EI vs. N Numbers

[ragnar58]

When you look at a Kodak publication for development times, you various times associated with different exposure indices. For a 400-speed film, you may see 200, 400, 800, 1600, and 3200. Is a change from the development time from EI 400 to EI 800 the equivalent to a N+1 development?
My quick look into seems that a N+1 is way too much for a 1-stop push but a N+2 is very close to a 3-stop push.
For roll film users, do you generally use the published times or do testing to get more precise times for the EIs listed? If you do the test, on what feature are you basing your findings?
Thanks

 

[ic-racer]

The minimum gradient of the toe of the film curve to produce good prints in 'average' conditions is always 0.3 times the slope of the straight portion. That principle does not change when you alter contrast (ie development time). By rating the film at a higher exposure index you are saying that your scene doesn't need shadow detail 4 stops below what the meter indicates.

Various N-permutations of development are useful when printing in the darkroom when a wide range of print contrast control options is not available.

 

[MattKing]

When you look at a Kodak publication for development times, you various times associated with different exposure indices. For a 400-speed film, you may see 200, 400, 800, 1600, and 3200. Is a change from the development time from EI 400 to EI 800 the equivalent to a N+1 development?
My quick look into seems that a N+1 is way too much for a 1-stop push but a N+2 is very close to a 3-stop push.
For roll film users, do you generally use the published times or do testing to get more precise times for the EIs listed? If you do the test, on what feature are you basing your findings?
Thanks

I think that it might be muddying the water a bit to try to correlate expansion and contraction procedures with "push" development aimed at dealing with under-exposure issues.

The Kodak times for non-ISO exposure indices reflect Kodak's recommendations on how best to deal with the compromises inherent in under-exposing their films. As such, they reflect Kodak's opinion on how best to weight the various factors involved.

Any time you under-expose film, you lose shadow detail, and the near-shadow detail that is on the film will generally be of low contrast. Increasing development will boost the contrast of that near-shadow detail but, for subjects with a wide range of brightness, that may be at the expense of pushing the highlights into the shoulder of the film, resulting in a lower quality rendering of the highlights. The increase in development will not assist with the shadow details that are still lost.

I find it informative that Kodak recommends the same development time for T-Max 400 metered at the ISO speed of 400 and at an EI of 800. That tells me that Kodak considers that, for EI 800, increasing development time will decrease the quality of the highlights more than it will increase the quality of the near-shadow detail.

You might disagree with Kodak on the various weights to be assigned to the criteria involved - that is one big reason to do your own testing.

In comparison, when you decide to use N + 1 or N + 2 development to expand the contrast of your negative, you generally have a scene with a relatively narrow range of brightness. You expose the negative for full shadow detail, but increase the development time to increase the density on the negatives of the highlights, which would otherwise fall far short of the shoulder.

In my case, I shoot roll film. For most rolls, I use a development time that I've found works well with the ISO speed of the film. For those few rolls that I want to expand or contract the development, I use the ratios of times that Kodak recommends: if Kodak recommends a 20% increase for a one stop "push" I use a 20% increase from my time for a N + 1 expansion.

 

[markbarendt]

Changing "your" EI or adjusting "your" N are controls for making "your" printing easier.

There are times when I still use plus development, but they're pretty rare and probably will become nonexistent. I left N- development behind long ago.

When needed I adjust contrast with paper grade changes, rather than film development changes, even that is becoming more and more rare as I gain more experience.

N development, box speeds, and grade 2 printing, with a bit of burn and dodge, gets me prints I like.

Using burn and dodge, instead of adjusting film or paper contrast, allows me to keep the contrast rate look I like and improve the placement of the subject matter. Burn and dodge makes printing "to my specifications" easier. This is something you just have to experiment with to see what works best for you.

 

[polyglot]

N+1 means a 1-stop expansion of contrast in the Zone II-VIII range. It will produce an increase in speed, but generally less than one stop. Conversely for the contractions, etc, N+k is in no way equivalent to a k-stop push.

