Effective film speed (as in Lambrecht's method)

aparat · Feb 12, 2023

Bill Burk said:
The fallacy with BTZS sliding EI with development comes from it’s assumption that the 0.1 density above base+fog is the point where exposure is to be indexed.

It’s not. It’s just an easy point to find.

The point to index from is the point where the curve reaches a third of its average gradient.

Our friends at Kodak went to a lot of trouble to find out this tidbit of info, but finding this third of average gradient was hard. DIN campaigned with the standards organizations to call the speed point 0.1 above base plus fog. Any darkroom tech could find that.

They compromised in the end and said “fine”. You can have the 0.1 speed point but only when you develop the film to a specific contrast. Any other contrast doesn’t count.

When film is developed to ASA parameters the 0.1 “speed point” is 0.29 away from the one-third average gradient point.

That’s really the point under which you don’t want any shadow of your picture to fall, and the ASA/ISO speeds all make it so (for black and white negative film).

This actual point Kodak says we’re looking for is further down the toe, where the curve is turning so flat you can’t get anything more out of it.

If you develop film less, the whole curve gets flatter. You print on higher contrast paper that can get detail out of the slight detail, you can slide further down from the 0.1 speed point.

If you develop more, then you need to use lower contrast paper (assuming normal contrast subject and the goal to make an excellent print - not talking of pushing when you don’t care about print quality so much). Now your detail falls off closer to the 0.1 speed point.

In other words, effective speed of the film is fairly inflexible, it’s pretty close to the rated film speed under a wide range of development times.

Stephen Benskin wrote about this in threads about Delta-X.

If you accept this idea then you can just call the speed only when it’s met ASA parameters and don’t change the speed for different development times.

Bill, thank you for this thorough explanation. I just want to make a couple of additional points, if you don't mind. Phil Davis recommends deriving EFS values not from the 0.1 criterion, but from what he calls "fractional gradient IDmin values." His definition of "fractional gradient" speed is a little idiosyncratic, but it works within the BTZS system. In fact, almost everything Davis recommends in his book should be, in my opinion, considered within the BTZS system, rather than as universal sensitometric concepts. Otherwise, we get discrepancies, inaccuracies, and potential confusion. For example, the fractional-gradient-based EFS is best understood as potentially leading to higher quality negatives, i.e., negatives that are less prone to underexposure errors, rather than being "theoretically correct," if applied withing the BTZS exposure-development system. That is Phil Davis's message.

Regarding the idea of Effective Film Speed, in general, it is fairly well-documented in the literature, as these relative lateral speed-point displacements have been observed within curve families. It's what one does with that information that matters, as you point out.

Stephen Benskin · Feb 12, 2023

Bill Burk said:
The fallacy with BTZS sliding EI with development comes from it’s assumption that the 0.1 density above base+fog is the point where exposure is to be indexed.

It’s not. It’s just an easy point to find.

The point to index from is the point where the curve reaches a third of its average gradient.

Our friends at Kodak went to a lot of trouble to find out this tidbit of info, but finding this third of average gradient was hard. DIN campaigned with the standards organizations to call the speed point 0.1 above base plus fog. Any darkroom tech could find that.

They compromised in the end and said “fine”. You can have the 0.1 speed point but only when you develop the film to a specific contrast. Any other contrast doesn’t count.

When film is developed to ASA parameters the 0.1 “speed point” is 0.29 away from the one-third average gradient point.

That’s really the point under which you don’t want any shadow of your picture to fall, and the ASA/ISO speeds all make it so (for black and white negative film).

This actual point Kodak says we’re looking for is further down the toe, where the curve is turning so flat you can’t get anything more out of it.

If you develop film less, the whole curve gets flatter. You print on higher contrast paper that can get detail out of the slight detail, you can slide further down from the 0.1 speed point.

If you develop more, then you need to use lower contrast paper (assuming normal contrast subject and the goal to make an excellent print - not talking of pushing when you don’t care about print quality so much). Now your detail falls off closer to the 0.1 speed point.

In other words, effective speed of the film is fairly inflexible, it’s pretty close to the rated film speed under a wide range of development times.

Stephen Benskin wrote about this in threads about Delta-X.

If you accept this idea then you can just call the speed only when it’s met ASA parameters and don’t change the speed for different development times.

