The Ins and Outs of D-76 -- (The Film Developing Cookbook)

[Kino]

About half-way through the 2nd Edition of "The Film Developing Cookbook" by Bill Troop and Steve Anchell and am really learning a lot.

The section on D-76 kinda stopped me dead in my tracks.

I never knew there were so many variants and that the fresh stuff I mixed was different enough from Kodak's mix to have a real impact on image quality.

So if I am reading this right, Kodak added their "supplements" to prepackaged D-76 to overcome the tendency to rise in PH over time when stored in powdered form and "activate the Hydroquinone", which was otherwise inactive.

Huh?

Why have HQ at all in the formula if it is not "activated".

Did they mean enhance the effect of HQ?

What am I missing?

 

[Trask]

Interesting questions — i’ve been cogitating along the same lines. Came across a shop with lots of 1-liter bags of D-76 for $8/each, but decided not to buy because I can mix “D-76” from raw chemicals at maybe 1/10th the price. But, like you, I wonder about what Kodak has added that I’m not adding. I’ve read that one addition may have to do with helping the metol dissolve properly, since when you use a bag of developer you can’t put in a pinch of sodium sulphite before adding the metol; the metol, hq, SS, and accelerator all hit the water at the same time. Or perhaps Kodak has added some agents to deal with tap water impurities? Long story short, I look forward to the responses I hope you get.

 

[mshchem]

I am really enjoying this new book. I have used XTOL from it's debut. The book praises XTOL. They don't seem to appreciate the Jobo process. I have had excellent results with just about every combination I've worked with.

Modern film is so different from what I started with, this is what I have noticed.

I suppose Kodak may have made changes in the decades since these formulas were published. Not sure I would notice much difference. Think about the huge variety of light sources and enlargers.

It's sure fun exploring. Think how much fun the cell phone photographers are missing!

 

[Anon Ymous]

@Kino
The formula used by Kodak in their prepackaged D76 is probably a better buffered one. Using a combination of compounds, one can make a pH buffer that will resist, to some extent, pH swings. D76d is a better buffered variant that uses a mix of borax and boric acid instead of just 2g/l borax. It is rumored that this is close to what you get when buying D76.

Now, regarding the "activation" of hydroquinone, I really don't understand what they mean from what you said. Metol and hydroquinone is a superadditive combination and hydroquinone certainly works in D76. Perhaps they mean the "activation" of quinone, an oxidised form of hydroquinone, by converting it to hydroquinone monosulfonate, which is a mild development agent, but they certainly don't need to add anything special to D76 to do so. Sodium sulfite, which is already present in D76 takes care of this.

 

[john_s]

There's some extra reading here, saved from the now-defunct silvergrain web site of Ryuji Suzuki:

https://web.archive.org/web/20090530193028/http://silvergrain.com/labs/D-76

 

[Ian Grant]

It's often forgotten that the way D76 is used today is often quite different to it's original origins as a replenished D&P and motion picture developer. It was never designed for amateur use diluted at 1+1 or 1+3.

Changes in buffering improved it's use at 1+1 and 1+3 making it less prone to variations in water supply. The major issue with all Metol & MQ based film developers and replenishment is Bromide build up which suppresses Metol activity meaning you have to use a bleed system of replenishment. This was overcome by Ilford in the late 1950's who introduced Autophen a Phenidone PQ variant of ID-11/D76 for D&P/Photo finishing labs, this needed top up replenishment which is far more economic and not prone to issues with Bromide build up.

Kodak had experimented with Phenidone and Ascorbic acid developers but were prevented from commercial use by a Swedish company's Patent and were only able to introduce Xtol when this expired.

Ian

 

[bnxvs]

@Kino
Now, regarding the "activation" of hydroquinone, I really don't understand what they mean from what you said. Metol and hydroquinone is a superadditive combination and hydroquinone certainly works in D76. Perhaps they mean the "activation" of quinone, an oxidised form of hydroquinone, by converting it to hydroquinone monosulfonate, which is a mild development agent, but they certainly don't need to add anything special to D76 to do so. Sodium sulfite, which is already present in D76 takes care of this.

