Film Speed from development times

[Sirius Glass]

Sirius, that's wrong.
Not only middle gray value and optimal EI change with extended development: even shadows do, in a small amount.
You can see it yourself the day you decide to do that homework.
Speed depends also on the developer.

Juan,
Extending development does not increase film speed (aka sensitivity to light) appreciably.
It does alter the characteristic curve though, which can result in more usable results from exposure that is less than optimum.

As Matt noted, the speed increase is so small that it is not noticeable and can only be shown it best. In real life, there is no noticeable difference.

 

[ic-racer]

You, who has all the sensitometers in the world (except maybe a Kodak Type 1B), shoots a plain target?

Eactly, and that value ( the EI that leads to 0.1 log D) calibrates any sensitometer test of the film for use in the field. It is the missing link between film testing and determining camera settings that make a negative that prints well.

 

[ic-racer]

Now that I think about it, I don't believe I know what you mean by "0.1 log d". Can you explain?

Those are the technical units of the readout of most sensitometers and the units of the Y- axis of the H&D curve. D is density which is the inverse of transmission. That is why the H&D curve gets higher to the right as LESS light can pass through the film.

 

[ic-racer]

In the Fred Picker version, yes one tests for speed BEFORE testing for development. But if one has access to a sensitometer ( even makeshift ) development time can be determined irrespective of the speed test by observing the slope or contrast index and ignoring the X-axis values.

 

[BobUK]

John Finch has an informative channel on Youtube named Pictorial Planet .
He has also written an excellent book called The Art of Black and White Developing.

He explains in detail a simple way of determining film speed for your chosen developer, and in the second video he shows how to determine the developing time for correct contrast.
Have a look at the links

 

[tom williams]

Those are the technical units of the readout of most sensitometers and the units of the Y- axis of the H&D curve. D is density which is the inverse of transmission. That is why the H&D curve gets higher to the right as LESS light can pass through the film.

Understood. And thanks. So, using a densitometer, one measures a density 'D' of the negative uniformly exposed at Zone I, computes the product 0.1*log D .... and does what exactly with that number? I think I'm missing some context or background here.

 

[Juan Valdenebro]

Juan,
Extending development does not increase film speed (aka sensitivity to light) appreciably.
It does alter the characteristic curve though, which can result in more usable results from exposure that is less than optimum.

Sure, Matt: but as you imply and I wrote, in practice it gives us a bit more speed, and that's why I use EI640.
Obviously for soft light only.
Film's real sensitivity, of course can't change.

 

[Stephen Ryde]

John Finch has an informative channel on Youtube named Pictorial Planet .
He has also written an excellent book called The Art of Black and White Developing.

He explains in detail a simple way of determining film speed for your chosen developer, and in the second video he shows how to determine the developing time for correct contrast.
Have a look at the links

I am trying this method as we speak.
I shot a test roll of Ilford Pan F in 35mm and a roll of HP5+ in 120mm
The negatives are drying at the moment. I will do a Dmax test using the fb+f negative on silver gelatin paper and on Agfa Ortho Green xray film to find an iso.

@Andrew O'Neill
Would you be able to read these negatives with your densitometer for me as well?
I would like to know what comes from this angle as well!!!

 

[Andrew O'Neill]

I am trying this method as we speak.
I shot a test roll of Ilford Pan F in 35mm and a roll of HP5+ in 120mm
The negatives are drying at the moment. I will do a Dmax test using the fb+f negative on silver gelatin paper and on Agfa Ortho Green xray film to find an iso.

@Andrew O'Neill
Would you be able to read these negatives with your densitometer for me as well?
I would like to know what comes from this angle as well!!!

It'll cost you one apple fritter.

 

[Andrew O'Neill]

But I'm not bringing my densitometre to Timmy's!

 

[MattKing]

But I'm not bringing my densitometre to Timmy's!

Too bad - that might be the definition of Canadian, photographic nerdism!

 

[Andrew O'Neill]

Too bad - that might be the definition of Canadian, photographic nerdism!

If I bring it along, everyone will want a go! Hmmmm.... how dense is the icing on this Honey Glazed?

 

[Stephen Ryde]

It'll cost you one apple fritter.

Only one!!?!!
I was thinking more like a 6 pack
I have two rolls to measure

 

[Stephen Ryde]

Too bad - that might be the definition of Canadian, photographic nerdism!

 

[Andrew O'Neill]

Only one!!?!!
I was thinking more like a 6 pack
I have two rolls to measure

Are you trying to kill me? 6 pack is fine, but TWO rolls??!!

 

[ic-racer]

Understood. And thanks. So, using a densitometer, one measures a density 'D' of the negative uniformly exposed at Zone I, computes the product 0.1*log D .... and does what exactly with that number? I think I'm missing some context or background here.

0.1 is the value and ‘log D’ is the nomenclature for the units: the logarithm of optical density.