I pretty much agree with everything with only a slight caveat on the last sentence. All curves have a correlating film speed, it's just that changes are minimum when defined using the Delta-X criteria and it's not really necessary to adjust the camera EI for most changes to development. Especially with increased development. This is because a fixed density criterion tends to underrate films that are developed to a lower average gradient and to overrate films that are developed to a higher average gradient. The Delta-X criterion uses the fixed density of 0.10 over film base plus fog as a point to determine the fractional speed point based on certain contrast parameters. In and of itself, it is not the speed point. The ISO standard appears to only use a fixed density method, but that's because the Delta-X equation is built into the contrast parameters. Any processing differing from that specified in the standard needs to use the Delta-X equation in order to determine accurate speeds.

The reason film speed doesn't change as much with the Delta-X method as the fixed density method is because the fractional gradient speed point tends to move in an inverse direction as compared to the fixed density method in response to changes in development. It moves to the right with increased development while the fixed density point moves to the left. This tends to compensate for any apparent speed increases. On a side note, this brings into question the concept of a true proof as the negatives will not have a consistent shadow density with different levels of development. Nor should it as density isn't determinative in defining quality. It's about contrast: overall and local.

And here is an example of speeds obtained using the fixed density method and Delta-X method.

Bill Burk · Feb 12, 2023

aparat,

Can you point me to this? "fractional gradient IDmin values."

I have a couple Davis’ references but didn’t see this.

Chuck_P · Feb 12, 2023

My 2 cents.....the BTZS WinPlotter analysis allows for factoring the effect of subject flare up to a full stop so that the final curve has information that you can apply in actual photographing.

aparat · Feb 12, 2023

Bill Burk said:
aparat,

Can you point me to this? "fractional gradient IDmin values."

I have a couple Davis’ references but didn’t see this.

Sure. Sorry, I should have included a citation. In my book (Third Edition), it's on pp. on pp. 93-94 and 102-103, with regard to EFS.
In my program, I have tried to reverse-engineer Win Plotter, and, by and large, I can set it up to give virtually identical analysis. But I also implemented many other methods, just to let the user decide what to use. The BTZS system is very opinionated, as I am sure you know. That is my main criticism of it. It requires a very literal application of its concepts to benefit from. I tried it a few years ago, and was quite pleased with the results. But then again, any other decent film exposure-development-printing methodology/system can be used successfully.

aparat · Feb 12, 2023

Chuck_P said:
My 2 cents.....the BTZS WinPlotter analysis allows for factoring the effect of subject flare up to a full stop so that the final curve has information that you can apply in actual photographing.

Yes, it does. I implemented a very similar concept of flare density in my program. Whether to use it that way or not is subject to debate. You'll read different opinions here on Photrio.

I am currently working on a project comparing film curves obtained with and without lens flare to see if I can get the same kind of flare density effect as the theory predicts. It'll be a few weeks before I am ready to present the analysis.

Bill Burk · Feb 12, 2023

Thanks, my copy is older. It didn’t have flare.

I’d look to Stephen Benskin’s explanation or pointers to references about flare.

He might agree, despite your best plans, flare will keep you from accurately predicting the results you are going to get in real life.

I look at a fixed flare chart that shows an aim contrast of 0.62 for a 7 stops subject luminance range, with 1 1/3 stop flare, to grade 2 paper in a diffusion enlarger.

I like the simplicity of the fixed flare model but Stephen has shown me a better, more practical flare model.

Stephen Benskin · Feb 12, 2023

Doremus Scudder said:
Bill,

A question for you (and maybe Stephen Benskin if he feels like addressing it):

Both you and Stephen advocate a testing method that removes flare from the equation. What, then, do we do with our new-found film speed when we go out to make photographs in real-life situations? Do we just ignore the flare, which is sure to be there to some extent or other? Or do we compensate for it somehow?

My more empirical testing, as you likely know, uses all the flare inherent in my system: taking lens, enlarger lens, etc. I'm doing just fine with exposure and development using my methods, I'm just curious how the sensitometry under controlled circumstances gets translated into practice. Are we really just ending up with a lot of pre-flashed negatives due to camera flare regardless of how we rate our film?

TIA,

Doremus

Doremus, it's great that you have a method that works for you, and if your purpose is to have greater control over your personal shooting then there's nothing more required, but that is a different subject than one on testing methodology, and we shouldn't conflate the two. This is a frequent occurrence in many of these types of discussions. Discussing a film speed methodology, for example, is different than determining what EI setting to use, even though there is a lot of other lap. Almost without fail, whenever I write about film speed, a subsequent post will counter with how they obtain good results another way. It's in effect changing the subject.

