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What can cause this optical problem?


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#1 Sean Emer

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Posted 06 August 2009 - 06:15 PM

Got footage back from my first 35mm shoot. Exposure, focus, framing, etc. were all spot on, but we did run into an odd problem. For some shots, bright offscreen objects were mirror reflected as a mild fog in the lower right part of the frame. Has anyone seen something like this before? It was not apparent in the viewfinder while shooting, which initially led us to think something might have been wrong with the gate... but we can't shake the feeling that this must be a lens issue of some kind. Below are links to a still frame and a short clip of one of the shots.

Thanks for any help!

Still Frame

Video Clip
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#2 John Sprung

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Posted 06 August 2009 - 07:09 PM

Hmmm..... What kind of camera was it? Can you get your hands on the camera and lens again? The reflection is definitely of the green and yellow balls, but there are so many it's hard to know which ones.

Assuming that this is a mirror shutter reflex camera, and given that the problem doesn't appear in the finder, I'd suspect a path that only exists when the shutter is open.

The reflection is in quite good focus and only on the far right of the picture. Because the image is inverted, this is the left side of the camera aperture, next to where the ground glass would be. So, I'd suspect that this lens covers a circle quite a bit larger than the aperture, and that some of that extra light reflects off the back of the ground glass and onto the film. The early Arriflex cameras had thin sheet metal light baffles behind the glass to prevent this. Of course it could be some other reflective surface in that general area, too.

If this is the problem, you might want to discuss it with the camera rental company.




-- J.S.
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#3 Tom Jensen

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Posted 06 August 2009 - 07:20 PM

What kind of camera, lens, filtration were you using?
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#4 Sean Emer

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Posted 06 August 2009 - 07:22 PM

Thanks for the reply, John.

We shot on an old Arri IIB with a rented set of Zeiss superspeeds. The camera we still have access to, and we checked the baffles around the gate, all of which were intact. The lenses came from a rental company that we can contact if needed.

some of that extra light reflects off the back of the ground glass and onto the film.


Did you mean the shutter, or are you saying that the actual groundglass is reflecting light onto the film? I don't understand how that is possible since when the film is being exposed the groundglass won't receive any light because the shutter is open. We did notice during the shoot that when the lens was pointed at very bright objects I could see a small amount of ambient light inside the gate when the shutter was open (and the groundglass was staring past it into the gate itself), but it was a diffused light, probably from the highlights bouncing around 30 year old not-so-matte material. I can see how this might cause a fog, but not an image. Any ideas?

Edited by Sean Emer, 06 August 2009 - 07:23 PM.

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#5 Sean Emer

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Posted 06 August 2009 - 07:25 PM

Tom, it was an Arri IIB, Zeiss Superspeeds, and with no filters.
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#6 John Sprung

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Posted 07 August 2009 - 12:21 AM

Interesting that the baffles are there. A lot of them got bent up, and eventually people just ripped them out. They make those five vertical black lines in the finder that some operators can't stand. Their purpose is to prevent reflections off the ground glass itself. With the shutter open, what can happen is that the cone of light coming out of the lens is much wider than the aperture, and some of it bounces off the GG at a very shallow angle and onto the film. The GG is perpendicular to the film. But with the five little baffles in place, that's impossible. There are those horizontal surfaces just above and below the aperture, if they get shiny for some reason, they're also perpendicular to the film and could kick some light. Take the lens out, and look in the front while you inch the shutter open. You should see the baffles over the GG. Look around for anything else that might be shiny.

If that doesn't show up an obvious answer, take a little scrap of film -- six inches is enough -- and wash all the emulsion off until you have clear plastic. Then take ordinary kitchen scouring powder, like Comet cleanser, and make a wet paste of it. Put this on some surface like the edge of a sink, and use it to grind one surface of the film into the equivalent of ground glass. Holding the film by the ends and using a sort of toweling motion works. Clean and dry it, then put it in the gate, ground side towards the lens. With the pressure plate open and the shutter open, you should be able to see an image on this ground film. A magnifying glass and dentist's mirror will make it easier to see. (This is how they used to set up shots on the old silent hand crank cameras.)

