Homemade BTZS tubes, excessive 'center' density

[ken_miller3]

I've been using a set of homemade BTZS tubes made from ABS pipe. They work fine, except every now and then a 4x45 negative exhibits slightly higher density in the *center*. Edge density is fine.

 

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Anyone have any idea how this can happen?

 

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Let me stress that the density is in the center of the negative, and is not a 'streak' running width-wise across the negative. If you look at a negative, you'll a circular section (about 2" in diameter) of slightly higher density in the center of the negative.

 

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A friend who has a Jobo couldn't explain it either.

 

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Ideas?

 

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BTW: HP5+ 4x5 sheets, Ilfotec HC 1+31, 7:30.

[jorge_gasteazoro]

Could it be your lens? that you need a center filter? If you are

using some of the wide angle lenses and you are seeing higher

densities in the center, and it is a circle then I say it is not the

developing but the lens.

If it walks like a duck, quacks like a duck, and swims like a

duck...maybe it is a duck..:-))

[jim_chinn1]

Ken,

 

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A few years ago a made a set of rollers and tubes for 8x10 negs from

ABS. I had a similar problem, especially when longer development was

required. My rollers allowed the tube to travel laterally while it

was being rolled. A friend suggested that if the movement was

excessive back and forth, the developer would slosh slightly side to

side and in the middle their would be an increase in turbulence where

the wave fronts met. He was a mechanical engineer, and I don't know

if he really knew anything about fluid dynamics, but after I put a

couple of stops on each end to hold the tube stationary while rolling

the problem was eliminated. If you notice sideways "walking" of the

tube this may be your problem.

[ken_miller3]

Jorge:

 

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The lens is a Roddenstock Grandagon-N 75mm. I have noticed some

falloff around the ledges, but never excessive density in the middle.

I've taken a reasonable number of shots with the lens, and have not

noticed this center density in all the shots.

 

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James:

 

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I run the tubes in a fairly narrow container, and so they don't have

a lot of movement left-to-right. However, I'll try and make sure that

the rotation is a bit 'smoother', and that I don't bounce the tubes up

and down too much (which may cause the sloshing you mention).

 

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Thanks!

[jim_chinn1]

Ken i apologize for not reading the post abit more closely. I would

agree with Jorge that it may be an exposure problem since you

describe the density change in the middle of the neg.

One other possibility could be film holders. I recently bought a 4

used riteway film holders and after using them had a couple negs with

the same problem you describe but showing overexposure indicating

some sort of a light leak. In any event once a narrowed it done to

the offending holders the problem was eliminated. Just another

possibility.

[ken_miller3]

No problem James.

 

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I'm starting to suspect that the 75mm is the cause. I was under the

impression that you would only see fall-off if you did any sort of

movements, but I guess that isn't the case; the fact that you can by

center grads for the 75mm illustrate that point. Searching on Usenet

yields lots of hits on just tihs subject...

 

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I will, however, veryify that it's not the holders that are causing

the problem. I'll take a test shot or two of a single-colored object

(a wall) and inspect for fall-off.

 

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We'll get there.