If you notice, I don't advocate doing any testing for speed if the end purpose is to shoot. There are way too many variables and bad techniques to come up with anything determinative. You don't have to look any farther than ISO vs Zone System testing methodology for san example. My suggestion has rather consistently been to make film curves to determine development and then simply going out to shoot to determine an EI. Determining film speed; however, is done under no flare conditions. Two different subjects.

To test a film, it's best to eliminate as many variables as possible in order to be testing only the film's characteristics. Flare is part of the camera system and should be kept as a separate variable. Just because film speed is determined under no flare conditions doesn't preclude it's interpretation includes real world conditions or that those conditions cannot be factored in. Testing breaks everything down into it's constituent parts which can then be reintegrated after they are defined.

If you create a film curve that contains flare, how do you control the amount of flare, or determine what degree of flare should be applied, or how to communicate the amount to others? Without flare, you have how the film curve responds to various levels of Illuminance when developed to specified development conditions. Flare can then be applied along with camera Illuminance which is why Jones invented the tone reproduction diagram, and on the seventh day, he rested.

aparat · Feb 12, 2023

Bill Burk said:
Thanks, my copy is older. It didn’t have flare.

I’d look to Stephen Benskin’s explanation or pointers to references about flare.

He might agree, despite your best plans, flare will keep you from accurately predicting the results you are going to get in real life.

I look at a fixed flare chart that shows an aim contrast of 0.62 for a 7 stops subject luminance range, with 1 1/3 stop flare, to grade 2 paper in a diffusion enlarger.

I like the simplicity of the fixed flare model but Stephen has shown me a better, more practical flare model.

Thank you! Great idea. I had already implemented fixed, variable, and Stephen's practical flare models, with any enlarger type (condenser, diffusion, LED, etc.). I still want to test the theory with my own flare data using the method described by Henry (Controls in Black and White Photography, p. 160).

Chuck_P · Feb 12, 2023

Bill Burk said:
You can download and use Ralph Lambrecht’s spreadsheet to input your results. It’s a great tesoirce.

BTZS method of sliding the EI with development time is due to a fallacy.

Where can I get it...maybe Ralph will respond.

Chuck_P · Feb 12, 2023

aparat said:
The BTZS system is very opinionated, as I am sure you know. That is my main criticism of it. It requires a very literal application of its concepts to benefit from. I tried it a few years ago, and was quite pleased with the results. But then again, any other decent film exposure-development-printing methodology/system can be used successfully.

I have been getting the feeling myself that I'm not sure the BTZS program is what I need to be using. I'm not sure I understand the Personal Speed Point value I am supposed to input between a log exposure 2.1 and 2.4.........and I wish I could change the horizontal axis to read in increasing log exposure from left to right starting at 0.0.

aparat · Feb 12, 2023

Chuck_P said:
I have been getting the feeling myself that I'm not sure the BTZS program is what I need to be using. I'm not sure I understand the Personal Speed Point value I am supposed to input between a log exposure 2.1 and 2.4.........and I wish I could change the horizontal axis to read in increasing log exposure from left to right starting at 0.0.

You can use Win Plotter to get you a lot of very useful information, but some of the details are kind of idiosyncratic, as @Bill Burk and others pointed out. I do consider it to be an excellent tool.

The idea of the Personal Speed Point (PSP) is very simple in theory, but hard in practice. It is a way of calibrating your curve family (and the entire program) to create a speed point reference, based on which film speed of this and all other films can be estimated. You need to pick a film and developer that gives a certain speed. For example, Ilford Delta in XTOL consistently gives me ISO 100. If you follow the BTZS guidelines, your Delta 100 speed point will fall somewhere along the exposure axis, and that will be your reference, i.e., your PSP (it "should" fall somewhere between 2.3 and 2.5). From now on, you can use the PSP to estimate the speed of other films, in reference to your PSP and Ilford Delta 100. That is it, in a nutshell. I would be more than happy to help you along if you are willing to stick with film testing and Win Plotter (or any other tool). You can also reach out to Fred Newman at View Camera Store. He is familiar with the BTZS method probably more than any photographer out there.

Chuck_P · Feb 13, 2023

aparat said:
You can use Win Plotter to get you a lot of very useful information, but some of the details are kind of idiosyncratic, as @Bill Burk and others pointed out. I do consider it to be an excellent tool.

