Chasing down film scratches in a 6x7 film back?

[Steven Lee]

One of my film backs for the Bronica GS-1 started to scratch film. Strangely, only some film stocks are affected. Perhaps this depends on the thickness of the film base and robustness of the emulsion. FP4+ and Kentmere 100 seem to be the most prone to scratching.

I have examined every square millimeter of it and I failed to identify a single place where film touches anything other than the backing paper. The pressure plate (behind the backing paper) presses film against the rails. The spools and rollers in the back all interact with the packing paper only. There's no contact between film's emulsion side with anything else.

Also, there's usually a single frame (or two) that are affected.

What tricks have you used in the past to chase down the source of scratches?

[EDIT] Full sized scan.

 

[mshchem]

White lines would, IMHO, indicate that the film base is creased, or it's a static streak type light strike.

If the emulsion was scratched off it should show up as black

 

[Dustin McAmera]

If it didn't do it before, and it does it now, that would suggest wear might be involved. I'd look for sideways play in the rollers at each end of the inner part of the holder; can they move far enough to bring the film surface to brush a surface of the shell?

 

[Steven Lee]

@mshchem The color of scratches is interesting indeed! When I examine the negatives with a 12x loupe I see faint and sharp black lines. I too wonder why they're black... I would expect an emulsion scratch to leave a clear mark. Unfortunately, they are so faint that I am unable to tell which side of film they're on: the emulsion or the base...

@Dustin McAmera Actually I cannot confidently say that the back did not do this before. I mostly shoot color on 6x7 and these scratches are only visible on FP4+ and Kentmere 100. I just recently started using these films with this camera. It is entirely possible that it has always been this way. The back itself looks nearly brand new. GS-1 accessories in general tend to be in excellent shape due to the relatively young age of the platform. Mine was rated EX when it came from KEH.

 

[mshchem]

I remember from my old life, a term, "stress whitening" in regards to stressing plastic. The whitening refers to a white opaque mark that appears when you bend plastic. I've seen this when I've bent a tail end of film fumbling in the dark trying to load a developing reel. Doesn't take much
I wonder if either your back or Ilford is pressing the acetate base enough to create a streak???

The line is so fine and uniform. I've had a dozen different Bronica (SQ and ETR) backs over the years, I've never seen anything similar to this.

 

[reddesert]

When the lines are perfectly straight and horizontal it seems most likely to be something that happens during film transport.

Damage doesn't have to be scratching the emulsion off, if some small piece of debris is caught against the film maybe it can press on the film enough to cause a local pressure point and a black line on the negative, similar to the way that buckling the film causes fingernail-shaped crescent marks.

You might be able to localize the damage to front or back of the film by pressing the film against a piece of glass and looking for parallax between the scratch and its reflection.

I don't have any concrete ideas, but inspect the pin rollers on either side of the film gate and make sure there isn't any dirt or junk in the back (perhaps shed from the felt film trap) that could be getting caught between the large rollers, the film, and the pin rollers.

 

[petrk]

I had similar occasional scratches. The cause was an amateurishly installed velvet strip that seals the back against the light. Adjusting the seals fixed the problem, at least so far.

 

[Steven Lee]

@petrk was it a GS-1 back?

 

[petrk]

Yes, sorry i forgot to mentione it.

 

[Sirius Glass]

Photographs of the negatives might help although it sounds as though the scratches are lengthwise on the film. Yes? No?

 

[mshchem]

I had similar occasional scratches. The cause was an amateurishly installed velvet strip that seals the back against the light. Adjusting the seals fixed the problem, at least so far.

This sounds like a promising idea. Good idea to inspect. I've settled on Hasselblad and the seals of the older backs can deteriorate get very grimy.
Good idea!

 

[Steven Lee]

@Sirius Glass I posted the high resolution scan above.

 

[Sirius Glass]

Photographs of the negatives might help although it sounds as though the scratches are lengthwise on the film. Yes? No?

@Sirius Glass I posted the high resolution scan above.

Sorry, I missed the scan.

 

[Steven Lee]

@Sirius Glass if you wanted to see the negative photo without an inversion, something like an iPhone taking a picture of a negative on a light table, you won't see the scratches. I can hardly see them with a 12x loupe. They are extremely fine: as you can see in the 8K scan a scratch only takes 2-3 pixels.

 

[Sirius Glass]

@Sirius Glass if you wanted to see the negative photo without an inversion, something like an iPhone taking a picture of a negative on a light table, you won't see the scratches. I can hardly see them with a 12x loupe. They are extremely fine: as you can see in the 8K scan a scratch only takes 2-3 pixels.

Good, I thought that my vision was suddenly getting bad!

 

[MattKing]

You might try running some backing paper removed 120 film (or 220 film I guess) through the camera, to see if that makes the scratches more visible.
I'm assuming you haven't recently started wearing shirts with scratchy buttons when you load your film reels .
My Dad had a story about chasing down scratches in the Kodachrome lines at the Kodak Canada processing lab where he was customer service manager for a couple of decades. They eventually determined they were caused by a very small change in how one of the women who worked (in total darkness) in the pre-splice area. That required opening each 35mm cassette and splicing the customer film on to the mile of spliced film that was then loaded (with a mile each of leader and trailer) into the processing machines.

 

[Steven Lee]

Hehe, I have unknowlingly tested for this theory! I've exposed 3 rolls on 3 different backs wearing the same shirt. I quite hopeful that petrk's clue will work out. There's indeed a light seal material near the emulsion side. Looking at the geometry of the back it doesn't seem like it should touch it, but if there's some kind of slack/bulging... who knows.

 

[Steven Lee]

Quick update: I fixed the scratching.

However, I wasn't able to determine the exact cause. Geometrically it is quite obvious that the only surface capable of touching the emulsion side is the back's interior wall around the spools. One of them features the light seal mentioned by @petrk

However, when the film roll is mounted, there's clearly a few millimeters of space between the film path and the interior wall. So my theory goes: if there's some slack in the film it may bulge outwards touching the back's interior wall.

I have glued some camera light seal foam onto those surfaces, and also tightened the leaf spring which keeps pressure on the film roll to prevent it from giving slack. I do not know which one of these actions did it, but I just developed another test roll of Kentmere 100 and there are no scratches.

Documenting it for future generations