Bokeh striations in ZWB2

[Andy Perrin]

Yeah, I think probably yes. If it only appears with this filter, then it's probably the filter.

[KhanhDam]

can you see these defects on the filter surface? seems like this manufacturer doesn't have very good quality control. another post complainted about 8mm thick filter bluring the photos. 

[Andy Perrin]

None of the ones from China are great at quality control, that’s why the prices are lower. But even Schott has a certain tolerance for striations and bubbles. It’s all about probabilities and tolerances. Some people here like to take their chances and pay less, with the hope that their filter will be one of the good ones. Some have no choice if the budget is low. 
 

I think at least five or six of us got the 8mm filters you refer to, and all but mine were sharp. 

[Nate]

One way to tell for sure, is to rotate the filter while observing the lines. I once had a S8612 that did that even when focused, very disappointing as the image never got sharp.

[ulf]

9 hours ago, KhanhDam said:

can you see these defects on the filter surface? seems like this manufacturer doesn't have very good quality control. another post complainted about 8mm thick filter bluring the photos. 

No you cannot see striae on the surface of a filter.

It is a phenomenon where the filter material is not completely homogenous and not something you will test for in an individual filter.

It is done for the glass blocks that are cut to sheets by the material producer like Schott, Hoya-Optics...

 

Some info about striae in optical glass and how to test it:

https://wp.optics.arizona.edu/optomech/wp-content/uploads/sites/53/2016/10/tie-25_striae_in_optical_glass_us.pdf

This article is a bit dated and there might be more modern methods.

 

I suppose that it is a bit more difficult to check visually black filter glass materials for UV-pass and NIR-pass.

---

I took a series of photos to test and compare my 8mm ZWB1 against my U-340 2 x 4mm stack.

I used my EL-Nikkor 80mm old metal lens at f/5,6, F/8, F/11 and F/16.

I tested the filters on my full frame Sony A7 III

There was very little difference in sharpness. 

 

I have also compared the surface flatness  a 8mm ZWB1 glass round, from Tangsinuo, with the flatness of my quality filters U-340, 4mm made by UVIROptics.

The flatness of all six surfaces were similar.

In the future I will show some pictures of such a test.

 

My reference optic flats for this test are at least flat to 0.09um or better.

---

The filter producing companies refine the raw material by grinding the sheets to the desired thickness and cut or shape them to fit the intended ring or holder.

Hoya sells sheets of filter materials in the shape of 4mm x 165mm x 165mm, to be refined further into filters.

Naturally if you are a very big customer needing very many identical filters, Schott can refine the sheet material too.

 

There are companies that sell refined filter material for the scientific market.

One of them is

https://www.uqgoptics.com/catalogue/filters/?gclid=CjwKCAjw6fyXBhBgEiwAhhiZslb1IKY_SfZQ0-P4XH-MSbWD1STxNopwP_man7PV32pmSYw-KXGu3hoCsLQQAvD_BwE 

Then you have optical scientific suppliers like Thorlabs and Edmund Optics that also carry their filters, among many other things.

 

 

[Ming]

13 hours ago, KhanhDam said:

can you see these defects on the filter surface? seems like this manufacturer doesn't have very good quality control. another post complainted about 8mm thick filter bluring the photos. 

Nothing I can see.  It's clean and black.

 

12 hours ago, Andy Perrin said:

None of the ones from China are great at quality control, that’s why the prices are lower. But even Schott has a certain tolerance for striations and bubbles. It’s all about probabilities and tolerances. Some people here like to take their chances and pay less, with the hope that their filter will be one of the good ones. Some have no choice if the budget is low. 
 

I think at least five or six of us got the 8mm filters you refer to, and all but mine were sharp. 

It is a bargain if only one out of five is bad, except for the person who got the bad one.

 

11 hours ago, Nate said:

One way to tell for sure, is to rotate the filter while observing the lines. I once had a S8612 that did that even when focused, very disappointing as the image never got sharp.

I will try rotating it.  Thanks.  It'd be cool to control the orientation of striations.

 

[Kai]

Interesting idea to do shadowgraphy with UV-light to detekt fluctuations in optical density!
I´ll post an experiment to this seperately.

 

 

[Andrea B.]

As a side note here:  I would really, really like to see that Tangsinuo replaces any filter which turns out to be bad !!

 

 

[Ming]

56 minutes ago, Andrea B. said:

As a side note here:  I would really, really like to see that Tangsinuo replaces any filter which turns out to be bad !!

 

 

That would be wonderful.  However, buyers pay return shipping according to their terms https://www.ebay.com/itm/274412098463.  If @Andy Perrin's statistics is in the ballpark (1 out of 5 of the 8mm ZWB1 is bad), it's faster just to buy a replacement for a little extra money over return shipping.  So I bought four more ZWB2 -- there is bulk saving.  It turns out that one of the additional four is still bad.  So my bad-luck statistics is 2 out of 5.  Anyway, now I have three good ZWB2 for half the price of one Hoya U-360 https://www.ebay.com/itm/153462684520.  It's a good deal to me.  Forgive my complacency.

 

[Andy Perrin]

Yeah, that's always been the argument in favor of the Tangsinuo filters. On the other hand, if you want to troubleshoot a problem, knowing more precisely what transmission you have (at least at the specified wavelengths) is very useful, and I would still recommend everyone have at least a few pieces of Schott or Hoya in their collection, for testing purposes if no other reason. People who own actual spectrometers and can measure the transmission may ignore this suggestion.

[Ming]

Does this image show that the ZWB2 filter that I bought from Tangsinuo has excessive striations?  It was taken with EF 40mm f/2.8 STM, ZWB2, and BG39.  I can reliably reproduce such striations with this filter.  However, the bokeh from the ZWB1 filter that came with Igoriginal 35mm f/3.5 is always smooth.  So I suspect that the Tangsinuo filter is defective.