In one of the old patents on the technology for the production of powder compositions of developers, I read that in order to improve solubility and keep them dry, the developing substances are preliminarily “activated” in various mixtures of reagents. For example, in a mixture of sulfite and regular sugar. Then the prepared solution is dried and the resulting powder is used in the preparation of the final product.
Perhaps this is precisely what was meant by the term "activation of hydroquinone."

 

[Kino]

@Kino
The formula used by Kodak in their prepackaged D76 is probably a better buffered one. Using a combination of compounds, one can make a pH buffer that will resist, to some extent, pH swings. D76d is a better buffered variant that uses a mix of borax and boric acid instead of just 2g/l borax. It is rumored that this is close to what you get when buying D76.

Now, regarding the "activation" of hydroquinone, I really don't understand what they mean from what you said. Metol and hydroquinone is a superadditive combination and hydroquinone certainly works in D76. Perhaps they mean the "activation" of quinone, an oxidised form of hydroquinone, by converting it to hydroquinone monosulfonate, which is a mild development agent, but they certainly don't need to add anything special to D76 to do so. Sodium sulfite, which is already present in D76 takes care of this.

That is what is implied and seems to have come fairly early on, just about the time D76 was superseded by other formulas in motion picture work (such as D96) and D76 was adopted more or less as a standard for still photographic negative developers.

The sentence was just confusing, because if there is no superadditivity initially, only after the PH rises, it's kind of contradictory. I guess it's just a way of saying the buffering of the hydroquinone was minimal and subject to exhaustion unless used shortly after compounding the developer.

As to hydroquinone monosulfate, the book implies that that is a byproduct of the actual developer reaction to exposed sliver, so as you suggest that is highly doubtful.

All of this is very helpful, as we run about 750 gallons of D96 in our continuous negative processor and are constantly "chasing the dragon" in fighting bromide build-up, even with what should be adequate replenishment. However we are also constantly being pressured to lower our waste stream, so it is a delicate balance. I am hoping to better understand the mechanisms to assist in this process...

 

[Kino]

There's some extra reading here, saved from the now-defunct silvergrain web site of Ryuji Suzuki:

https://web.archive.org/web/20090530193028/http://silvergrain.com/labs/D-76

Thank you! I had forgotten about Silvergrain Labs! Thank you Internet Wayback Machine!

It's often forgotten that the way D76 is used today is often quite different to it's original origins as a replenished D&P and motion picture developer. It was never designed for amateur use diluted at 1+1 or 1+3.

Changes in buffering improved it's use at 1+1 and 1+3 making it less prone to variations in water supply. The major issue with all Metol & MQ based film developers and replenishment is Bromide build up which suppresses Metol activity meaning you have to use a bleed system of replenishment. This was overcome by Ilford in the late 1950's who introduced Autophen a Phenidone PQ variant of ID-11/D76 for D&P/Photo finishing labs, this needed top up replenishment which is far more economic and not prone to issues with Bromide build up.

Kodak had experimented with Phenidone and Ascorbic acid developers but were prevented from commercial use by a Swedish company's Patent and were only able to introduce Xtol when this expired.

Ian

Thank you.

So the buffering is a function of that adaptation from motion picture to still film development; just to enable longer term storage without PH shift and to overcome variations in water quality for small tank, diluted, one-shot development.

I often wondered why labs in Los Angles waited so long to convert to Ascorbic Acid developers; now I understand.

Kodak and Patents; what a complicated story...

 

[Anon Ymous]

In one of the old patents on the technology for the production of powder compositions of developers, I read that in order to improve solubility and keep them dry, the developing substances are preliminarily “activated” in various mixtures of reagents. For example, in a mixture of sulfite and regular sugar. Then the prepared solution is dried and the resulting powder is used in the preparation of the final product.
Perhaps this is precisely what was meant by the term "activation of hydroquinone."

I think it was @Photo Engineer in this forum who said that Kodak make a blend of chemicals that go in a developer sachet and they cover the development agents used with sulfite. This should reduce oxidation of these agents drastically while the product stays on the shelf.

