Confusion about ND filter density, transmission and uses.

[Premier]

Hi, It seems that I am trying to understand something puzzling.

I have read @Bill Burk's resource for using a .30 ND filter to confirm film development. What confuses me, and I hope someone can clarify, is why his use of a .30 ND filter is called for when he is making the difference two stops between compared negatives to verify film development. As I understand it, according to Ansel Adams in The Negative, 1/3 of a stop is equivalent to .10 density of additional exposure on a negative (pg. 86). By that reasoning, shouldn't one stop be equivalent to .30 ND filtering (which AA also states on the same page), and so two stops require a .60 ND filter? My quandary/question is bolstered by use of a .60 ND filter on my Beseler 23C condenser enlarger. I place one of these in the contrast filter tray to multiply exposure times by 4, or otherwise put reduce light transmission by two stops. So why should Bill's test to verify film development require a two stop bracketing shot and a .30 ND filter?

Am I misunderstanding something about log exposure units, or rather something about enlargers?

I am very new to all of this, and trying to orient myself. Thanks for suffering a puzzled reader!

 

[MattKing]

@Bill Burk might be the best person to ask! This post will probably alert him.

 

[koraks]

@Premier there's a small catch that's hinted at here in @Bill Burk's pdf:

If the result reveals both negatives look the same, the development has a good, approximately 50 percent gradient

So what he's aiming for is a "50% gradient", or a slope of 0.5. I.e. 1 unit of change in exposure will have 0.5 units of change in negative density.
This means that if you give 2 stops more exposure, you should get 1 stop more density in the negative, = 0.3logD.

 

[xkaes]

Since I haven't read Bill's writings, I have no way of knowing if he is suggesting a 0.5 slope. He might only be trying to give a simple example.

That said, just as it's important to determine your own ISO for your films and developers and technique and gear, it's just as important to determine your own C.I. for your papers and developers and technique and gear.

 

[RalphLambrecht]

Hi, It seems that I am trying to understand something puzzling.

I have read @Bill Burk's resource for using a .30 ND filter to confirm film development. What confuses me, and I hope someone can clarify, is why his use of a .30 ND filter is called for when he is making the difference two stops between compared negatives to verify film development. As I understand it, according to Ansel Adams in The Negative, 1/3 of a stop is equivalent to .10 density of additional exposure on a negative (pg. 86). By that reasoning, shouldn't one stop be equivalent to .30 ND filtering (which AA also states on the same page), and so two stops require a .60 ND filter? My quandary/question is bolstered by use of a .60 ND filter on my Beseler 23C condenser enlarger. I place one of these in the contrast filter tray to multiply exposure times by 4, or otherwise put reduce light transmission by two stops. So why should Bill's test to verify film development require a two stop bracketing shot and a .30 ND filter?

Am I misunderstanding something about log exposure units, or rather something about enlargers?

I am very new to all of this, and trying to orient myself. Thanks for suffering a puzzled reader!

I'll leave the "why Bill..." part for Bill to answer, but I can tell you that your understanding of 0.1 density being 1/3 stop is correct. 0.3 density being a stop and 0.6 being 2 stops is correct.
N=
x=10^d
N=log2(x)
So, you must first calculate the extension factor x and from that the equivalent f/stop.

 

[Chan Tran]

Hi, It seems that I am trying to understand something puzzling.

I have read @Bill Burk's resource for using a .30 ND filter to confirm film development. What confuses me, and I hope someone can clarify, is why his use of a .30 ND filter is called for when he is making the difference two stops between compared negatives to verify film development. As I understand it, according to Ansel Adams in The Negative, 1/3 of a stop is equivalent to .10 density of additional exposure on a negative (pg. 86). By that reasoning, shouldn't one stop be equivalent to .30 ND filtering (which AA also states on the same page), and so two stops require a .60 ND filter? My quandary/question is bolstered by use of a .60 ND filter on my Beseler 23C condenser enlarger. I place one of these in the contrast filter tray to multiply exposure times by 4, or otherwise put reduce light transmission by two stops. So why should Bill's test to verify film development require a two stop bracketing shot and a .30 ND filter?

Am I misunderstanding something about log exposure units, or rather something about enlargers?

