Issues while using two thread in filters together?

[Michal Kedracki]

Hi,

Apologies if the anwsers to my questions are obvious, my background is in lighting and the stuff I found online only raised more questions. 
I’m about do a short on S8mm (Canon 814, 7.5-60 mm) and I’ll be using a thread in 85 filter for some exterior stuff.  I’m wondering about "safety" of adding another filter on top of it (ND).

My two worries are:

1. Is there a high risk of the two filters creating reflections and flares? I’ve read a lot about Matte Box filters potentially creating issues like that.
2. Could there be any Vignetting on the wide end of the zoom? I unfortunately was not able to test that as I had no access to both filters at the time.

The filter size on this particular lens is 58mm, if that’s relevant at all.

Many Thanks,

Michal

[David Mullen ASC]

Most of the time you’re fine but if you have a bright edge on something you might see a ghost reflection of it.

[Johanan Pandone]

If you're worried about vignette you could always go the route of putting a step up ring (58-77 for example) and then stack 77mm filters (which are common and cheap) on the step-up ring. As far as adding ghosts, sure, every extra peice of glass you place in the optical path creates an opportunity for more reflections, but most problems come from light hitting the glass sideways, so if you've got any control over lighting, you can adjust the placement of your backlights, flag off any unwanted spill, and adjust you camera angle to minimize these reflections. Don't be afraid to make your own hood out of gaff tape and anything material you have on hand that is flat black (cinefoil, posterboard, etc.) it might look silly, but I've done this many times to get rid of unwanted reflections. You could also back up and use a longer lens (which would allow for a longer hood) to get out of the path of offending light. There's so many things you can do to control it. Start by holding your hand out over and infront of the lens until you figure out which side you need to block then cobble something together. Get the shot. throw it away!

[David Mullen ASC]

The ghost reflections I’m talking about can’t be fixed by using a sunshade. It’s the double reflection of something bright in the image itself, like a hot rim light around a person or a bright lamp in the frame. Normally you solve it by taping the two filters together to get rid of the air gap, which gets rid of the double image, and then if there’s a flare you can tilt the two taped filters if the matte box allows it. Neither of these two tricks work with screw-on round filters stacked together — you can’t tape them glass-to-glass and you can’t tilt them.

[Eric Eader]

Michal,

If you anticipate doing this a lot, and not just as a onetime thing, then consider renting or purchasing combined 85N3, 85N6, 85N9, filters in a series 8 or series 9 utilizing step-up rings from your 58mm lens size.

85N3 = 1and2/3 stops 

85N6 = 2 and 2/3 stops

85N9 = 3 and 2/3 stops

(The Orange correction is combined with Neutral Density to make one filter).

Eliminates stacking problems outlined above.

Tiffen Filters is a starting point.

Since you are in London, that might not be as easy to do as in the USA, though. But for long term film shooting it is much more convenient than stacking.