ND Filter Causes "Fringing" Effects? HELP!

[John Layton]

Been having problems with peripheral fringing (double-image) effects while using my 6 stop (64x) B+W MRC neutral density filter. Problem crops up with axial movements, and seems to increase as degree of movement increases. Possibly worse with rear base tilts vs front or rear axials...but not sure. Also, with base tilts, fringing effect noticeable mostly in foreground. Happened previously with "cheap" Hoya nd filter - so moved to B+W...and, lo and behold - the problem persists! Web search reveals nothing. Am desperate! Anyone?

[LabRat]

Usually double highlights with filters happen due to the inside filter plane reflecting between another flat plane in the lens, where one plane might be tilted slightly, and reflects (esp lights at night) into multi images (instead of in register)... There might be a misalignment in the filter ring/threads that allow some tilt... Or if the lens has a flat element, it might be loose and tilting a little... (Give the lens a shake to hear if there might be a little element play somewhere...) Do a night test on some distant light with a filter on to see if it happens with other filters or lenses...

Good Luck!!!

Steve K

[vinny]

I've never had this issue with any of my nd's either round glass or square resin. Can you post some examples?

[Steve Goldstein]

If it happens with two different filters on the same lens then why do you say it's the filter? Do you get this effect using the same filter(s) on different lenses? How close together are the fringes? Is there any chance it's a pinhole light leak in a lensboard? This might not have much impact in normal use but with a 6-stops filter it could be significant.

It seems to me unlikely that it's light reflecting off the front of the lens and back off the filter. Uncoated glass reflects about 4%, and a multicoated filter less than 1%, so that would produce a reflection with at most about 0.04% of the intensity of the original source, about 11-1/3 stops down relative to the primary image-forming light after it's gone through the filter, and another stop down for a coated lens. I'd be surprised if this was it.

[LabRat]

If it happens with two different filters on the same lens then why do you say it's the filter? Do you get this effect using the same filter(s) on different lenses? How close together are the fringes? Is there any chance it's a pinhole light leak in a lensboard? This might not have much impact in normal use but with a 6-stops filter it could be significant.

It seems to me unlikely that it's light reflecting off the front of the lens and back off the filter. Uncoated glass reflects about 4%, and a multicoated filter less than 1%, so that would produce a reflection with at most about 0.04% of the intensity of the original source, about 11-1/3 stops down relative to the primary image-forming light after it's gone through the filter, and another stop down for a coated lens. I'd be surprised if this was it.

Yes Steve, strange but true as I found out as I do a lot of night/tele work, and one lens would produce multi bright highlights when shooting distant very bright lights, and I scratched my head trying to figure what was up... I finally found that a flat element had been a little loose and with the CC filter on, it would set this up... A curved surface element would not set-up this reflection, but a flat element would reflect MUCH more back.. (I also noticed this effect when aligning telescopes using star images, so it does exist...) John's problem might be different, but I barely remember using heavy ND's years ago, but sometimes strange things crept into the images, but I'm thinking it was reflections then...

Steve K

[Jac@stafford.net]

[...] Happened previously with "cheap" Hoya nd filter - so moved to B+W...and, lo and behold - the problem persists! Web search reveals nothing. Am desperate! Anyone?

To cover all possibilities - there are counterfeit B+W filters, enough that B+W now has a special holograph and QR code combination to determine genuine products. I hope you did not get a counterfeit.
.

[Sal Santamaura]

Which lens? Filter mounted to front or rear cell?

[Corran]

6-stop ND filter means you are probably making fairly long exposures, right?

My bet is that your filter is just fine, but the film is falling slightly after getting dislodged from sitting sideways slightly for a long period of time (it can get slightly stuck under the film holder's side rails, especially when shooting in landscape orientation). I've had this happen several times recently because I keep forgetting to rap the film holder in the direction it will sit in the camera on the palm of my hand to dislodge the film if it is stuck.

This is a common problem when shooting really long exposures.

I was looking for an example but I appear to have already deleted the latest one, from just last week.

[John Layton]

First noticed this with an 82mm Hoya 6 stop Multicoated ND...particularly when mounted on a 120mm f/8 Super Angulon with 5x7 format, and particularly when shooting vertically with fairly strong rear base tilts. Also with a 58mm B+W 6-stop Multicoated mounted on a 135mm Sironar-N with 4x5...again mostly when shooting vertically with rear base tilts. In each case, foregrounds affected most - upper 2/3rds of the image hardly at all.

