Attaching filter for cross polarization on Laowa 25mm

[AlexC]

I have a Laowa 25mm ultra macro, and I'm wanting to tame some of the reflections on minerals with cross polarization. Seeing as the Laowa 25mm doesn't have conventional filter threads, is there any way I could do this?

[Lou Jost]

Filters are light, it would be easy to epoxy a step-up ring onto the front of that lens and then mount a filter to it.

[Ichthyophthirius]

Hi,

Is the lens cap made from plastic? Then you could get a spare lens cap, cut a circular hole and use that as a filter holder.

Or is the lens front magnetic? Then you could use that with a CPL filter with magnetic mount.

Or what's the diameter of the front of the lens? You could mount an adapter with 3 points of attachment (like an Exakta-M42 adapter https://www.ebay.co.uk/itm/383050615706 ; replace metal screws with plastic screws!) and use that as a base for step rings.

Regards, Ichty

[AlexC]

Hi,

Is the lens cap made from plastic? Then you could get a spare lens cap, cut a circular hole and use that as a filter holder.

Or is the lens front magnetic? Then you could use that with a CPL filter with magnetic mount.

Or what's the diameter of the front of the lens? You could mount an adapter with 3 points of attachment (like an Exakta-M42 adapter https://www.ebay.co.uk/itm/383050615706 ; replace metal screws with plastic screws!) and use that as a base for step rings.

Regards, Ichty

It's metal, but not threaded. It snaps in place.

[Macro_Cosmos]

Double tape the polarisation film to the rear of the lens, have another in a rotation mechanism of some sort for the light source. One can be made out of Lego or removing the optics of a cheapo polariser.

[Pau]

Double tape the polarisation film to the rear of the lens, have another in a rotation mechanism of some sort for the light source. One can be made out of Lego or removing the optics of a cheapo polariser.

I never had this lens in my hands but checking pictures at the web the rear rectangular mask of the lens seems protruding past the bayonet mount so your suggestion doesn't seem doable, at least for DSLRs

Placing the polarizer (analyzer in Microscopy jargon) in the rear side of the lens is in principle a good idea in many cases* for magnifications over 1X -less optical issues like spherical aberration and better WD, for example- and almost indispensable with most microscope objectives*

* This will depend of the design of the lenses or objectives: cover glass correction, mechanical fitting...

[Lou Jost]

Note that for cross polarization it is important to have access to the filter, in order to rotate it to the desired degree of extinction, so I would think front mounting would be preferable if the filter does not cause optical problems. It is usually much more difficult to rotate the polarizing sheets of the lights, though it is not impossible.

[JLyle]

The lens cap bayonets onto the lens. I bought an extra cap when I bought the lens. Drilled a hole in the center. Glued on a threaded ring and right from the start using a 39mm BW polar for cross polarization was effortless.

[Macro_Cosmos]

Double tape the polarisation film to the rear of the lens, have another in a rotation mechanism of some sort for the light source. One can be made out of Lego or removing the optics of a cheapo polariser.

I never had this lens in my hands but checking pictures at the web the rear rectangular mask of the lens seems protruding past the bayonet mount so your suggestion doesn't seem doable, at least for DSLRs

Placing the polarizer (analyzer in Microscopy jargon) in the rear side of the lens is in principle a good idea in many cases* for magnifications over 1X -less optical issues like spherical aberration and better WD, for example- and almost indispensable with most microscope objectives*

* This will depend of the design of the lenses or objectives: cover glass correction, mechanical fitting...

You aren't wrong, but the rear recesses into the lens, so it's totally possible to cut out a rectangle larger than the opening and tape it on. I used a piece of plastic here.


Tried taking a pic at 2.5x, no issues. Unless one wants to rotate both polarisers (I think only one is necessary, right?), this should be the easiest method.

Note that for cross polarization it is important to have access to the filter, in order to rotate it to the desired degree of extinction, so I would think front mounting would be preferable if the filter does not cause optical problems. It is usually much more difficult to rotate the polarizing sheets of the lights, though it is not impossible.

I might be wrong, but I thought rotating one sheet suffices? I haven't done much cross polarisation,

[Lou Jost]

Right, it is only necessary to rotate one polarizer. But for most set-ups, the polarizer in front of the light is a hard-to-rotate largish rectangular sheet. A normal polarizer filter on the front or behind the lens is easier to rotate.

I would be afraid to use non-optical-quality polarizing film behind the lens...

