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eBay Rheinberg filters first impressions

 
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Olympusman



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PostPosted: Wed Jul 22, 2015 2:38 pm    Post subject: eBay Rheinberg filters first impressions Reply with quote

My recent purchase of eBay Rheinberg filters is that they are terrible and you should, in my opinion, spend your money elsewhere.
Here is a rotifer Platyias patulus shot with a Zerene stack in darkfield with a 20x objective.



Here is the same specimen shot in Rheinberg illumination with a filter I made myself by printing out an image made and printed in Photoshop and shooting it in sunlight from an 8X10 printed image on Ektachrome transparency film with a 6X6 centimeter twin reflex camera and cut to 32mm. The filter thickness is 0.14mm. The image has a red center area and a blue surround.



Here is the same specimen shot with the eBay filter set with a red center spot and a blue surround. The filter thickness is 2.85mm.



It appears to me, the refraction between the substage illuminator and the condenser are being compromised by the thickness of the Rheinberg filters.
In all fairness, from what I have found is that Rheinberg illumination depends upon the condenser diaphragm be set to its widest aperture.
I would appreciate any discussion from those with optical physics knowledge to weight in.

Regards
Mike
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zzffnn



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PostPosted: Wed Jul 22, 2015 3:39 pm    Post subject: Reply with quote

Mike,

I don't have much knowledge of optical physics. Though I did play with those Rheinberg filters for quite a few months.

I have those eBay Leica filters (made for my Leica condenser adapted to Nikon scope) with various center stop diameters. I also have some (eBay) custom made Rheinberg filters made for 40x, 60x objectives and above.

I did not record many Rheinberg photos, but my diatom Rheinberg photos shot with 40x (NA 0.65) and 60x (NA 0.85) objectives had more blue/red contrast than what you showed here using 20x objective with that eBay filter. My filters were made by the same eBay seller as your. This is unexpected, as Rheinberg at 20x (typical NA 0.40) objective should be easier and produces more contrast than 40x above.

My filter thickness is 2.5 mm. Scope is a Nikon Labophot 2 with 30w halogen light and 4 (teaching) eyepieces. So my light source is pretty weak. I did try both long exposure and flash and results are comparable.

Is your rotifer moving, are you using flash? Long exposure? Did you provide enough light for Rheinberg? I assume your condenser diaphragm is fully open and substage lamp diaphragm is sufficiently wide. If there is enough light output, thicker filter should still work (that >2 mm thickness worked for me).

You may want to try different center stop size. Higher NA needs a larger center stop. Though 20x objective's Rheinberg stop size is not a lot larger than that of 10x. And I think you meant "blue center stop and red surround". (Blue) center stop provides background, while (red) surround provides subject's color.

What is your center stop size used in the third photo? I am guessing your blue center stop is way too big for 20x objective, as there was too much blue (background color from center stop) and not enough red in your third photo. Please try a smaller center stop that works well for 10x. On my scope, 20x objective can use Rheinberg filters made for 10x objective. I think the blue (Rheinberg) center stop I used for 20x objective is 10.3 or 11 mm in diameter (I use 10 mm stop for 10x). Effective diameter of my red surround is 26.5 mm.

Thicker sample usually works better with Rheinberg light (my thicker diatoms show up much better than thinner diatoms under Rheinberg light). Your rotifer seems to be thicker than my diatom (judged from objective used and image size). So again, it is surprising why yours did not work so well.
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Charles Krebs



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PostPosted: Wed Jul 22, 2015 8:17 pm    Post subject: Reply with quote

Mike,

As zzffnn mentioned,
Quote:
I think you meant "blue center stop and red surround". (Blue) center stop provides background, while (red) surround provides subject's color.


Pull an eyepiece take a look at the back focal plane of the objective. The size of the center stop is important. Ideally, just like a darkfield stop, whatever the center stop color is it should just "fill" the rear focal plane. So if the center stop is blue, the rear of the objective, as viewed down the empty eyepiece tube, should be almost all blue (except for the subject) and you should not see the red outer annulus visible around the edges. (And just like darkfield, it's best to keep the condenser wide open). But you want it to just fill the rear aperture. If the center stop is too large it can extend greatly into what should be the annulus color and potentially dominate the color of the light illuminating the subject. (If this is a problem you can add an opaque black ring to cover the excess center stop).

Another very common problem is the density difference between the center stop and outer annulus. The amount of light passed by the outer annulus should usually be considerably greater than the level of light passed through the center stop. It can sometimes be tricky to get a nice balance, and can vary quite a bit with subject transparency. Very often the center stop is not dense enough and its color overwhelms the outer annulus color. (When I did Rheinberg I would sometimes put a plastic circle of polarizing material under the center stop. Then by placing another polarizer over the base light and rotating it I could vary the transmittance of the center stop).
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zzffnn



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PostPosted: Wed Jul 22, 2015 8:56 pm    Post subject: Reply with quote

Mike,

I just noticed that the filter set you bought contains only one center stop size. Maybe that center stop is too large and is optimized for 40x objective ( unless your 20x has NA much larger than 0.40)?