I am very new to all of this, and trying to orient myself. Thanks for suffering a puzzled reader!

He used the 0.30 density filter to evaluate the negative density. When you increase the camera exposure by 2 stop the actual density of the negative increases about 1 stop because the negative film has the slope close to 0.5. (I think it's more like 0.6).

 

[koraks]

Since I haven't read Bill's writings, I have no way of knowing if he is suggesting a 0.5 slope.

It's one or two pages. I edited a link into post #1. Give it a read, you'll see he literally mentions a .5 slope. There's no need to second guess here; he was very clear about it.

That said, just as it's important to determine your own ISO for your films and developers and technique and gear, it's just as important to determine your own C.I. for your papers and developers and technique and gear.

That's a whole different topic though. What @Bill Burk attempted to do, the way I read it, is give a quick, practical and easy method to determine whether your film development is in the ballpark of being correct. A quick read of the source material would preclude the need to reiterate what he has explained himself very clearly.

 

[DREW WILEY]

Density is calculated logarithmically. Each .30 of added density equates to one extra EV of needed exposure (either open up one stop or give double the exposure time). There are other modifying factors : long exposures also need to be corrected for film reciprocity failure; (20) labeled density might not be the actual density of the filter involved (depends on the quality control of the filter brand involved).

I'm not going to comment on the specific application of the theory in this case. That's yet another set of variables ("slope", gamma, characteristic curve shape, etc). I don't buy into generic statements about this kind of thing anyway,
since black and white films can differ quite a bit from one another.

 

[Bill Burk]

Everyone’s right.

It’s just a ballpark feeler gauge.

I kind of wandered away from this project because the filters aren’t cheap enough for me to cut and mail to anyone who wants them, and it was harder than I thought it would be to describe a light box that fulfills the same purpose.

 

[xkaes]

Since it's just a "ballpark", there's no need to buy a Wratten 0.3 ND filter -- that you might never use again. Any 2X ND filter -- that most shutterbugs have -- will keep you in the ballpark.

 

[Bill Burk]

Sure.

Another approach I tried was putting a box over my light box with a divider under a frosted acrylic surface (a clipboard from Office Depot) and filling one side with cotton balls until the meter reads one stop’s difference.

Also tried printing a checkerboard on overhead transparency material.

If anyone can dream up an easier way I would be interested.

 

[Premier]

Thank you for all of your replies.

Through them, I understand that my confusion is about development rather than filters, and that "normal development" (50% to 60% slope) aims to compress the density range of what might be a 0.0-3.0 log scale to a density range of 1.50. I presume that this is in order to make them conform to film sensitivity and paper sensitivity technologies.

I will be reading and re-reading the many great books that form the basis of your knowledge, and I understand that so many of you have studied and even written!

I appreciate your willingness to kindly respond and make more understandable the complexity of the science involved in photography!

 

[Bill Burk]

Yes, you understand exactly.

 

[koraks]

"normal development" (50% to 60% slope) aims to compress the density range of what might be a 0.0-3.0 log scale to a density range of 1.50. I presume that this is in order to make them conform to film sensitivity and paper sensitivity technologies.

Yes. Very simply put, the negative is tailored to suit the output medium. For B&W negatives, that means in practice, and for the most part, a density range that comfortably fits in the range of 'paper grades' offered by variable contrast paper, or historically as discrete variants.

From this you can deduce that there's no absolute 'truth' - there's no specific gamma or slope that's "correct". It depends on the output medium, and, if that's not very flexible, the scene brightness range. For most B&W photography, you can basically ignore this, but it becomes a key issue if you start working with fixed-contrast output media, like many 'alternative' printing processes. E.g. classic cyanotype works well with a less contrasty negative than what you'd typically get with a slope of 0.6 (unless the original scene happens to be on the flat side), whereas salted paper demands a much steeper slope in the negative.

 

[xkaes]

I will be reading and re-reading the many great books that form the basis of your knowledge, and I understand that so many of you have studied and even written!

If it's not on your list already, I'd suggest Richard Henry's "Controls in Black and White Photography". He advises photographers to run their own tests -- because everyone's conditions are different -- and starts out FIRST by testing the paper, the end product.

It's up to you how easy or complicated you want to make it.