 

[Kino]

In one of the old patents on the technology for the production of powder compositions of developers, I read that in order to improve solubility and keep them dry, the developing substances are preliminarily “activated” in various mixtures of reagents. For example, in a mixture of sulfite and regular sugar. Then the prepared solution is dried and the resulting powder is used in the preparation of the final product.
Perhaps this is precisely what was meant by the term "activation of hydroquinone."

Interesting to see how the basic formulas might diverge as a function of their utility; one a fresh mix for on-demand development (replenished or not) and the other stable for storage and use as a dilute developer for still film.

 

[138S]

The book praises XTOL.

Kodak had experimented with Phenidone and Ascorbic acid developers but were prevented from commercial use by a Swedish company's Patent and were only able to introduce Xtol when this expired.

Also Kodak praises Xtol in his popular commercial chart:



Phenidone-Ascorbic was a major technical leap forward in the Kodak product range ! That chart is pretty clear.

Xtol made D-76 technically obsolete, add Xtol's low intrinsic toxicity. Xtol was formulated 70 years later, Kodak may had been learning something in 7 decades.

Still D-76 is classic stuff with some particular aesthetic footprint, of course grain and tonality can also be worked in the processing, but straight D-76 results are nice and one may want exactly that, D-76 is loved by many and a reason has to be there.

They don't seem to appreciate the Jobo process.

Not a surprise, while rotary processing is flawless and consistent it only allows a single kind of processing: continuous agitation. If one wants some kind of compensation from agitation pattern then rotary cannot do that. If one wants adjacency effects then rotary cannot do that also.

And also rotary tends to mix more air's oxygen in the chem, which may have some side effects sometimes.

But of course rotary is great if one just wants continuous agitation.

IMO Troop/Anchell tend to say thinks like they are, sometimes generating controversy about religious matter, and sometimes they also make statements that are debatable, but anyway DCB and FDC are a very, very good reference!!

I'm grateful because I've learned a lot with their books.

 

[Kino]

Still D-76 is classic stuff with some particular aesthetic footprint, of course grain and tonality can also be worked in the processing, but straight D-76 results are nice and one may want exactly that, D-76 is loved by many and a reason has to be there.

Not a surprise, while rotary processing is flawless and consistent it only allows a single kind of processing: continuous agitation. If one wants some kind of compensation from agitation pattern then rotary cannot do that. If one wants adjacency effects then rotary cannot do that also.

And also rotary tends to mix more air's oxygen in the chem, which may have some side effects sometimes.

But of course rotary is great if one just wants continuous agitation.

IMO Troop/Anchell tend to say thinks like they are, sometimes generating controversy about religious matter, and sometimes they also make statements that are debatable, but anyway DCB and FDC are a very, very good reference!!

I'm grateful because I've learned a lot with their books.

This at first suggests a corollary that machine processing is for precise, repeatable development that can be banal and that hand processing is artisanal and inherently variable, but I would say that both are capable of being artisanal.

Restraining the development to a known, fixed variable allows some people more flexibility prior to exposure, where as hand processing allows both prior and post exposure manipulation possibilities.

It's just a generalization and like most generalizations, it's shot full of exceptions...

 

[138S]

This at first suggests a corollary that machine processing is for precise, repeatable development that can be banal and that hand processing is artisanal and inherently variable,

Rotary reportedly has top consistency, but IMO there are several factors:

> In summer (for BW) the rotary requires a modification in the Thermostat to cool the bath by allowing to source sub 20ºC watter (opening an electrovalve).

> Temperature control in trays is also perfect if we nail the darkroom air temp at 20ºC, or if we use a thermostatic bath.

> Agitation in trays can also be totally consistent if taking care.

> Trays are also daylight is we use a paper safe as the tray, or if we put it inside a paper safe.

> Total consistency in development (time/temp/agitation) is not that critical if anyway we later have to adjust paper grade for each shot. Personally I try to be as consistent as possible, but IMO 20 seconds more or less of effective development are mostly irrelevant.