I actually tend to let the surround bleed a bit, rather than letting the center stop to overwhelm the surround. That way, your subject stands out at least. In my experience on my scope, Rheinberg center stop size is slightly smaller than that required by darkfield at same NA (it may be a personal preference though). But note that my Rheinberg filter is 2.5 mm thick (same thickness for center and surround).

As Charlie said, most people make center stop thicker and darker (and srurround thinner and lighter) to enhance contrast. But of most importance is the size of center stop. The one-sized eBay filter (10mm) that I bought only work from 10x up to 20x objective. That was why I got custom-made ones with different center stop sizes of up to 20 mm.

Your DIY filter looks quite good.

It should be easy to make Rheinberg filters out of camera flash gel/filters too. This circle template may be useful (it has size increment of 0.8 mm, which is slightly too big a jump, but I have not found anything better):

http://www.amazon.com/gp/product/B00290L1PM?psc=1&redirect=true&ref_=oh_aui_search_detailpage
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Olympusman



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PostPosted: Thu Jul 23, 2015 8:24 am    Post subject: Rheinberg Reply with quote

Thanks for the tips, guys. I'll print this out and try your suggestions this afternoon on a Cat Flea slide I've had for awhile.
The center stop in my self-made Ektachrome Rheinbergs are 12.5mm in a 32mm filter.
The center stops from the eBay set are 16.25 in a 32mm surround.
The test shots were a red center stop and a blue surround.

Mike
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zzffnn



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PostPosted: Thu Jul 23, 2015 9:17 am    Post subject: Reply with quote

Mike,

Maybe you can show us photos of your filters and back focal plane of your objective with filters on.

If you are really using red center and blue surround with a 20x NA 0.4 objective, is it possible that you did not center the filter optimally? Back focal plane photo will tell us.

With your DIY filter, did you stack red center on a solid circle of blue or on a ring (empty center) of blue?

You may want to change the center stop sizes and compare effects.

When I used 16mm BLUE CENTER and red surround with 20x NA 0.4, I got mostly blue and not enough red on subject. Just like your 3rd photo. But you were using RED CENTER and blue surround, which should produce the opposite color effect.

20x NA 0.4 is best with 10.3 mm (less than 11 mm for sure) on my scope.

Your 3rd might be produced with 20x NA >= 0.65 objective (very high NA for such low magnification) and red center/blue surround. You can confirm by looking at back focal plane of objective, once you peek into empty eye tube. You can also show us a photo of that. But then with a high NA 20x, your 2nd shot with smaller 12.5 mm red center should look worse (if 16.25 mm red center cannot cover back plane of that objective, 12.5 mm red center will only do worse).

But you got better result with that smaller center stop (which indicates a regular NA 0.4 20x objective was used, or filter was not positioned centrally). And then your color effect on the 2nd photo is the exact opposite of what I expected, from red center and blue surround (I expect blue subject on red background, if everything was set up optimally). Decentralized filter can produce strange effects.

I know thickness plays a role, but I won't expect such an impact.
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Charles Krebs



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PostPosted: Thu Jul 23, 2015 11:23 am    Post subject: Reply with quote

Mike...
Quote:
The center stops from the eBay set are 16.25 in a 32mm surround.
The test shots were a red center stop and a blue surround.


Well then something is really off in the "geometry". What is the NA of the objective used here, and can you give some specs on the condenser as well?
In general, how are you determining the "correct" condenser height?


Looking at the back focal plane should provide the best answers. Sometimes it is a little hard to see this clearly enough without a centering telescope (Bertrand lens). If you don't have one, a useful method is to pull one eyepiece out (or use a spare eyepiece) and carefully hold it up, top facing top (eye-lens to eye-lens) to the other eyepiece that is still inserted. Look into the back of the eyepiece you are holding, and use it as a loupe to look thorough the other eyepiece. Move it closer and farther until you get good focus (in my case there is about 1.75 inches of space between the two eyepieces). You should clearly see any of the stop/annulus size issues we've discussed, and can see the results of changing the condenser height as well.
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Olympusman



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PostPosted: Fri Jul 24, 2015 8:37 am    Post subject: Rheinberg filters Reply with quote

Charlie and Fan,
Here are notes on some of the tests you asked to conduct and requested specs.
1. Fan -- the original shots were actually a filter with a blue center spot with a red surround.

2. The condenser is an Abbe 1.25 N.A. I had to shim the pivot for the filter holder to accomodate the thickness of the new Rheinberg filters. The condenser was newly centered after the modification.
The condenser rack and pinion has a vertical travel of 10mm.

3. The numerical apertures of the objectives being used are:
4X 0.10 N.A. 10x 0.25 N.A. 20x 0.40 N.A. 40x 0.65 N.A.

4. Filters

After testing various combinations, these are the ones that tested out the best.