I strongly recommend you read Davis' BTZS book as it makes all this very clear. But the ultra-condensed version is:
- N development has nominal contrast for printing on #2; there is a specific density ratio on the film between zones II and VIII (6 stops Subject Brightness Range on the spotmeter)
- N+k development will give you the same density ratio with 6-k stops of SBR (positive k values are expansion, negative k values are contraction)
- changing development changes the speed, a little. You plot that for all your k values (usually -2 to +2) and determine an appropriate EI for each N+k
- you pick the k according to the metered SBR so that the contrast comes out nicely on the film
- you look up the appropriate EI for your chosen k
- you make an exposure or two
- you develop according to the chosen k

Note that the causality under BTZS etc is "contrast defines speed" and not the converse, i.e. contrast is the free variable. If you're pushing film in order to obtain a particular speed, then quit thinking about N+k factors because you're not doing Zone-anything, you're just pushing for speed and will have to bear the contrasty consequences. If you've done your BTZS testing then you might well know how much expansion comes from doubling the EI.

The other thing to consider is that Zone practitioners are aiming for full shadow detail. No matter how much you push a film, you're never going to get 3 stops of true extra speed from it so when you see development instructions for crazy pushes (e.g. Tri-X at 3200) then the basis for those instructions is usually to preserve midtone density assuming you metered from a midtone - and that is a very not-ISO, not-Zone definition of film speed.

It's a little hard to visualise without diagrams, but think of the film response as a straight line, slope being contrast. If you do N+2 then the exposure difference between the film's shadow-speed point and midtones will be one stop less for the same density achieved in the midtones. Therefore if you metered a midtone and developed for N+2, you would get slightly more than 1 stop extra "speed". Note that the film is not achieving that speed, but that the midtones will print to the same brightness. You will lose a stop of both shadow and highlight detail on the print unless you change paper grades.

 

[markbarendt]

Technically no expansion comes from doubling the EI. Changing EI only changes the placement of the subject matter in relationship to the toe of the film.

Adjusting development controls the expansion or contraction, how many measured stops from the scene will "straight print"; 5,6,7 or whatever with no burn or dodge.

 

[ragnar58]

When I started to look into this issue, I wasn’t interested in the Kodak tables. I knew Kodak bases the times on the resulting slope (contrast index). I have never reliably been able to calculate CI, so comparing my test results to published CI’s has not been very successful. I was on vacation recently and I still have several rolls to process. Some of the rolls were in narrow streets with open shade while others were in harsh sunlight. I was looking to tweak the times to gain some benefits. This is when I started to ponder these tables.
If you push 1 stop then you underexpose 1 stop and increase the development to compensate. Why wouldn’t this compensation be the same as a N+1? Wouldn’t this be the same as having Zone VIII exposed as VII (-1) and developed up to VIII (+1)?

 

[polyglot]

If you push 1 stop then you underexpose 1 stop and increase the development to compensate. Why wouldn’t this compensation be the same as a N+1? Wouldn’t this be the same as having Zone VIII exposed as VII (-1) and developed up to VIII (+1)?

Because you meter at V not VII. If you reduce exposure by 1 stop at the midtone, the expansion required to bring what was IV back up to V will push VI up to VIII. So that's approximately N+2. I say approximately because increasing development does slightly increase true film speed, so a 1-stop push doesn't require a whole N+2 expansion.

Edit: draw out some H-D curves, even if they're imaginary.

 

[markbarendt]

I was on vacation recently and I still have several rolls to process.

Practice with unimportant rolls

 

[ragnar58]

In low-light street scenes, I normally just use my mini tripod, use the normal EI, and let what movement there is be blurred as part of the image. I had avoided the push. But I have one roll that I needed to keep shorter exposure times and this led to questioning the subject. I realize this topic may end up more like a recipe than a formula,

I was thinking that the development compensation was aimed at the necessary increase to restore the high values. If you meter at V and expose for a IV value (-1), then all zones are dropped by one. The values above V can be restored to near normal values while values the below V will not be completely restored and the lowest will be lost. I have a densitometer, spreadsheets, and graphs; so I can see this effect.

While searching the topic, I have seen an assortment of “rules-of-thumbs” such as:
- “for 1 stop increase 1.5x, for 2 stops increase 2.25x, and for 3 stops increase 4.5x”
- “increase 50% for each stop of push (meaning if 6:00 is normal then increase to 9:00, 13:30, and 20:15)”
- “add 10%, then 20%, and 30% for traditional film and 5%, 10%, and 15% for T-grain film”
I also saw the issue declared as voodoo, which makes more sense the further one looks into it.