Restraining the development to a known, fixed variable allows some people more flexibility prior to exposure, where as hand processing allows both prior and post exposure manipulation possibilities.

Yes... in the highlights !!!

But this is allowed by tray/tank development. With a reduced agitation pattern (say 10s each 3 or 4 min) and extending a bit development (30-50%) we allow a regular development for mids/shadows but we limit developement in the highlights, conserving better highlight texture.

Contrary to what is often said, compensation in the highlights does not mainly come from developer exhaustion, but from not removing Bromide by-product from the emulsion as long we not agitate. In the highlights more silver bromide reacts to deliver more free bromide, and bromide is exactly a restrainer that slows development, as equilibrium is displaced.

In fact by controlling agitation we restrain more or less development in the highlights by allowing to accumulate temporary Bromide locally in the highlights, while rotary removes bromide inmediately.

Of course this is irrelevant in many scenes in what "rotary or not rotary" has little effect, but IMO there are scenes where it can be important to not blow extreme highlights, or to make them printable in an easier way. In particular I find that controlling agitation is a very good resource for night photography, as shadows have higher LIRF then we easily blow highlights if exposing to get the shadows, it's no secret that stand (or semi) processing can be powerful in that situation.

 

[Rudeofus]

I think it was @Photo Engineer in this forum who said that Kodak make a blend of chemicals that go in a developer sachet and they cover the development agents used with sulfite. This should reduce oxidation of these agents drastically while the product stays on the shelf.

If you dissolve more than a few g/l Sodium Sulfite in water, it becomes next to impossible to dissolve Metol. The biggest accomplishment of Kodak's D-76 was, that they managed to pack D-76 into a single powder bag, and prepare the powder in a way, such that the Metol fully goes into solution before the Sodium Sulfite is exposed to water. All the other D-76 powder kits require two bags - number one containing Metol and some stuff, number two containing the Sodium Sulfite and the whatever bag #1 did not provide.

AFAIK Kodak also added a powerful sequestering agent to their D-76 kit to both deal with water hardness and to prevent premature developer decomposition catalyzed by trace metals. Now, that the developer had meaningful shelf life, they finally may have improved the Borax buffer to prevent pH changes over time and with developer reuse. If you mix your developer fresh every time, and if you use it single shot, you may not benefit from any of these Kodak modifications. Most sequestering agents are also weak silver solvents (comparable to Ammonia), and together with the stronger buffering this modified version of D-76 may give somewhat different results than the original formula.

Why have HQ at all in the formula if it is not "activated".

If you compare Haist's version with the original one, you can trade 5g HQ for about 0.5g extra Metol and still get the same activity. This tells me, that the HQ is not overly active at this pH. However, none of these proponents of D-76H ever showed developer capacity compared to regular D-76, owing largely to the fact, that many of these publications (including FDC and FDC2) vehemently oppose developer reuse. If only a small fraction of HQ is active at pH 8.5, then the inactive HQ may well replace active HQ used up during development, keeping the developer more stable over time or with reuse.

My conclusion is, that it's best not to get hung up over some small differences in developers. You won't fall of a cliff, if you mix your D-76 with 80 or 120 g/l Sulfite, or if you add 4 g/l instead of 2g/l Borax. There may be detectable differences between classic D-76 and Kodak's single bag version, but neither version will make or break a negative.

 

[Anon Ymous]

@Rudeofus Few years ago, I had mixed a batch of homebrew Perceptol, which started out crystal clear, but developed a precipitate after the first use. I needed to know if my raw chemicals were the culprit, so I mixed 100g/l sulfite with 30g/l sodium chloride and left it sit unused for a day. No precipitate was formed, so I thought about adding the amount of metol needed to make some Perceptol. I had read numerous times that metol won't dissolve in a sulfite solution, but decided to go ahead and try it. It turned out that it was just a bit harder, but nothing too hard. If you take into account the recommended mixing temperature (50°C or thereabouts), it should actually be fairly easy to dissolve. I honestly don't know if this thing is real, or a myth perpetuated without confirmation. Or was I somehow lucky?

