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12-06-2016, 06:09 PM   #1
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Purple cross on a Variable ND Filter

Today I took my new 21mm prime lens to my local lake hoping to do some long day time exposure shots using my variable Promaster ND Filter. Now, I needed to use a step up ring as the thread is 49mm and the filter 52mm, no problem, screwed on the step up ring. Next goes on the filter voila, all looks and feels fine.

Everything is going to plan with set up etc, however I noticed that when tweaking the darkness close to max I'm getting (on the viewfinder) a purple cloud/fringing cross like pattern. If taking a photo this is also reproduced.

I recall using my ND filter with another 52mm thread lens (no step up ring required), and i was getting a similar purple effect coming in from the edges also, it never was quite as bad as this however. I surmised that perhaps I was screwing the filter onto the lens poorly, perhaps some light leakage coming in and ruining the filters objective?

Anyway, i took it all apart several times and screwed everything back together, again and again, nothing got rid of this purple cloud coming in whenever I really cranked up the darkness.

It meant that even at f22 iso 100 I could only manage about 3secs exposure, nothing like the 20secs I've managed before (without the step up rings).

What's going on?

I can try and take a picture of the viewfinder and what it's showing later on to help trouble shoot. Is this a common issue? Step up ring naff? Variable ND bad idea? Not sure where to turn next...

Cheers,

Bruce

12-06-2016, 06:20 PM - 1 Like   #2
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A variable ND filter is actually two polarizing filters stacked. The filter is at its darkest when the polarization axis of the two filters are perpendicular to each other. What you are seeing is an interference pattern. You may also see a splotchy diamond pattern or large polka-dots. There is no good way to avoid it with variable ND filters. Invest in a set of regular ND filters and stack them to get varying densities instead.
12-06-2016, 06:28 PM   #3
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QuoteOriginally posted by AquaDome Quote
A variable ND filter is actually two polarizing filters stacked. The filter is at its darkest when the polarization axis of the two filters are perpendicular to each other. What you are seeing is an interference pattern. You may also see a splotchy diamond pattern or large polka-dots. There is no good way to avoid it with variable ND filters. Invest in a set of regular ND filters and stack them to get varying densities instead.
I have a B&W Variable ND filter that's never had interference pattern issues...
12-06-2016, 06:35 PM   #4
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It's a well known limitation of variable ND filters at their maximum settings. If you dial it down a bit, it probably will disappear.

Usually the max on a NDX is about 5 or 6 stops or so. If your old one was stronger, that would explain a lot.

12-06-2016, 06:36 PM   #5
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QuoteOriginally posted by AquaDome Quote
A variable ND filter is actually two polarizing filters stacked. The filter is at its darkest when the polarization axis of the two filters are perpendicular to each other. What you are seeing is an interference pattern. You may also see a splotchy diamond pattern or large polka-dots. There is no good way to avoid it with variable ND filters. Invest in a set of regular ND filters and stack them to get varying densities instead.
Wish i knew this before forking out $160AUD for it!
12-06-2016, 08:17 PM   #6
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QuoteOriginally posted by BruceBanner Quote
Wish i knew this before forking out $160AUD for it!
I feel your pain. I paid $149 for a 67mm 1-6 stop from B&H. Made by Formatt Hitech. Supposed to be very good. The first time I used it on my DA*50-135, I saw a pattern of splotches in the sky. I immediately recognized it as the same pattern I see when I wear polarized sun glasses in a car with polarized window tint. I sent the filter back to B&H. Now I use Cokin P or Lee square filters for most situations because I can use a graded ND and adjust where the grade is.
12-06-2016, 09:48 PM   #7
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Common problem with variable ND especially on wide angle lenses. There are several articles on the the forum. It seems the issue come from the fact that the ND filter is basically two polarising filters and some interference pattern is generated. It seems to affect wide angle lenses more and the quality of the filter may have some affect but I understand even the most expensive can suffer to some extent. Pity the sellers of these things do not make the limitations clear to buyers.

I bought cheap ones just for the heck of it. They are now in the junk bin.

12-06-2016, 09:51 PM   #8
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If i'm seeing no issue at light to mid darkness, can I buy a ND filter that's mid to dark in strength (not sure what the actual stop might be), use THAT instead of the step up ring, and then add on the expensive VB ND filter ontop of that? Would that work?

EDIT: Here's some examples;

1) What the lake I was photographing should look like (ie no filter); https://www.flickr.com/photos/95230640@N04/31360756391/in/dateposted-public/

2) Here's what I was getting cranking the darkness to max (the picture has been resized and visited lighroom, auto tone applied just to show the purple cross better); https://s16.postimg.org/k7uhdac51/IMGP3805_1.jpg

3) Here's what happens along the way, this purple shadowing creeps in from both edges and gets worse the higher the stop; https://s3.postimg.org/lny2ffac3/IMGP3803.jpg

I'm pretty unimpressed, $160 down the drain pretty much, I can get maybe 1-2sec max exposure from this thing.... but it can go much MUCH darker but doesn't give any usable results...

Last edited by BruceBanner; 12-06-2016 at 10:20 PM.
12-07-2016, 11:34 AM   #9
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I'd suggest getting a decent set of standard ND filters of varying strength. If you get 2X, 4X and 8X filters you can stack them to create even greater reduction in exposure. Consider getting much bigger ones than your lens(es) require to avoid vignetting when stacking.
12-08-2016, 12:27 AM   #10
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Today I took my Promaster Variable HGX ND Filter back to the shop I purchased it from to complain how bad it was. I mentioned that even with 50mm lens the effect you see above is apparent, but of course much worse in a 21mm.
They were very sympathetic and offered a credit note in return, not bad for a product I had possession of for a couple of months.
We switched it over for 2 regular non variable ND filters, that when stacked give almost the same amount of darkness as the max on the variable (one stop away). For those interested I'll post back here my results next chance I get to the lake, see if that purple clouding exists or not etc.

The shop owner did mention that essentially to avoid that clouding you may have to pay quite alot of money for a very VERY nice ND filter, he threw out the figure $400AUD... ouch!
12-08-2016, 06:04 PM   #11
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That is a crazy price for ND filters. The Hoya HMC and Kenko Zeta are both excellent quality and you can get even the big 77mm sizes for well under US$50. There are tons on ebay. I've bought from this seller before and they offer shipping down under Kenko - Tokina Zeta 77mm Neutral Density ND-4 (0.6) - ZR Super MC - KZ-77ND4 4961607037744 | eBay
12-08-2016, 10:55 PM   #12
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The Joel Tjintjelaar long-exposure kit from Formatt cost about $350 US in 67mm. It consists of three ND filters, 3x, 6x, and 10x, which also block IR at their rated density.

QuoteOriginally posted by BruceBanner Quote
Today I took my Promaster Variable HGX ND Filter back to the shop I purchased it from to complain how bad it was. I mentioned that even with 50mm lens the effect you see above is apparent, but of course much worse in a 21mm.
They were very sympathetic and offered a credit note in return, not bad for a product I had possession of for a couple of months.
We switched it over for 2 regular non variable ND filters, that when stacked give almost the same amount of darkness as the max on the variable (one stop away). For those interested I'll post back here my results next chance I get to the lake, see if that purple clouding exists or not etc.

The shop owner did mention that essentially to avoid that clouding you may have to pay quite alot of money for a very VERY nice ND filter, he threw out the figure $400AUD... ouch!
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