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Best lens focal length for extension tubes?
Posted By: Zephos, 02-02-2015, 10:15 PM

In the past I used an old 18-55 mm with extension tubes, but do I get more magnification with longer focal lengths? What's the best in your experience?
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02-02-2015, 10:59 PM   #2
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I haven't done too much Macro in the last couple of years, but I think from memory you get higher magnification with a shorter focal length. Though your working distance is much better on a longer focal length. - Could be confusing this with reversal rings though.

Someone else with a bit more experience might correct me on this.
02-02-2015, 11:25 PM - 2 Likes   #3
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Chaos_Realm is correct. More specifically (vis-a-vis amount of extension):

-- Adding 100 mm extension to a 100mm FL lens (set to infinity) gives a magnification of 1.0, while only 50mm extension is need with a 50 mm FL lens to get magnification of 1.0.

-- If you add 1/2 the FL in extension you get magnification of 1/2 (0.5) and adding twice the FL in extension you get magnification of 2.0. And so forth.
02-03-2015, 05:56 AM   #4
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Macro on a tight budget:

extensionrings and a smc M 28 og M 50
More magnification: Reversal ring for Pentax. (49mm for for M 28 and M 50)

You can use the reversal ring with or without the extension rings.

02-03-2015, 07:10 AM   #5
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there is also the option of lens stacking, reverse the smaller lens and mount it to the larger one using a double filter ring.
02-03-2015, 07:11 AM   #6
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General Rules of Thumb on Macro (naturally there are exceptions):
  1. The closer the lens to the subject the more magnification - shorter focal lengths focus closer to the subject
  2. The longer the extension the closer you can focus and the greater the magnification - Extension equal to the focal length of the lens = 1x (1:1) magnification.
  3. Reversing the lens allows closer focusing - Lenses are normally designed so the distance from the subject to the lens is greater than the distance from the lens to the image sensor or film plane. Reversing the lens reverses this relationship allowing you to focus closer as well as improving image quality at magnifications greater than 1x.

The Diagram below from the Pentax Auto Bellows-A manual illustrates #3 above. Figure 1 shows the typical distance relationships. Figure 2 shows the relationship at 1x magnification and why the lens need not be reversed. Figure 3 shows the lens reversed and how the distance relationship is also reversed.

02-03-2015, 07:19 AM   #7
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Another rule of thumb... to get proper macro, make sure that the focal length of the lens is smaller than the focal length of the extension tube (or the total of extension tubes) you are using.
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