Two questions re extension tube

Have questions about the equipment used for macro- or micro- photography? Post those questions in this forum.

Moderators: Pau, rjlittlefield, ChrisR, Chris S.

bs0604
Posts: 40
Joined: Tue Feb 19, 2013 8:09 am
Location: Sarasota FL

Two questions re extension tube

Post by bs0604 »

I built an extension tube setup with adapters and tubes following fairly closely the recommendations elsewhere in this forum. I am using a APS-C camera and 10x mitutoyo infinity focused objective.
1) the extension set up is about 200 mm long mimicing what I saw in the forum. Since the microscope objective is infinity focused all the light rays coming through it will be parallel. It would seem to me that an identical result on to the sensor would be achieved whether the extension tube is 200 mm long or 50 mm long. The shorter version would be less subjective to sagging, bending and vibrations. ??
2) In the set up is the rayox lens reverse mounted. What is the purpose of the raynox lens? How would the image differ without it?

JKT
Posts: 222
Joined: Fri Oct 28, 2011 9:29 am
Location: Finland
Contact:

Post by JKT »

The purpose of the Raynox is to form the image to the sensor. Without that you don't get a picture ... or at least not a good one as the Mitutoyou would be forced to act as finite objective.

The distance between Mitutoyo and Raynox can vary, but the distance from Raynox (from the nodal point of Raynox to be exact) to the sensor should be equal to focal length of the Raynox. This depends on which Raynox you have.

ChrisR
Site Admin
Posts: 8586
Joined: Sat Mar 14, 2009 3:58 am
Location: Near London, UK

Post by ChrisR »

The Raynox is replacing the lens in a microscope which would be called a "Tube" lens, so that's the name it's usually given in the extension tubes arrangement.
A more general and more descriptive term is "converging lens", but it's not used a lot.

Remember that at "infinite" objective is still just a lens, it'll focus at any distance a "finite" lens would. It's just designed for the object and image to be in the specified places. Using other focus positions just means the lens isn't being used at its best, which in some cases is dramatically less well.

Reversing a Raynox has been found to be a bit better, which makes sense if you think about the distances in front and behind for which it's designed. (Called Conjugatre Distances.) Most of the improvement is away from the center.

Sure you can use 50mm of tubes, with a 50mm tube/converging/Raynox lens, but then the magnification and coverage on sensor, will be 1/4 of what they would be with a 200mm lens.
The Raynox 150 is 208mm, by the way. To get the correct amount of extension tubes, get it focused on a very distant object. Being that precise is difficult with tubes, but get within a few mm of the "infinite" extension or you may impact the results. That's more critical with higher magnification/higher NA objectives.
Chris R

bs0604
Posts: 40
Joined: Tue Feb 19, 2013 8:09 am
Location: Sarasota FL

Post by bs0604 »

I have the Raynox 150. So I should have 208 mm from the sensor to the nodal point of the rayox? Where is the nodal point on the Raynox?

mjkzz
Posts: 1237
Joined: Wed Jul 01, 2015 3:38 pm
Location: California/Shenzhen
Contact:

Post by mjkzz »

you can try to focus to infinity, ie, distant object, (without the objective), like Chris said, no need to find the nodal point.

ChrisR
Site Admin
Posts: 8586
Joined: Sat Mar 14, 2009 3:58 am
Location: Near London, UK

Post by ChrisR »

It's slightly "telephoto", so you can't measure 208mm anywhere useful!
Chris R

Post Reply Previous topicNext topic