 

[Lachlan Young]

Rotary reportedly has top consistency, but IMO there are several factors:

> In summer (for BW) the rotary requires a modification in the Thermostat to cool the bath by allowing to source sub 20ºC watter (opening an electrovalve).

> Temperature control in trays is also perfect if we nail the darkroom air temp at 20ºC, or if we use a thermostatic bath.

> Agitation in trays can also be totally consistent if taking care.

> Trays are also daylight is we use a paper safe as the tray, or if we put it inside a paper safe.

> Total consistency in development (time/temp/agitation) is not that critical if anyway we later have to adjust paper grade for each shot. Personally I try to be as consistent as possible, but IMO 20 seconds more or less of effective development are mostly irrelevant.




Yes... in the highlights !!!

But this is allowed by tray/tank development. With a reduced agitation pattern (say 10s each 3 or 4 min) and extending a bit development (30-50%) we allow a regular development for mids/shadows but we limit developement in the highlights, conserving better highlight texture.

Contrary to what is often said, compensation in the highlights does not mainly come from developer exhaustion, but from not removing Bromide by-product from the emulsion as long we not agitate. In the highlights more silver bromide reacts to deliver more free bromide, and bromide is exactly a restrainer that slows development, as equilibrium is displaced.

In fact by controlling agitation we restrain more or less development in the highlights by allowing to accumulate temporary Bromide locally in the highlights, while rotary removes bromide inmediately.

Of course this is irrelevant in many scenes in what "rotary or not rotary" has little effect, but IMO there are scenes where it can be important to not blow extreme highlights, or to make them printable in an easier way. In particular I find that controlling agitation is a very good resource for night photography, as shadows have higher LIRF then we easily blow highlights if exposing to get the shadows, it's no secret that stand (or semi) processing can be powerful in that situation.

In a double blind situation, I don't think you'd be able to tell the difference between a continuously agitated negative and one with extended agitation cycles, if they were developed to matched contrasts. All that reduced agitation seems to give in my experience is a better chance of landing on an exact aim contrast vis-a-vis your ability to precisely hit process timings & temperatures. It doesn't make negatives necessarily easier to print. Most of the claimed benefits of reduced agitation are better achieved by more sensible means: masking; and/ or aiming negatives to G4+; learning how to selectively fog-in highlights. You can pull back the highlights all you want in processing, but unless you punch-in the shadow contrast via a specialist mask, you're going to have to go to a sufficiently higher grade of contrast in printing to make the shadows look 'right', and then you're still going to have to burn & fog-in the highlights! That isn't to say that a little reduction in highlight density isn't a bad thing (makes burning easier & keeps the separation by shoving the shouldering a bit later - depending on materials), but you can definitely go too far!

There may be detectable differences between classic D-76 and Kodak's single bag version, but neither version will make or break a negative.

This is probably the most important comment that everyone fiddling around with D-76 (or many other film developers) should be aware of! Secondary to which I'd suggest that without a microdensitometer - or at the very least some extreme enlargements with optimised lenses, almost all claims about 'sharpness' should be totally disregarded, as they'll almost always be distorted by changes in overall granularity, without looking at the consequences that any rise or fall in granularity has on fine detail rendition (aka, the effects of noise on the fine rendering of signal quality).

 

[Kino]

My conclusion is, that it's best not to get hung up over some small differences in developers. You won't fall of a cliff, if you mix your D-76 with 80 or 120 g/l Sulfite, or if you add 4 g/l instead of 2g/l Borax. There may be detectable differences between classic D-76 and Kodak's single bag version, but neither version will make or break a negative.

Agreed. My question was more academic than practical.

After reading the Suzuki D-76 article from Slivergrain Labs (from post #5 -- which appears to be the primary source for this entry of the Film Developing Cookbook), the original issue of HQ "activation" appears to be an editorial simplification of a very complex interaction between PH, superadditivity and bromide byproducts production during active development using the stock original D-76 solution.

The subject of D-76 and all it's variants in this article is complicated by the fact that the author jumps around between the implications of various versions for both continuous agitation motion picture processing of negatives and small tank/tray development for still film.