Top row: eBay Rheinberg filters Middle row: "Ektachrome" copy filters
Bottom row: Darkfiled stops and UGF filters

5. Darkfield stops stop diameters 4X/10X objectives 9.7mm 20x objective 12.8mm

6. Bertrand lens test results
The only time could get a trace of the surround by racking the condenser up and down was with a 40x objective with the 13.2 center stop with the "Ektachrome" filters. Never saw a trace of the surround with any other objectives and filters.

Looks like I need a set of Rheinberg filters with smaller center stops.

Thatnks for your guidance.

Mike
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zzffnn



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PostPosted: Fri Jul 24, 2015 8:58 am    Post subject: Reply with quote

Mike,

Your latest post (blue center/red surround and using smaller center for Rheinberg) makes sense to me. I suggest using 10-10.3 mm center stop for Rheinberg with your 20x NA 0.4 objective (I think the eBay filter is way too big with its center stop, even your DIY one is still on the bigger side - your center/background color still bleeds too much into your subject/foreground).

My eBay Leica Rheinberg filter came to me with 10 mm center stop size (6.25 mm [significantly] smaller than your eBay set). I am guessing the seller's intent is for buyer to start with brand-specific filters (like my Leica set) by using the easiest combo (lowest magnification/smallest stop) first, then progress to a bigger DIY set like the one you bought for higher NA (0.65 and/or above) / higher magnification (40x and/or above).

Rheinberg is usually done with condenser at highest possible position and diaphragm fully open.

This may be out of topic, but I think your UGF is missing black tape masking about half of the circle (you may want to mask the upper or lower half of your filters as shown in the above photo). That means you are missing most of the oblique three diementional effect (your subject image would not stand out as much from background). If you loookat Litonotus ' original filters, there is always black masking tape on half of the filter IN ADDITION to gradient. So what you have there is only gradient (without oblique mask).

With that said, you may not notice lacking of the oblique/3D effect, when you are doing lots of focus stacking. The benefit of oblique over bright filed or plain gradient is more obvious when you compare single shots (not stacked).

And I understand that you may not want to modify your condenser further to allow UGF get closer to condenser diaphragm, along with UGF's complete sliding/masking action. If you want to dig further with UGF though, here is some details on optimization (note I always have half of the UGF masked by black tape/oblique mask and condenser diaphragm should be blocked completely with BOTH oblique mask and gradient): http://www.photomacrography.net/forum/viewtopic.php?p=173507&highlight=#173507
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Olympusman



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PostPosted: Fri Jul 24, 2015 1:44 pm    Post subject: Rheinberg filters Reply with quote

I'm giving up on Rheinberg illumination for a number of reasons. Mainly, I am dissatisified with using my condenser wide open. While the effect may be interesting, the compromise in detail is very unsatifactory. In addition, I would have to go to the expense of a set of custom filters.
I contacted a maker of Rheinberg Filters (Mike Shaw at mike@mikeshawtoday.com) and he recommended his book on makng your own filters.
As for Fan's suggestions on getting more out my UGF filters, instead of black electrical tape, I have cut out a set of half-moon and crescent moon oblique filters from black paper that I can sandwich with my filters cut out from Cokin Grad gray filters on the filter holder on my condenser. I'll let you know how these work out.

Thanks for all the guidance Charlie and Fan.

Regards
Mike
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zzffnn



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PostPosted: Fri Jul 24, 2015 1:54 pm    Post subject: Reply with quote

Sounds good Mike.

Your way of making UGF will surely work and enhance contrast and depth. I would recommend using bigger crescent (more oblique masking) for high NA. UGF benefits from wide open condenser too and in that case focus depth and contrast will not suffer, as UGF acts similarly as condenser diaphragm.

Not closing down condenser diaphragm for Rheinberg may reduce contrast and focus depth a bit, though stacking and Rheinberg filter's inherent color contrast may make it up sometimes. But I understand your relutance to go for custom Rheinberg.
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Charles Krebs



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PostPosted: Fri Jul 24, 2015 6:37 pm    Post subject: Reply with quote

Mike,

If you get the urge to give it another go, you might try this...

Cut a "donut" of some black opaque card stock and add it to your Ebay filter. Something like this...



This would effectively reduce the inner circle (background in shot) which appears to be too large and is causing the background color to extend into the region were you want your subject color. You really need to have an appropriately sized filter for each objective, and it may take some trial and error to get it right.
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g4lab



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PostPosted: Sat Jul 25, 2015 2:20 am    Post subject: Reply with quote

http://paedia.com/Diabloc.html

This is related and interesting
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zzffnn



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PostPosted: Sat Jul 25, 2015 5:43 am    Post subject: Reply with quote

Mike,

You can also try:

Oblique crescent + your DIY Rheinberg filter + condenser at highest position and fully open (+ flash for more light)

The oblique crescent will act similarly to condenser diaphragm there, so your contrast and focus depth should not degrade. You just need to add a crescent to your existing Rheinberg set-up, which should be quick and easy.
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