Look at the ground film image with whatever lenses you have available, and see if you can reproduce the problem. If not, pack up the stuff and take it to the rental house (phone first to be sure the lenses you used are in). Show them the images you have, and try to reproduce the problem on the ground film. Any good rental company will be very helpful in this, as they'll want to find any NG stuff they have and get it fixed before it gives any other customer trouble.



-- J.S.
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#7 John Sprung

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Posted 07 August 2009 - 12:19 PM

Duh! When I woke up this morning, I thought of what I shoulda thought of first: Have you had a negative cutter look at the camera original? Maybe we're looking too far upstream. It could be that the film is fine, and this happened in the telecine or scan. That's the first thing to do, it'll tell us whether to keep looking at the camera and lens or not.





-- J.S.
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#8 Rafael Rivera

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Posted 07 August 2009 - 12:41 PM

I'd check the negative with a lupe, but I'm inclined to say is a telecine issue.
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#9 Sean Emer

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Posted 07 August 2009 - 02:55 PM

Excellent idea!

I was there for the supervised transfer on a DaVinci in the city, so I assumed that whatever problems we saw were our fault... it never occured to me that they could have messed something up. I'm not in the same state as the OCN right now, but I'll ask the director to take a look. I'll report back soon(ish)
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#10 boy yniguez

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Posted 10 August 2009 - 02:30 AM

Got footage back from my first 35mm shoot. Exposure, focus, framing, etc. were all spot on, but we did run into an odd problem. For some shots, bright offscreen objects were mirror reflected as a mild fog in the lower right part of the frame. Has anyone seen something like this before? It was not apparent in the viewfinder while shooting, which initially led us to think something might have been wrong with the gate... but we can't shake the feeling that this must be a lens issue of some kind. Below are links to a still frame and a short clip of one of the shots.

Thanks for any help!

Still Frame

Video Clip

the key words to your query are the "bright OFFSCREEN objects" getting mirrored into the frame. this strongly suggests improperly aligned two or more filters in front of the lens that were causing reflections in between them. this occurs a lot when using more filters than a matte box can accomodate and a filter is taped onto the front of the filter array of the box. did that happen at all?
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#11 John Sprung

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Posted 10 August 2009 - 02:38 AM

the key words to your query are the "bright OFFSCREEN objects"


Ah, but if you look at the images, the things in the reflection are the green and yellow balls, of which there are many both in and out of frame. I don't think we can identify for sure whether the balls in the reflection are in or out of the intended frame. The OP also mentioned that there were no filters.




-- J.S.
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#12 Sean Emer

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Posted 10 August 2009 - 08:11 AM

Ah, but if you look at the images, the things in the reflection are the green and yellow balls, of which there are many both in and out of frame. I don't think we can identify for sure whether the balls in the reflection are in or out of the intended frame. The OP also mentioned that there were no filters.




-- J.S.


The example is only one instance of the problem. It happened in other shots as well, some of which weren't transferred. It was definitely caused by offscreen objects. In one instance, a bright red minivan was just off screen to the top right and a mirror image of it fogged the top right corner. Every other reflective problem occurred in the lower right, but was caused by the same offscreen bright object.
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#13 John Sprung

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Posted 10 August 2009 - 01:38 PM

It was definitely caused by offscreen objects. In one instance, a bright red minivan was just off screen to the top right and a mirror image of it fogged the top right corner.


Oy. In that case, we can rule out telecine as a possible cause of the problem. It has to be somewhere between the subject and the film in the aperture. Was there any kind of matte box or lens shade? Did it happen with more than one lens? What we're looking for are vertical shiny surfaces roughly parallel to the optical axis.






-- J.S.
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#14 Sean Emer

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Posted 10 August 2009 - 07:28 PM

Thanks for sticking with me on this one John,

I believe the problem persisted whether we had the mattebox on or off. I would have to check the camera reports that are, of course, with the ocn, but I think it may have been specific to one or two lenses, the 50mm and maybe a slightly wider one. What I have trouble understanding is how it could have been a problem in front of the shutter if I didn't see it in the viewfinder. I think your telecine idea may still be valid, as I recall that during the transfer he zoomed and panned our negative area to fit framing references i had marked at the head of the first roll. So, its possible that the offscreen objects were recorded on the film, but not directly seen in the telecine. Of course, if it was the telecine I would have expected to see the error more often.

