Solution to Variable ND Filter with step up rings experiencing ghosting

[johnnyk]

Hello Shooters!
Here's a little challenge I'm having..

I'm predominately shooting with 3 lenses on a GH2 - Leica 50mm Noctilux (60mm thread), Leica 28mm Summicron (46mm thread), and an SLR Magic 12mm Hyperprime (58mm thread). All three of these lenses are pretty fast, and it would be nice to be able to use those apertures in daylight for the shallow depth of field. I was attempting to get a vari ND filter to cover them all and use step up rings to make it fit.
The 60mm and 46mm threads are kind of odd sizes. I initially purchased a 62mm Heliopan vari ND filter from the good folks at 17th Street Photo and got the necessary rings to make it cover the 3 lenses - (Noctilux = 60mm-62mm ring, Summicron = 46mm-48mm + 48mm-58mm + 58mm-62mm rings [whew!], and Hyper Prime = 58mm-62mm ring).

Okay. Sorry for all the math.

First issue, vignetting on the HyperPrime. Even though it's a 58mm filter thread being stepped up to a 62mm filter, either it's because the lens is so wide or that the vari ND is so deep that it's catching the edges.
I can solve this issue by moving up to a 72mm vari ND filter. Easy.

Second issue that I can't seem to solve, major ghosting problems, especially with the Noctilux and Summicron. I'm assuming that since the filter is stepped out from the lens that the light is being caught between it and the ND, unable to escape and thus reflecting onto the image? I have a B+W ND 0,9 for the Noctilux that requires no step up ring and it gives me no problems. Unfortunately, they don't seem to make vari NDs for 60mm and 46mm threads, and again, the goal was to only purchase one quality filter instead of three.

As I mentioned, I tried moving to a 72mm vari ND and even eliminating an interim ring with the Summi, but I still have a ghosting issue. I've read about people converting all of their lenses to 77mm so that every filter and lens cap would fit and they would be that much quicker in the field, but I've not come across any accounts of the problem of ghosting when using vari ND with DSLRs. Is anyone experiencing this? Am I barking up the wrong tree with this solution? Perhaps it's the dual glass nature of the vari's that is causing this. I wonder if I used step up rings to make all of the lenses the same filter size could I potentially just get a set of ND filters to cover it? I haven't tried a single ND filter with multiple step ups to see if the distance creates the ghosting.

Greatly appreciative of any ideas or advice.

thanks,
jk

 

[treynolds]

i would expect that your problem is coming from the distance between the front element of the lens and the filter. Like you said. Before buying anymore filters i would try getting a single adapter for each lens to make them 62mm

like one of these
http://www.ebay.com/itm/46-62-mm-46...gs_Holders&hash=item3cc694358b#ht_4348wt_1163

The vignetting i think might just be a problem with a lens that wide with a Vari-ND. I thought on FF cameras you cant use anything wider than 24mm (which would mean 12mm for you would be the cut off).

You also have to keep in mind that these variable ND filters are 2 pieces of glass where as the nd0.9 you were using was a single piece which is obviously less likely to cause problems.

I would say for the $2 its worth getting the single step up ring to keep that filter as close to the front element as possible.

 

[Samuel H]

I don't think the distance creates the reflection. It separates the highlight from its reflection, and if they were close enough you might not notice the reflection is there and might not care about it. But it's bad coatings that are creating that reflection.

 

[johnnyk]

thanks for the reply, treynolds.

i did try a single adapter to the 62mm vari ND with the noctilux and still had the ghosting issue. i'm not sure if it changes if i find a filter that fits directly without an adapter (not possible as far as i know, 60mm filter threads are an odd size and i don't believe a vari ND was made for them).

the vignetting on the 12mm lens is fixable by moving up to a 72mm filter. i already tested that. i'm aware of the vari's being 2 pieces of polarized glass vs. one piece for the Nd0,9. perhaps it's the 2 piece nature that is causing the ghosting effect, however i've yet to hear anyone else complain about this with DSLRs. in fact i normally hear how vari ND filters are a must have for DSLR shooters.

puzzled,
jk

 

[johnnyk]

Samuel H,

do you think it's bad coatings? i mean, i'm going from Leica lenses to Heliopan filter. i wouldn't expect either of those to have bad coatings.

thanks,
jk