EXTENSION TUBES

[HarryMac]

I have some old Nikkor, manual focus AIS lenses that I use regularly on my X-Pro 2 with the NK-FX adapters. I'm thinking of getting an extension tube for the 50MM f/1.4 to try some macro work. My questions:

 

1. Does the adapter itself affect the images that would be created? 

2. Is there any way to calculate the size in millimeters of a tube that would bring the image at, or as close as possible, to a 1:1 ratio?  

 

Thank you; any help will be appreciated.

[Doug Pardee]

1. Does the adapter itself affect the images that would be created?

 

It increases the focal length of the lens/adapter system by the length of the extension tube. This increases the magnification, which is kind-of important if you want to get to 1:1 magnification.

 

2. Is there any way to calculate the size in millimeters of a tube that would bring the image at, or as close as possible, to a 1:1 ratio?

 

With the lens focused at infinity, an extension tube of length equal to the focal length of the lens does the job. In the case of your 50mm lens, a 50mm extension tube increases the focal length to 100mm, and changes the focus distance of the lens to 100mm when set to infinity (note: measured from the optical center of the lens, not the focus plane marker engraved on the camera -- that would be 200mm). This produces 1:1 magnification.

 

Shorter extension tubes can be used for 1:1 by focusing the lens closer than infinity and moving the camera/lens closer to the subject. How short you can go will depend on how close the lens can focus and what its focal length is when close-focused. Virtually all affordable lenses exhibit focus-breathing, where the focal length varies depending on the focus distance -- the marked focal length is at infinity focus.

Edited February 22, 2018 by Doug Pardee

[HarryMac]

Thank you, Doug. I appreciate it.

Harry