Tube lens / Variable Focus Assembly

[rjlittlefield]

I am using these "tube lenses" with 20mm separation in case of the DCR's and 30mm separation in case of the X6 lens.

Would you recommend adding extra separation?Normally I use an Iris, would you recomend the Iris to be closer to the tube lens or to the microscope objective?

For example, I feel I am geeting poor corner performance with the DCR-125 and nikon CFI 10/0.25 , could this improve with extra separation?

Yes, I think more separation could help. I did a quick test just now looking at a distant target through the 11 mm aperture with various separations. On APS-C, corner quality was pretty bad with minimum separation of two adapter rings (49-52 and 52-42, with the aperture bored in a 42mm body cap). It got much better with 28 mm added, improved a little more with 56, then degraded just a hair at 84. Here are actual-pixel crops:

Normally I use an Iris, would you recomend the Iris to be closer to the tube lens or to the microscope objective?

I would put it as close as possible to the objective. If you put it back at the tube lens, you'll be using off-center parts of the objective away from center of the image. Combine that with added separation and you might even get vignetting.

Also, is it possible for a tube lens to perform well with some objective and not so well with others?

Yes, that seems possible. Aberrations can either add or subtract. They could add with one pairing but cancel with another.

--Rik

[seta666]

Thank you Rik for your answers; one think I still can not visualice is the set up you have used for the test. EDIT: Now I think I understand it, you just add an iris in front of the tube lens and take a picture of something, right? I will try it myself then

You say that that 11.2mm aperture would be like using and effective f18 combo, like the mitu's 5X and 10X. Thas it mean that the effective apperture fixes those aberrations the tube lens may have on its own? So, what happens whe you use a 10/0.25 at 5X (f10)? You need a better corrected tube lens to start with, right?

One more thing, have you used the Raynox in normal position or reversed? in my first test it showed better performance in normal possition but this advantage could change with added spacing, right?

Regards

[rjlittlefield]

Thank you Rik for your answers; one think I still can not visualice is the set up you have used for the test. EDIT: Now I think I understand it, you just add an iris in front of the tube lens and take a picture of something, right? I will try it myself then

Right. Since you have an adjustable iris, the easy solution is to set the iris at 11 mm and add extension tubes between the Raynox and the iris. In my case I wanted something thin and I also got tired of bumping the iris control, so I just bored out a body cap.

You say that that 11.2mm aperture would be like using and effective f18 combo, like the mitu's 5X and 10X. Thas it mean that the effective apperture fixes those aberrations the tube lens may have on its own? So, what happens whe you use a 10/0.25 at 5X (f10)? You need a better corrected tube lens to start with, right?

Yes, that's a good summary.

One more thing, have you used the Raynox in normal position or reversed?

Normal position, with the Raynox's male threads toward the camera.

in my first test it showed better performance in normal possition but this advantage could change with added spacing, right?

Yes, and that's a very important thing to have realized.

The Raynox's are odd beasts. Simple theory predicts that they should work best reversed, but that's not what the tests show. Why is that??

The answer is aperture position. In normal use as closeup lenses, the Raynox's play nicely with a telephoto lens whose entrance pupil is several cm in back of the Raynox's male threads. Reversing the Raynox but moving the aperture close to the Raynox messes up that relationship.

I did test also with the Raynox 150 reversed and found that it worked work well that way IF the relative aperture positioning was preserved by putting the aperture several cm in front of the Raynox's male threads. However, that made a long setup whose performance (in brief testing) did not seem to justify the reduced convenience. For full frame it could also introduce vignetting issues, which I did not test.

--Rik

[seta666]

So many thing learn about tube lenses behavior

Well, this afternoon I did some quick test both with the 5D mkII and the NEX-5N

These Tube lenses I have used and the extension to the iris, those are minimum extension then +27mm and +41mm tubes; differences in minimum extensions are due to the way they are adapted:

DCR-250 (125mm FL) Min 12mm Med 39mm Max 53mm
Raynox 6X (169mm FL) Min 18mm Med 45mm Max 59mm
Morfanon (172mm FL) Min 10mm Med 37mm Max 51mm
DCR-150 (208mm FL) Min 12mm Med 39mm Max 53mm

Do not pay to much atention to the exposure because is messed up

First Top right corner crops on the 5D mkII, chart 6m away from the camera



Full size

http://farm9.staticflickr.com/8380/8635 ... abe9_o.jpg

The raynox seem to perform nice with +27mm or +41mm extension, depending on the lens.
I am surprissed by the poor quality of the morfanon. It has given me pretty good results in the past.

Then the NEX-5N crops, chart 10m away from camera



Full size
http://farm9.staticflickr.com/8241/8635 ... e81d_o.jpg



I would say DCR-250 is best with +27mm tube, DCR-150 with +41mm tube and the 6X either with +27 or +41 looks almost the same. Morfanon is more even but with bad quallity, maybe with +27mm tube

Regards
Javier

[seta666]

in my first test it showed better performance in normal possition but this advantage could change with added spacing, right?

Yes, and that's a very important thing to have realized.

The Raynox's are odd beasts. Simple theory predicts that they should work best reversed, but that's not what the tests show. Why is that??

The answer is aperture position. In normal use as closeup lenses, the Raynox's play nicely with a telephoto lens whose entrance pupil is several cm in back of the Raynox's male threads. Reversing the Raynox but moving the aperture close to the Raynox messes up that relationship.

I did test also with the Raynox 150 reversed and found that it worked work well that way IF the relative aperture positioning was preserved by putting the aperture several cm in front of the Raynox's male threads. However, that made a long setup whose performance (in brief testing) did not seem to justify the reduced convenience. For full frame it could also introduce vignetting issues, which I did not test.

--Rik

I can confirm that at least my Raynox DCR-250 works way better reversed; I can work at 3X with mitutoyo 5/0.14 and at 6X with Mitutoyo 10/0.28. It can be pushed up and down with good results

In normal position it gives me bad corner quality even with 27mm extension tube, I have used it reversed with that 27mm extension tube with good results

I am runing some stacks but I do not think I will be able to post them today

Regards