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December 03, 2002

Stephanie

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December 03, 2002

BTW, I do most of my indoor macros with a pair of monolights and a flash meter now. As you well know, generic monolights cannot be TTL controlled by a camera body. That has forced doing the rigorous math again to determine more accurate magnification and resulting light loss figures so I can compensate for it. Worth the pain as I now have much more exacting control over the lighting quality and direction.

Stephanie:
If you use flash for a macro made using extension tubes, or a hand-held meter instead of the TTL metering in your camera body, there is a light reduction related to the amount of magnification that must be compensated for. The farther the lens is extended (for greater magnification), the more the light loss. It's one of the reasons the focus helical on non-macro prime lenses only extends the lens to about 1/7th of the focal length. The light loss to that point is negligible. If the flash is TTL controlled by the camera body, no problem, the loss will be compensated for automatically by the amount of light passing through the lens. If it's not (as with monolights and metering with a hand-held flash meter), then exposure compensation is required. The equations for this using magnification are:

To compensate by adjusting lens aperture:
Compensation Factor = 2*log(1 + M)/log(2)
This gives the number of stops (or EV) by which exposure must be increased. I use this one most often and adjust lens aperture. When using flash that overwhelms ambient light levels, the effective shutter speed is more flash duration than it is the X-sync speed. Lens aperture can also be set between the f-stop markings, and partial f-stop settings can be estimated on most lenses to about 1/3 of an f-stop.

To compensate by adjusting shutter speed:
Exposure Factor = (1 + M)^2
M = magnification
Use this one by multiplying your shutter speed by the Exposure Factor. Possibly useful when metering ambient light using something other than TTL metering in the camera itself (which automatically compensates for the light loss), such as a hand-held light meter.

-- John

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December 04, 2002

Stephanie

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December 05, 2002