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Bleach bypass vs partial bleach bypass?


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I heard the term partial bleach bypass thrown around the internet a lot. I’m aware of various kinds of silver retention processes, a full skip bleach being and obvious one, But I’m also aware of more controlled silver retention processes used in the past like ENR, and processes that used to be offered at Deluxe (CCE and ACE), where B&W developer baths were used in between the bleach and fix stages.
 

But what is a “partial” bleach bypass which seems to still be offered nowadays? Does it retain a percentage of silver through some other means? Is it like a diluted or shorten bleach duration?

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Posted (edited)

We have done this in the past, ECN2 has allot of bleach and if you run some tests you can bypass enough racks to get partial retention.

It takes most of the bleach racks out if I remember correctly from when Bob did it so the film runs through about 1/5 of the normal bleach time.

Edited by Robert Houllahan
typo
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Posted (edited)
12 hours ago, Robert Houllahan said:

We have done this in the past, ECN2 has allot of bleach and if you run some tests you can bypass enough racks to get partial retention.

It takes most of the bleach racks out if I remember correctly from when Bob did it so the film runs through about 1/5 of the normal bleach time.

Great thanks Bob.

Is this silver retention process controlled to a set aim? As I understand this can be achieved with an infrared densitometer. Is a particular relative percentage of silver retention expected from a 1/5 bleach duration?

Edited by Seth Baldwin
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No set AIM for a partial bypass it is a test to suit process and thus kind of labor intensive.

We ran a series of tests for the client and they settled on the one they wanted, it was not exactly linear in terms of number of racks bypassed as the ECN2 process has "extra" bleach in the process design most likely for consistency in process as the bleach gets more exhausted.

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Partial Bleach Bypass is mostly a myth. If you just dip the film into the bleach for a few seconds, yes, then you will have a set amount of silver retention. The time needs to be extremely short though. Less than 15 seconds. This may introduce uniformity issues though. Not only according to me, but also according to Kodak, 95% of the bleaching action happens in the first 30 seconds of the bleach. With camera original negative, a partial bleach bypass is next to impossible. Current UL ECN-2 kit bleach is super active and holds up very well. If you'd want to run a partial BBP, you'd most likely have to use a modified bleach, or a more diluted ferri bleach at lower temperature. If a lab runs on ECN-Kit chems, no one will even give a thought to it.

It is not impossible, but near impossible given the ramifications of day to day lab operation. Very hard to do on scale, and with a self set aim. 

Additionally, bleach bypass as a whole is very proportional to the actual image on the film. If you have a dimly lit night scene you won't notice the bbp at all, while at a bright day scene it will be all the more noticeable. 

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In case anyone is interested: Here is a pretty much definitive guide on several silver retention techniques. Please note, before you add "but you said partial camera original neg silver retention is impossible": I was referring to actually skipping or shortening the bleach time. ENR, which is resilvering the neg, is possible and controllable. This basically means that after bleaching, which does nothing else than rehalogenize the metallic silver, you can redevelop a tiny amount of that rehalogenized silver before fixing, resulting in a more controllable amount of metallic silver in the negative. 

cheers

https://theasc.com/magazine/nov98/soupdujour/pg3.htm

 

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13 minutes ago, Ludwig Hagelstein said:

In case anyone is interested: Here is a pretty much definitive guide on several silver retention techniques. Please note, before you add "but you said partial camera original neg silver retention is impossible": I was referring to actually skipping or shortening the bleach time. ENR, which is resilvering the neg, is possible and controllable. This basically means that after bleaching, which does nothing else than rehalogenize the metallic silver, you can redevelop a tiny amount of that rehalogenized silver before fixing, resulting in a more controllable amount of metallic silver in the negative. 

cheers

https://theasc.com/magazine/nov98/soupdujour/pg3.htm

 

Very interesting, thanks for sharing these resources. I've heard about that silver retention demo from Fotokem, but I can't seem to find it anywhere on the internet? Did they ever publish these demos publicly because I'd love to see them.

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On 7/28/2022 at 7:52 PM, David Mullen ASC said:

Most silver retention processes are for projection prints, so a video transfer isn't really of much use for understanding how they looked on the big screen.

I`d argue that modern scanners make BBP with camera original material more viable though in a shoot, develop, scan, hybrid workflow - and creatively relevant - than in the past, when either scanners weren't that good or you had to print through too much highlight density with too little shadow density at the same time, resulting in too strong effects. 

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7 hours ago, David Mullen ASC said:

I’m just saying that the Fotokem demo was of projection contrast prints so a telecine transfer or scan of them would not necessarily look accurate on a monitor to how they looked projected. 

of course. Just continued the chain of thought. Have you ever contemplated doing BBP with camera original material?

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