The mystery continues... still waiting for the director check the ocn.

Thanks for the support so far, I like this site more and more every day!
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#15 Doug Durant

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Posted 10 August 2009 - 10:19 PM

Thanks for sticking with me on this one John,

I believe the problem persisted whether we had the mattebox on or off. I would have to check the camera reports that are, of course, with the ocn, but I think it may have been specific to one or two lenses, the 50mm and maybe a slightly wider one. What I have trouble understanding is how it could have been a problem in front of the shutter if I didn't see it in the viewfinder. I think your telecine idea may still be valid, as I recall that during the transfer he zoomed and panned our negative area to fit framing references i had marked at the head of the first roll. So, its possible that the offscreen objects were recorded on the film, but not directly seen in the telecine. Of course, if it was the telecine I would have expected to see the error more often.

The mystery continues... still waiting for the director check the ocn.

Thanks for the support so far, I like this site more and more every day!


Sean, how you been buddy? Saw the trailer for the feature you shot, great job on that! Sorry to hear about this. I actually just spoke to George about it, and he said that it maybe could have something to do with the video tap optics leaking light into the light path. Definitely something to investigate as the camera has probably been modified quite a few times over the years.
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#16 boy yniguez

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Posted 10 August 2009 - 11:34 PM

Sean, how you been buddy? Saw the trailer for the feature you shot, great job on that! Sorry to hear about this. I actually just spoke to George about it, and he said that it maybe could have something to do with the video tap optics leaking light into the light path. Definitely something to investigate as the camera has probably been modified quite a few times over the years.

a light leak from the video tap or from elsewhere in the camera body would just produce flares in the negative, not focused images. now if the tap was actually bouncing back images, then it would be from within the scene visible through the finder, not outside the frame!
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#17 John Sprung

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Posted 11 August 2009 - 01:57 PM

I actually just spoke to George about it, and he said that it maybe could have something to do with the video tap optics leaking light into the light path. Definitely something to investigate as the camera has probably been modified quite a few times over the years.


There's a video tap? I thought this was kind of a bare bones old Arri II-B. Hmmm -- At this point, the thing to do is arrange with the rental house to get the exact same lenses together with your camera body on their premises and try that ground film test. Watch while someone walks a flashlight side to side into and out of frame. BTW, I'm in LA -- the valley. Where are you guys located?




-- J.S.
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#18 Sean Emer

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Posted 11 August 2009 - 07:58 PM

To Doug: Hello, sir! It's been a while... I'm glad to see you on this site as well. I'll have to talk to George more... he was the first person I went to.

To John:

It is an incredibly old camera, don't worry :)

It does have a videotap attached to the front of the viewfinder, not on the opposite side, so both of them use the same optics as far as I know. Again, I have trouble visualizing how the tap could project an image onto the film if it only receives one when the film is covered.

The director is, as I am now realizing, on vacation... so that explains why he never answered my messages. Once he is back we will get this whole thing sorted out. The film is on Long Island, and I am in New Jersey. Hopefully I'll be in LA within a year or two. Maybe then I'll have to buy you a drink to thank you for all the help!

Edited by Sean Emer, 11 August 2009 - 07:59 PM.

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#19 John Sprung

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Posted 12 August 2009 - 03:04 AM

Thanks, Sean. If you have access to the camera, could you post some pictures of it? Macro shots looking into the empty lens port with the shutter open and closed might be helpful. BTW, what's the serial number on the camera?

Another thought -- how are the outer circular edges of the mirror shutter? We had a guy here a while back who was getting a strange kick off a chipped shutter edge. But that was easier to diagnose because the flare was on every other frame. If the silver is missing on the outboard edge of both blades, maybe, but it's a long shot.



Thanks again --



-- J.S.
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