The idea of the Personal Speed Point (PSP) is very simple in theory, but hard in practice. It is a way of calibrating your curve family (and the entire program) to create a speed point reference, based on which film speed of this and all other films can be estimated. You need to pick a film and developer that gives a certain speed. For example, Ilford Delta in XTOL consistently gives me ISO 100. If you follow the BTZS guidelines, your Delta 100 speed point will fall somewhere along the exposure axis, and that will be your reference, i.e., your PSP (it "should" fall somewhere between 2.3 and 2.5). From now on, you can use the PSP to estimate the speed of other films, in reference to your PSP and Ilford Delta 100. That is it, in a nutshell. I would be more than happy to help you along if you are willing to stick with film testing and Win Plotter (or any other tool). You can also reach out to Fred Newman at View Camera Store. He is familiar with the BTZS method probably more than any photographer out there.

Thanks. The reason I've become unsure about it is because I'm not going to be using my enlarger to contact the step tablet and he (Davis) seems to put all the emphasis on doing that to be able to accurately use the PSP value.

Bill Burk · Feb 13, 2023

PSP or personal speed point sounds like relative calibration of sensitometer.

If you’re not “making” a sensitometer you will have to work hard to be consistent with your exposure.

Just “check your PSP” frequently and if it doesn’t change much then you are probably alright.

You might add a UV filter to the test camera (and when shooting) to rule out seasonal variations in the amount of UV in sunlight.

aparat · Feb 13, 2023

Chuck_P said:
Thanks. The reason I've become unsure about it is because I'm not going to be using my enlarger to contact the step tablet and he (Davis) seems to put all the emphasis on doing that to be able to accurately use the PSP value.

I didn't know that. Sorry. Actually, I am currently working on a project comparing contact and projection making of test negatives. I'll share my results once they're available. From what I have seen so far, contact printing gives higher contrast, but that's all I can say so far, a very preliminary finding. Good luck and keep at it. I will be curious to see what you find out. And yes, you can still use PSP, even if you're not contact printing the negs.

aparat · Feb 13, 2023

Chuck_P said:
Thanks. The reason I've become unsure about it is because I'm not going to be using my enlarger to contact the step tablet and he (Davis) seems to put all the emphasis on doing that to be able to accurately use the PSP value.

I just looked it up, HP5 Plus in XTOL-R gives CI of 0.47 with the Lambrecht method and 0.7 with my sensitometer (11 minutes development). Again, this is very preliminary. The difference in contrast in mostly due to lens flare. I was using my Pentacon Six Tl 80 mm lens with extension tubes, photographing the step tablet on a light table. Your results will be different due to your lens having different flare resistance. Please, do not take these results too seriously, I am still not finished.

Chuck_P · Feb 13, 2023

aparat said:
And yes, you can still use PSP, even if you're not contact printing the negs.

Good to know. In the Plotter, I am choosing the 0.1 fb+f fixed density criterion, that's what I want to use instead of CI. My mind is geared to view it in ZS terms.......not trying to be informative here just saying......the personal EI that gives a density of 0.1 above fb+f density at Zone I. My intention is to use the Plotter to graph just net densities, subtracting fb+f density (Schafer method), just like I've graphed them some years ago when hand drawing them.......having the added benefit of being able to factor in a reasonable flare factor. Something I failed miserably at years ago, but grasp much better now. Can I do this with the Plotter and it still be effective, or am I pissing into the wind, as they say, lol.

Perhaps I'm viewing the PSP incorrectly......the way the horizontal axis is labeled from left to right (3.0 down to 0.0 in 0.1 intervals..... and even going negative), I think 2.7 should be Zone I but the BTZS Plotter won't let me choose a PSP log value of 2.7. Am I looking at it incorrectly......that's probably a rhetorical question, I must be.

No matter how the X-axis is labeled in graphs that are presented, I always convert it to zones, and from left to right, 2.7 seems it should be Zone 1......since major log exposure divisions of 0.3 are a one zone change, unless I'm missing something, that should be easy to determine, right? I'll stop here, feels like I'm rambling.

Bill Burk · Feb 13, 2023

aparat said:
I didn't know that. Sorry. Actually, I am currently working on a project comparing contact and projection making of test negatives. I'll share my results once they're available. From what I have seen so far, contact printing gives higher contrast, but that's all I can say so far, a very preliminary finding. Good luck and keep at it. I will be curious to see what you find out. And yes, you can still use PSP, even if you're not contact printing the negs.

When I hear people talk of using an enlarger for film sensitometry I understand they are talking about contact printing under the enlarger.

In-camera testing is done a few different ways but I assume Chuck_P is placing film in contact with the step wedge in the film holder.

