Forgot Password
Pentax Camera Forums Home

Show Printable Version Search this Thread
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?
Views: 1,215
02-02-2015, 10:59 PM   #2
Site Supporter
Site Supporter
Chaos_Realm's Avatar

Join Date: Feb 2011
Location: Melbourne, Australia
Photos: Gallery | Albums
Posts: 1,251
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
Site Supporter
Site Supporter

Join Date: Aug 2011
Location: New York, NY
Photos: Gallery
Posts: 1,738
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
Senior Member
Runegutt123's Avatar

Join Date: Apr 2011
Location: LODINGEN, Northern Norway
Photos: Gallery | Albums
Posts: 274
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
Loyal Site Supporter
Loyal Site Supporter
UncleVanya's Avatar

Join Date: Jul 2014
Photos: Gallery | Albums
Posts: 13,799
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
Not a Number's Avatar

Join Date: Mar 2012
Location: Venice, CA
Posts: 5,320
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
Veteran Member
mrNewt's Avatar

Join Date: Nov 2011
Location: ON, RH
Posts: 2,170
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.

  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
extension, length for extension, lens focal length
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Similar Threads
Thread Thread Starter Forum Replies Last Post
Best lens focal length(s) for Albuquerque Hot Air Balloon Festival? GlennG Pentax SLR Lens Discussion 4 05-18-2014 07:30 PM
Best Focal Length for macro work Redundant Banker Pentax SLR Lens Discussion 41 08-07-2013 01:29 PM
What is the Best Focal Length for a Portrait Lens? Drom Pentax SLR Lens Discussion 94 04-22-2012 02:00 PM
what is focal length when using extension tubes. katanaismybike Photographic Technique 9 07-17-2010 01:44 PM
Best fixed focal length lens for architectural photography on a K10D nanter Pentax SLR Lens Discussion 29 10-25-2008 09:21 PM

All times are GMT -7. The time now is 05:30 AM. | See also:, part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]