This doesn’t explain what is measured with these adjusted times. A quick survey of Ilford and Kodak information shows that Ilford recommends a much higher extension compared to Kodak (on average 1.25x, 1.74x, 2.58x verses 1.17x, 1.43x, 1.78x). I suppose one could shoot a test roll and develop snips at increasing longer times until the resulting prints are deemed unacceptable.

What I wanted to see is something that would be more like “increase development to achieve a .10D in the original Zone I-1/2” or “increase development to restore Zone VIII to the intended density without the push processing.” I was looking for a more objective type of statement.

I guess the question to someone would be: if you were given a film and a developer that has no published push times and asked to determine these times, what criteria would you use to judge the extended times?

 

[polyglot]

This doesn’t explain what is measured with these adjusted times.

And you've hit the nail on the head right there. People publish all sorts of times for all sorts of different EIs without defining what will actually happen. MDC is a particualrly bad offender there.

or “increase development to restore Zone VIII to the intended density without the push processing.” I was looking for a more objective type of statement.

My understanding is that outside of the formal zone systems, when people say "EI X", then the aim is that midtones will be right if you set your SLR meter to X and point it at a scene full of midtones, i.e. the no-thinking-while-exposing approach. So what it effectively means is "increase development to restore Zone V to the same density". That causes (for fixed paper grade) equal loss of both highlight (due to paper underexposure) and shadow (film underexposure) detail and leaves the scene at basically the same average brightness.

If the effect that you want is to restore Zone VIII then obviously less expansion is required. It would give you a one-stop speed increase if you meter at Zone VIII, but only a half-stop increase if you meter at Zone V, which is kind of what a meter does.

If you've done the formal film testing already then you can derive exactly the answer you want according to whichever formal definition you choose. And the definition you choose will depend on how you like to do your metering.

 

[RalphLambrecht]

unfortunately, It's not quite that simple. an increase in development often also means an increase in EI, BUT ONLY A PROPER FILM TEST(ala BTZorWBM) WILL TELL you the new Eis for N-3 toN+3.I wouldn't know how to do that without the test

 

[markbarendt]

I was thinking that the development compensation was aimed at the necessary increase to restore the high values. If you meter at V and expose for a IV value (-1), then all zones are dropped by one. The values above V can be restored to near normal values while values the below V will not be completely restored and the lowest will be lost.

Under-exposure, as you are suggesting, affects only the printability of the low tones, the high tones can be printed fine either way.

Compensation/film development adjustments, are intended to "fit" a certain measured SBR to a fixed or specific grade of paper: say grade 2.

If you are using a fixed grade paper, with experience and testing, you can make straight printing easier by adjusting development.

If you are using VC paper there is little need for + or - development; a well tested "N" will work fine for most situations and the adjustments are simply made in the enlarger.

 

[Bill Burk]

I've tried to come up with an idea to explain the difference between push/pull and N+/N- developing.

First off, the N times come from testing (as Ralph pointed out, WBM/BTZS testing and I'd add ZS other Sensitometry). You don't underexpose and develop to try to make up for it... You give plenty of exposure and work with development to get the most appropriate range of densities in the negative to fit your printing model.

Push is as you've imagined, a deliberate plan to underexpose (for example in low light to keep a reasonable shutter speed) and hope to get any detail. Maybe you will bring a mid-tone that was underexposed, up to where it might have been if it had been properly exposed.

While they both affect the development time in an attempt to alter the negatives, they are done for entirely different reasons.

I keep thinking it might be like comparing driving really fast to get from San Francisco to Los Angeles in less than 8 hours, versus driving really fast to win the Indy 500. You're driving really fast, but for totally different reasons.

 

[markbarendt]

The difference in intent between a "push" and an "expansion or contraction" is that;

1 - with a push you are purposefully sacrificing some of the subject detail and tonal relationships (detail and relationships that you would actually prefer not to give up) to get something, anything usable on your film. In my mind a push should only be done when when the the exposure settings you would really like to use would result in a blur.

2 - conversely, with expansion or contraction no compromise is made to shutter speed (other ways are found to avoid blur), expansion or contraction are purposeful choices done to fit a specific SBR from the scene to the printable range of a fixed grade of paper.

 

[Bill Burk]

markbarendt,

Your explanation sums it up nicely!

1 - Push/Pull Processing tries to mitigate problems caused by exposure mistakes (whether deliberate or accidental).

2 - Expansion and Contraction try to make the best possible negatives that you can get with perfect exposures.