Filters tight in their mounts. Mounted to front of lenses. Lenses performing up to par otherwise (years of long exposures in many conditions, without filters, and, so far, good results with 3 stop filters). 135mm lens has fairly strong front-element and rear element curvature.

Thing is (and this is what has me wondering)...each of these 6 stop filters was advertised (on B+H website) as simply "ND" - and yet each arrived also marked "digital." Question - is this just playing up to hype? Or...perhaps something is being messed with (to make them more digitally compatible?) - like the REFRACTIVE INDEX OF THE GLASS? (Bob Solomon...any comments?)

[LabRat]

I dunno, but maybe try shooting a test with the lens centered and image path on-axis and see if there is a difference... (Maybe the filter is too thick for WA use???) Strong ND's seem to set up reflections (as I remember???), so maybe with a lens that refracts more (and a dark filter that reflects back) smaller errors in the light path might form secondary images.. Or try shooting the filter with a normal or longer FL lens and see if it happens there... (Most WA's have flatish inner elements facing the iris/shutter blades...) These filters might work better as a gel, rather than glass...

I'm getting ready to shoot with strong ND's again, so let me see if the old problem (that I can't seem to remember right now) rears it's ugly head again, and I can report back in a few weeks about it...

Good luck/Don't give up...

Steve K

[Jim Andrada]

I know that digital cameras "prefer" circular polorizers to linear and but hadn't come across digital ND's yet. Who knows - some things can really throw digital auto focus systems off. Maybe there IS a difference.

[LabRat]

I was thinking and trying to remember what happened when I used heavy ND's some 20 years ago, and I still can't recall exactly, but I seem to remember an analogy I was trying to explain to myself then (that was a possibility) so I will give it a stab here and see if it makes any sense to anyone else...

If one were in a dark space looking out through a very dark tinted window, the view would be dim, but light hitting your face might be causing a reflection on the window that might obscure what you are trying to see... With a clear window, the transmittance of light would overpower the weak reflection, but with a dark window, maybe the reverse... (Sorta like being in a building with one way mirror windows, during the day, you can see outside and they can't see in, but at night, all you can see is reflections from inside the room, but you can see in from the outside...) So there might be a point where the filter works like a beam splitter glass...

What might also be happening is that the first rays entering the filter & lens might be reflected back to the filter and re-focused through the lens to the film plane, and the slight extra distance from the reflection is projected as a different size to the focal plane...

I also have seen this come up when using misaligned solar viewing filters on telescopes... (There was a slight halo of the sun when mounted on-axis, but would start to double the image off-axis...)

Again, this is only a guess, as I was searching for a solution... I'll see if it comes up again soon...

Steve K

[Jeff Keller]

Can you mount the filter behind the lens to see if the problem still exists?

jeff

[John Layton]

Update - I did a test today to help verify a hunch based on previously affected negatives...and sure enough - the presence of axial tilts alone (with a strong ND filter) does not seem to cause this fringing effect. The other element needed is the presence of direct sunlight.

My test was simple: vertical 5x7 negative exposed through 120mm SA with 6 Stop ND (brand new 82mm B+W) - with moderate front axial tilt. Frame 1: side lit grass lawn. Frame 2: grass lawn in shade. Results: Frame 1: bottom of negative frame displays fringing. Frame 2: No fringing present. These results are completely consistent with previously affected negatives.

Will try some further tests with different subject matter, lenses, sunlight angle, etc. But for now, I'll be very careful about using this filter, at least while shooting off-axis in the presence of direct sunlight. More later.

[John Layton]

PS - my results are simply too consistent to indicate any "popping" of negatives due to thermal issues.

[LabRat]

Here is another test;

Hang a black or white background outside in the bright sun... Hang a circle, or sheet of card, paper, etc in the middle that contrasts the background... Shoot a film image, and see if there is a ghost or fringe image along the edges of the card, or edges of the frame... Try it at different tilts... Shine a light towards lens slightly out of frame and see if fringing goes bonkers, or multiple image toward edges...

I still think it might be a form of internal reflection from the inside of the dark filter...

Steve K