On the diagram below the 0.1 log D point is ‘m’

 

[tom williams]

Got it, thanks. I didn't recognize the nomenclature, which now seems like a natural way to represent what I had filed as the "intercept of the film characteristic curve with the line 0.1 above FB+F", aka the ISO "speed point".

Do you use the manufacturer's H&D curve to place your measured density?

 

[Stephen Ryde]

Are you trying to kill me? 6 pack is fine, but TWO rolls??!!

 

[Bill Burk]

Understood. And thanks. So, using a densitometer, one measures a density 'D' of the negative uniformly exposed at Zone I, computes the product 0.1*log D .... and does what exactly with that number? I think I'm missing some context or background here.

The Arithmetic film speed is 0.8 (a constant) divided by the Arithmetic exposure the film received at the point where it has developed to 0.1 density above filmbase plus fog.

A 400 speed film takes -2.7 log meter candle seconds to meet that measure when developed to the ASA contrast parameters in standard developer like D-76.

Antilog of -2.7 log is 0.002 Arithmetic meter candle seconds.

So plug into the speed formula 400 = 0.8 / 0.002



0.1 density is an easy density to measure. You don’t actually “do” anything with it. But it’s near the area of dark shadow, key black of the print where detail is just emerging. It’s about a stop of exposure (0.29) above the 0.3x average gradient point (where the slope of the film curve flattens out to be a third of its average). 0.3x average gradient is the speed point. That’s the point found in Kodak studies where ordinary people judged prints (sorted a bunch of prints made from negatives with variety of exposures into just acceptable, good, excellent, unacceptable) and picked the first excellent print.

It’s traditional to graph exposure on the X-axis in logarithmic terms decreasing exposure as you go right-to-left and the Y-Axis is density in logarithmic terms increasing density as you go up from bottom. That makes X and Y the same scale. You would get a curve of 45 degrees if the negative was developed to give one to one density increase for exposure increase. Unity contrast is a special purpose, for example negative copying. For regular photography you want to go up one for two across (0.5 contrast index). That fits normal subject luminance range on a negative that doesn’t block highlights. That’s a decent contrast for normal: 0.5 to 0.6

 

[tom williams]

The Arithmetic film speed is 0.8 (a constant) divided by the Arithmetic exposure the film received at the point where it has developed to 0.1 density above filmbase plus fog.

A 400 speed film takes -2.7 log meter candle seconds to meet that measure when developed to the ASA contrast parameters in standard developer like D-76.

Antilog of -2.7 log is 0.002 Arithmetic meter candle seconds.

So plug into the speed formula 400 = 0.8 / 0.002



0.1 density is an easy density to measure. You don’t actually “do” anything with it. But it’s near the area of dark shadow, key black of the print where detail is just emerging. It’s about a stop of exposure (0.29) above the 0.3x average gradient point (where the slope of the film curve flattens out to be a third of its average). 0.3x average gradient is the speed point. That’s the point found in Kodak studies where ordinary people judged prints (sorted a bunch of prints made from negatives with variety of exposures into just acceptable, good, excellent, unacceptable) and picked the first excellent print.

It’s traditional to graph exposure on the X-axis in logarithmic terms decreasing exposure as you go right-to-left and the Y-Axis is density in logarithmic terms increasing density as you go up from bottom. That makes X and Y the same scale. You would get a curve of 45 degrees if the negative was developed to give one to one density increase for exposure increase. Unity contrast is a special purpose, for example negative copying. For regular photography you want to go up one for two across (0.5 contrast index). That fits normal subject luminance range on a negative that doesn’t block highlights. That’s a decent contrast for normal: 0.5 to 0.6

Thanks Bill. I'm pursuing this subject - just stuck now on the 1/2 stop shift you mentioned in your post #6. I re-calibrated my Sekonic L778 to match my Minolta M Spot meter (to within 1/4 stop, adjusting the Sekonic to reduce its reading by 0.6 EV ). I exposed a negative + step tablet sandwich at Zone X as metered by the Sekonic before recalibration, and then exposed a negative with the re-calibrated Sekonic, in circumstances as nearly identical as I could manage, and with the same equipment as before. Developed the negatives as before, at 10 minutes in xtol 1:1, 68F, SP445.

The 1/2 stop shift was not reduced substantially. Which got me to thinking about flare. Exposing a sheet of film with the camera pointed at a white sheet (about 1 foot away from the lens) in direct sun seems like a decent way to introduce significant flare. That seems to me to fit your comment that I seemed to be giving the film about a half stop more exposure than I claimed I was. So next on my list is to repeat the test, with the recalibrated Sekonic, but with less aggressive lighting. Unless someone here points out that I'm off the beam with the flare theory.

cheers
Tom

 

[Bill Burk]

I was going to say, you’re flooding the interior of a dark chamber with light to make a contact print. There’s no flare in this scenario because all the light is even and the image is formed in that contact sandwich.

