Tube length question

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Jmurphy18
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Tube length question

Post by Jmurphy18 »

I have a question for you more knowledgeable folks as I seem to have a case of the dumb dumbs here. Tube lens for an Infinity lens, I want to go up to 10x from my present fixed 4x. I just purchased a Raynox 150 that I want to use as my tube lens. Reading the material out there I need 208.33 mm as a length for the total tube and the Nikon flange length is 46.5mm. This leaves me 161.83mm for actual tube length. I have 112mm for my fixed lens so that means I need 49.83mm more. Does this ~ 50mm include the Raynox tube lens or is that in addition to the lens? Basically does the 208mm calculation include the tube lens length in the figure?
Thanks much
JohnM

JKT
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Re: Tube length question

Post by JKT »

It is measured from the optical center of the Raynox, which naturally is not marked anywhere.

What I've measured is 202.8 mm from sensor to rear flange of the Raynox. I don't claim the last digit is correct, but the result should be in the ball park.

rjlittlefield
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Re: Tube length question

Post by rjlittlefield »

The key thing is that the extension should make the Raynox focus at infinity by itself, with no objective in front.

This will be accomplished, give or take a few mm, when the physical center of the Raynox DCR-150 lens is 208 mm from the sensor.

Strictly speaking, infinity focus will occur when the "rear principal plane" of the lens is exactly one focal length away from the sensor. 208 mm is a good approximation to the actual focal length, and "physical center of the lens" is a rough approximation to the location of the rear principal plane. See viewtopic.php?p=258032#p258032 and the surrounding thread for a lot more discussion, if you're a glutton for punishment.

With only a 10X objective, the exact extension is not critical. You can change it by many 10's of millimeters to adjust the exact magnification, and the image quality will still remain good. The biggest problem, especially with inexpensive objectives, is that if you push the magnification down too far, by using a short extension, then the corners of the image will go bad because then the objective does not have a large enough field to cover the sensor.

--Rik

Jmurphy18
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Re: Tube length question

Post by Jmurphy18 »

JTK & Rik: Thank you both very much for taking the time and effort for answering met question. Rick, I did review the link you said and I will do again so I can understand better. Thanks so much for doing so.
JohnM

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