Some tests ask you to tape a step wedge to glass and photograph it. That’s certain to introduce flare. Projecting a Stouffer scale by enlarger, as if it were a negative, will also introduce flare. These are good ways to study how much flare you have in the “tone reproduction cycle.” You can compare to tests that you do without flare.

Bill Burk · Feb 13, 2023

Chuck_P,

While I say it’s based on a fallacy when you change EI with development, I don’t think it will hurt the image quality if you change EI with development.

If you enter spotmeter readings into a Palm Pilot and it recommends f/stop and shutter speed based on a sliding EI scale, it will be fine.

But if you’re on the edge of a cliff holding onto a branch of a snag and trying to get a shot quickly. Just base exposure on the speed of the film, take the shot and sit down in the shade afterwards to figure out the contrast range and development.

That’s what I did for this shot:

Chuck_P · Feb 13, 2023

Bill Burk said:
In-camera testing is done a few different ways but I assume Chuck_P is placing film in contact with the step wedge in the film holder.

Yes, that's what I do, following the procedure by John P. Schafer in the AA Guide Book 2.

And no Palm Pilot here....

Sirius Glass · Feb 14, 2023

Doremus Scudder said:
Bill,

A question for you (and maybe Stephen Benskin if he feels like addressing it):

Both you and Stephen advocate a testing method that removes flare from the equation. What, then, do we do with our new-found film speed when we go out to make photographs in real-life situations? Do we just ignore the flare, which is sure to be there to some extent or other? Or do we compensate for it somehow?

My more empirical testing, as you likely know, uses all the flare inherent in my system: taking lens, enlarger lens, etc. I'm doing just fine with exposure and development using my methods, I'm just curious how the sensitometry under controlled circumstances gets translated into practice. Are we really just ending up with a lot of pre-flashed negatives due to camera flare regardless of how we rate our film?

TIA,

Doremus

Flare is just part of the optics, I just learn to live with it.

tih · Feb 15, 2023

Chuck_P said:
Where can I get [Ralph Lambrecht's film evaluation spreadsheet]...maybe Ralph will respond.

I have the spreadsheet exactly as he published it back in 2010, along with the PDF of the relevant pages from Way Beyond Monochrome that describe how to use it. (He made both files available on the web back then.) I've also fixed a minor bug in the spreadsheet in the version I use.

I've recently tried to find those old Way Beyond Monochrome resources on the web, without luck. If none here can point to a new location for them, I can always make the copies I have available.

Doremus Scudder · Feb 15, 2023

Sirius Glass said:
Flare is just part of the optics, I just learn to live with it.

Sirius,

I live with flare just fine, and I've got a certain flair myself

My question to Stephen and Bill was really to find out their opinion on how such careful, flare-less testing relates to real-world practice, especially when it comes to finding a personal E.I. for one's particular meter, lenses, etc. We all know that ISO testing is carefully done, but many of us (especially those that use ZS metering techniques) end up finding that an E.I. different than box speed works better. And, most of us have different development times from one another for the same film in the same developer depending on our own personal tastes and techniques.

While I think it's great to do all the testing without flare, I also think that the results so obtained are not directly applicable to finding a film speed and development time that work well in practice. Much of the determination for those things depends heavily on precisely the variables that were eliminated for the testing procedure.

Got an old, uncoated lens without a hood and a meter that reads 2/3 stop slow? Well, you're going to end exposing more than needed in many cases.

I'm wondering just how Stephen and Bill add flare and other factors back into the equation in order to deal with real-world situations.

Doremus

Chuck_P · Feb 15, 2023

Doremus Scudder said:
..... but many of us (especially those that use ZS metering techniques) end up finding that an E.I. different than box speed works better.

I've certainly experienced that in the past using TMX100 with HC110.

But I have to say that with TMX100 in Xtol stock, that I'm about as pleased as I can be with my important shadow placements at box speed so far, developing at Kodak's recommended time.....and I haven't tested yet but looking forward to that.

Bill Burk · Feb 15, 2023

Flare plugs right into the simple formula that you use to decide the film contrast aim to fit the target paper grade.

Say paper grade 2 has Log Exposure Range 1.05

You have a classically normal Subject Luminance Range of 7 stops, 2.1

Take flare 1 1/3 stops, 0.4

(2.1 - 0.4) / 1.05

0.62 is the aim to develop your film for normal.

Now suppose the scene has one stop less contrast, 6 stops, 1.8

(1.8 - 0.4) / 1.05

0.75 is the aim contrast to develop the film.

Effective film speed (as in Lambrecht's method)

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