But there is off-axis light hitting the walls of the bellows increasing the overall exposure, that’s flare. So yes you could be getting increased expose above what you estimated.

 

[tom williams]

I was going to say, you’re flooding the interior of a dark chamber with light to make a contact print. There’s no flare in this scenario because all the light is even and the image is formed in that contact sandwich.

But there is off-axis light hitting the walls of the bellows increasing the overall exposure, that’s flare. So yes you could be getting increased expose above what you estimated.

Quite a delay here, sorry to take so long to come back - I wanted to try an experiment to reduce flare. To that end, I shot a new test negative and developed it as before. For the test exposure I used a larger target and a longer lens (with a smaller acceptance angle) so that I could shoot from farther away from the target: about 5' away from the camera. I used an extended lens hood, and shot at sunrise, when the target was just about level with and perpendicular to the sun's direct rays. I reasoned that these adjustments would reduce the number of off-axis rays entering the lens. Also, instead of a white target I used a grey matt board.

Metering was with the same Sekonic as before, which I had adjusted to match my Minolta Spot M (post #45) .

Attached, a plot showing the new curve - in black - and the curve from the last plot I showed a few weeks ago. The FB+F+0.1 value for the earlier test shot was 0.27, and for the current shot it's 0.21. The right-angle apex of the ISO triangle is still about 1/3 stop away from the nominal Zone V value. I'm thinking that if I shot this film at ISO 125, the alignment might improve.

 

[tom williams]

Quite a delay here, sorry to take so long to come back - I wanted to try an experiment to reduce flare. To that end, I shot a new test negative and developed it as before. For the test exposure I used a larger target and a longer lens (with a smaller acceptance angle) so that I could shoot from farther away from the target: about 5' away from the camera. I used an extended lens hood, and shot at sunrise, when the target was just about level with and perpendicular to the sun's direct rays. I reasoned that these adjustments would reduce the number of off-axis rays entering the lens. Also, instead of a white target I used a grey matt board.

Metering was with the same Sekonic as before, which I had adjusted to match my Minolta Spot M (post #45) View attachment 310308 .

Attached, a plot showing the new curve - in black - and the curve from the last plot I showed a few weeks ago. The FB+F+0.1 value for the earlier test shot was 0.27, and for the current shot it's 0.21. A difference due to flare, perhaps? The right-angle apex of the ISO triangle is still about 1/3 stop away from the nominal Zone V value. I'm thinking that if I shot this film at ISO 125, the alignment might improve.

I realize that attributing the difference in FB+F to flare doesn't fit. I measured FB+F in the blank patch left on the negative by the taped patch on the stouffer step wedge, and the density was 0.1 both the July and the May test shots. The rebate measurements were different - from each other and from the tape blank - in the two test negative runs, and I used those in my plots, to maintain continuity with earlier tests. Now it seems to me that I should have used the tape blank.

 

[Stephen Benskin]

Tom,

If the step tablet was sandwiched over the film, then flare wouldn't be a factor. It's flare free. If you are wondering about the difference in Fb+f between the two tests, that sounds like fog. You need to make sure a the Fb+f section measured receives zero exposure.

The two tests appear to have about the same gradient. The displacement is from exposure. The test from May had more exposure than the recent one. If there was any extraneous exposure striking the area of the film where you measured Fb+f, then that part would have received more exposure in the May test and would have a higher density than the recent test. Process a blank sheet to guarantee a baseline if you are wondering what is causing the difference.

Stephen

 

[tom williams]

Tom,

If the step tablet was sandwiched over the film, then flare wouldn't be a factor. It's flare free. If you are wondering about the difference in Fb+f between the two tests, that sounds like fog. You need to make sure a the Fb+f section measured receives zero exposure.

The two test appear to have about the same gradient. The displacement is from exposure. The test from May had more exposure than the recent one. If there was any extraneous exposure striking the area of the film where you measured Fb+f, then that part would have received more exposure in the May test and would have a higher density than the recent test. Process a blank sheet to guarantee a baseline if you are wondering what is causing the difference.

Stephen

Stephen, thanks for your comments. I applied a bit of opaque tape to the step wedge, which left a spot on my negative that was not exposed. That patch on my negatives has measured 0.1 or 0.11 consistently. I erred in taking measurements from the rebate when the blank patch was available. Using the blank patch to compute FB+F+0.1 (= 0.2), rather than using the rebate for the FB+F+0.1 (=0.27) measurement, causes the anomalous 1/2 stop shift in the ISO triangle to disappear (Bill Burke's post #6). The plot below shows the relocated ISO triangle with base at 0.2 density units.



My apologies to the respondents to this post for dumb errors. I have learned more than a bit though, so thanks. Lesson not the least: the initial topic - could I use an systematic analysis of developed negatives to derive a more appropriate film speed for future shooting - may be more relevant once I have greater mastery of basic ideas and practice.