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UltravioletPhotography

Omegaron No Leak Helicoid Mounting


JCDowdy

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Like Andrea, I have been experimenting with various ways to mount an enlarger lens that has an illuminated aperture window and light guides that leak light onto the sensor. The Rodenstock Omegaron 75mm f/4.5 lacks both front filter threads and a rotatable plate to cover the light guides as are found on newer EL-Nikkors. I have described some of the ways I thought such a lens could be mounted in other threads but this is the one I will most likely use. I like this setup because it involves no drilling and tapping, no glue, no sticky tape, wont mar the body of the lens and most importantly does not leak light as far as I can tell.

 

Here are the parts

post-24-0-10681500-1423792838.jpg

 

From left to right:

  • A standard 39mm to 42mm thread adapter. I recommend the ones with the slots which are handy when things get stuck. Remember to face them away from the most expensive part (lens).
  • The Omegaron enlarger lens, you can see the green and red light guides that pipe light to the aperture window so you can see to adjust an enlarger in the dark.
  • A 7mm short section of M42 extension tube, a 9mm is also a common short section and might also work for this lens.
  • A 1.25" Filter Adapter for T-Thread Capture, which allows a thin plate to be trapped inside a male female T-thread junction. The plate has a threaded inner 1.25" hole where a standard 1.25" telescope eyepiece filter can be mounted. The outside diameter of the filter adapter is 1.61" (just under 41mm) and is unthreaded, it drops into a female T-thread component and retained by the male threads screwed in trapping it. It also fits a standard M42x1mm junction just fine. These are also sold on eBay.
  • A 1.25" telescope eyepiece filter of your choosing. This one is actually empty because I remounted it into a shorter ring so it would clear my filter drawer, but that is a posting for another day.

Here is how it goes together

post-24-0-44482800-1423794458.jpg

  • The left panel shows the lens mounted on the extension tube without the 1.25" filter adapter so you can see the light guides.
  • The center panel shows the 1.25" filter adapter captured inside the extension tube by the 39mm to 42mm thread adapter ring.
  • The right panel shows the lens screwed into the 39mm adapter ring with the light guides now completely covered by the 1.25" filter adapter.

Filter mounting

post-24-0-68616500-1423795404.jpg

  • The left panel shows how a reversed 1.25" filter can screw into the 1.25" filter adapter plate which is ~1.7mm thick. With some filters it does not matter which way the filter faces, the Andrea U for example. For other filters however it does matter which way the filter faces. For these there is an adapter shown sitting atop the assembly. This is an M28.5 (1.25" eyepiece) male/male gender changer. This one is a Blue Fireball part # G-1M widely available from astronomy vendors, I got mine from eBay.
  • The right panel shows the filter mounted on the male-male M28.5 adapter so that it faces forward.

Final assembly

post-24-0-76252000-1423796713.jpg

  • The left panel shows a view through the helicoid as well as the location of my 1.25" filters in the shallow µ4/3 to M42 adapter for when the enlarger lens does not have an illuminated aperture window.
  • The right panel shows the complete assembly. It is quite sturdy and could easily be carried in the field. Obviously to change filters one must unscrew the extension from the helicoid. Te plus side is the aperture adjustment is unobstructed and you can read the setting in the window which remains uncovered.

Light leakage

To test for light leakage I performed a simple test. I used an unmodified Panasonic G3 and no filters. I placed a body cap on the camera and with the camera set to aperture priority mode, ISO 160 & +1 2/3 EV on the exposure dial, went into a dark room checked the shutter speed which was 60 seconds.

 

I covered the front of the lens completely so that no light could enter and went outside into mid afternoon sunlight. I oriented the aperture window to face directly at the sun and the shutter speed remained on 60 seconds.

 

Obviously I must repeat the leak test with the modified G3 to determine IR leakage. There may be light leaks yet discovered when I conduct more extensive leak tests. But, so far as I can tell, it does not seem to leak. Any suggestions as to how best to conduct a more definitive leak test are most appreciated.

 

The final proof will be in the shooting of course.

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Hey there JD !! Thanks for posting such a detail explanation with fotos of your setup for the Omegaron 75/4.5.

 

I like these kinds of EL lenses for UV close work because they seem to have a flat field and are sharp. Also cheap! They seem to be corrected just enough not to have too much chrom ab in very near UV. (Each one is different however and must be tested.)

 

We posted simultaneously last night with our Omegaron adaptions. :)

 

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When testing for light leaks, be sure to run thru all aperture settings - or at least the extremes and a couple inbetween. Then try as many extension lengths as possible - shortest, longest and a couple of inbetween settings. I have noted on my Velcro® post that I'm getting an as yet undetermined light leak which disappears at certain helicoid extensions.

 

Update: That light leak was from an uncovered aperture window on the lens body. It is not enough just to cover the light guides on the base of such a lens. Nor is it enough just to set the lens inside a covering helicoid or tube.

 

Check that any mount adapter fits tightly and that covers all mount slots or holes are completely covered. For example, the Pentax M42-to-Kmount adapter has slots that some lenses are not wide enough to cover.

 

If you get a light leak even though aperture windows are covered, try some kind of lens shade - a cardboard tube or similar - to determine whether it is off-axis light rays which might be causing it. Although usually that creates more of a hot spot effect.

 

Open ports on the camera can also leak light if the port is not in use.

 

If I think of anything else about testing for light leaks, I'll add it later.

If anyone else has some suggestions about light leak testing, please add 'em to this post & thanks.

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The electronic viewfinder on my little G3 doesn't leak.

Unfortunately, the LVF/LCD button just to the left of the EVF sometimes will if angled towards the sun.

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