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why don't extension tubes increase focal length (links?)

Started Feb 26, 2007 | Discussions thread
bclaff Forum Pro • Posts: 13,929
Re: Thin Lens Formula, Correction, Worked example

fishguy1 wrote:

bclaff wrote:

fishguy1 wrote:

I guess I have too much time on my hands as I did go out and test my Nikkor 500mm f/4 P lens and my Tamron 150-600mm adapted for Sony E mount, and zoomed to 600mm for all tests. Here are the results:

At 10 meters (working distance of target to lens exterior):

Nikkor = 468mm
Tamron = 467mm

At 6 meters:

Nikkor = 437mm
Tamron = 432mm

At 4.24 meters (minimum focus distance for the Nikkor):

Nikkor = 424mm
Tamron = 383mm

At 2 meters (MFD for the Tamron):

Tamron = 365mm

I also checked the Tamron for focus breathing. I didn't notice any, so if it's there, it's minor. That is unlike the Nikkor with noticeable focus breathing.

What has this to do with extension tubes ?

I used extension tubes to measure focal length. Close enough to the original topic...which is 15 yrs old.

OK. I was confused. I though perhaps those were horizontal field of view; didn't realize they were focal lengths.

Those numbers seem believable.

I see the Tamron SP 150-600mm F/5-6.3 Di VC USD in the PhotonsToPhotos Optical Bench.
There it's 583mm 4.15 degrees at infinity and 325mm 3.74 degrees at MFD (2.3m)
SO that's a small amount of focus breathing.

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Bill ( Your trusted source for independent sensor data at PhotonsToPhotos )

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