[Pau]

Nice solution, although...

What camera mount do you use? (A DSLR has longer flange distance and the mirror could touch anything protruding into the camera while with mirrorless bodies the situation is inverse)

I wouldn't use a thin plastic sheet polarizer in the imaging path, a good optical filter is a much safer (and often expensive) option

I might be wrong, but I thought rotating one sheet suffices? I haven't done much cross polarisation,

Is better to have the option of both polarizer and analyzer rotatable, although just to cross them to cut reflections rotating one often suffices.
If you use several light sources you need to have them rotatable at each one.
For subjects with oriented reflective surfaces like minerals and for partially crossed polarization having all filters rotatable can allow more control.

[AlexC]

Thank you guys for the info, I have some polarizing sheets, but didn'y think of applying them to the rear. I was leaning towards an optical quality filter, and I've seen some variable filters on the market.

My fear is that with crossed polarized sheets, that it wont be perfectly parallel with the lens. (I assume this could cause unwanted effects on the image?)

[AlexC]

The lens cap bayonets onto the lens. I bought an extra cap when I bought the lens. Drilled a hole in the center. Glued on a threaded ring and right from the start using a 39mm BW polar for cross polarization was effortless.

Awesome! I might have to give this a shot.

[Lou Jost]

I highly recommend an epoxied step-up ring instead, unless you desire to sell the lens someday. It is more sturdy and can be more carefully aligned, I think. This is what I do with most of my unthreaded lenses. This also lets you use standard filter sizes for all lenses.

Another trick: many plastic linear polarizing films, and some glass linear polarizers too, do not reach 100% extinction when crossed. I find that if I use two sheet of film in front of the light, with the axes almost but not quite parallel, I get better extinction (less purple highlights).

Also polarizing the light source must occur AFTER diffusion, not before.

[Macro_Cosmos]

Nice solution, although...
What camera mount do you use?

I use a D810, no issues at all. Even with objectives, I just cut polarising film and stick it to the rear of the lens.

Results aren't the best, and my slightly decentered 20x isn't helping.

Is better to have the option of both polarizer and analyzer rotatable, although just to cross them to cut reflections rotating one often suffices.
If you use several light sources you need to have them rotatable at each one.
For subjects with oriented reflective surfaces like minerals and for partially crossed polarization having all filters rotatable can allow more control.

Good point on light sources, I only use one light source and I achieve lighting through various methods involving reflective material. Yeah for anything above 1 light, having the film at the rear is gonna be a painful.

I do have another method... not a good one but it kind of works for me. Polarisation should be done after diffusion to get the cross polarised effect! For this one, I'm just using a single CPL to eliminate some annoying highlights. I titled it cross polarised, but nah. The polariser on the light source was essentially useless. If you want to get rid of some reflections, this method will work nicely.

Shown is a UV-IR cut, but the point is, my diffuser allows a polariser to just sit on top of it. I can rotate both. Here's an example, 2x, subject is sand from a local beach.

I would be afraid to use non-optical-quality polarizing film behind the lens...

The film I have is quite decent, it comes from a supplier in the UK. It depends on extinction VS transmission. Polarisation film in general does seem to have higher transmission. I do have a couple cheap EO optical-grade polarisers coming. Their high extinction one is very expensive, is buying 2 necessary? Can I get away with using a lower quality one on the singular light source I have and use the good one on the objective? I think it won't matter too much, but I could be wrong.

I highly recommend an epoxied step-up ring instead, unless you desire to sell the lens someday. It is more sturdy and can be more carefully aligned, I think. This is what I do with most of my unthreaded lenses. This also lets you use standard filter sizes for all lenses.

For unthreaded lenses, I just use SM2 tubes extended in front (they come with internal threads, allowing the adapted objective to be recessed into the tubes) and put a 52mm filter on. Nicely aligned! Of course, larger threadless lenses, this method won't work as well, if not at all. Would be better to order a custom made adapter.


I'd say, buy an extra lens cap, remove the front, and buy a step-up ring to glue onto it. 52mm is pretty common, so I recommend that. The cap is about 45-47mm in diameter it seems, 46mm should work nicely.
Ebay item: 283571694908
(Do be careful with buying from HK, as we all know, there's stuff going on there, the item just might not arrive. Some freight forwarder warehouses were burnt down a couple weeks ago, lots of losses for their customers)

Also polarizing the light source must occur AFTER diffusion, not before.

Yep... that's why my other method above isn't good.