Still it's fascinating stuff and the implication that some formulas entirely remove the HQ, substitute other developers and juggle even vary the amounts of HQ and Metol to little effect is pretty amazing...

 

[138S]

In a double blind situation, I don't think you'd be able to tell the difference between a continuously agitated negative and one with extended agitation cycles, if they were developed to matched contrasts.

An strong compensation requires always low agitation, but agitation alone may not be enough for an strong compensation, this is also true. Rotary does not alow compensation, tanks and trays allow it in the degree you want with right techniques.

Compensation has many factors: kind of developer, dilution, agitation and additional techniques. In the additional techniques you may use intermitent water bath, for example.

It is true that reduced agitation may not have a great compensation effect for many scenes, or better said simple compensation from agitation is seen only in areas of very high overexposure, but we may make very strong compensations for the highlights if you want and none of those can be made with rotary processing.

Regarding adjacency effects, we cannot overlook how textures greatly increase microcontrast in stand and semi-stand. To realize in which amount this happens what I did is taking scans of regular vs stand, with matching contrast, then with Ps I took crops of textures and compared histograms, the difference is more than remarkable, it's huge. Many times the tonal distribution of the texture was doubled in width, the histogram shows it quantitatively.

IMO it's not only what agitation control alone does, it's also what it allows to do with compatible techniques.

 

[Rudeofus]

To realize in which amount this happens what I did is taking scans of regular vs stand, with matching contrast, then with Ps I took crops of textures and compared histograms, the difference is more than remarkable, it's huge. Many times the tonal distribution of the texture was doubled in width, the histogram shows it quantitatively.

It is quite amazing, how far a histogram has to be off before the effect becomes visible. Your statement tells more about the sensitivity of measurement setups than the significance of subtle differences in the image. There are endless threads about modified C-41 processes, which give terrible control strips, but decent looking images.

 

[138S]

It is quite amazing, how far a histogram has to be off before the effect becomes visible. Your statement tells more about the sensitivity of measurement setups than the significance of subtle differences in the image. There are endless threads about modified C-41 processes, which give terrible control strips, but decent looking images.

By naked eye many times we may clearly perceive the acutance increase from stand development, the histogram analysis is useful to learn the quantitative effect from several parameters while adjusting a reduced agitation processing, but I cite it to make clear that's very easy to prove quantitatively that agitation has a real and remarkable impact on "microcontrast" in the textures, another thing is if one wants that or not.

 

[Rudeofus]

There is likely some visible - and noticeable - difference between regular development with full strength developer and stand development in dilute developer. But is the difference between Kodak's and Ilford's D-76 versions really noticeable?

 

[Lachlan Young]

It is quite amazing, how far a histogram has to be off before the effect becomes visible. Your statement tells more about the sensitivity of measurement setups than the significance of subtle differences in the image. There are endless threads about modified C-41 processes, which give terrible control strips, but decent looking images.

It's often the case that at a certain point, printing ability & knowledge will (within fairly broad parameters) overcome apparently significant sensitometric differences that some people - who are often unwilling to put in the effort to learn how to really print well from an aesthetic standpoint because doing so would apparently force them to interrogate their emotions and trite worldviews in ways they find uncomfortable - rely on as the sole analysis of 'good' and 'bad'.

An strong compensation requires always low agitation, but agitation alone may not be enough for an strong compensation, this is also true. Rotary does not alow compensation, tanks and trays allow it in the degree you want with right techniques.

Compensation has many factors: kind of developer, dilution, agitation and additional techniques. In the additional techniques you may use intermitent water bath, for example.

It is true that reduced agitation may not have a great compensation effect for many scenes, or better said simple compensation from agitation is seen only in areas of very high overexposure, but we may make very strong compensations for the highlights if you want and none of those can be made with rotary processing.

Regarding adjacency effects, we cannot overlook how textures greatly increase microcontrast in stand and semi-stand. To realize in which amount this happens what I did is taking scans of regular vs stand, with matching contrast, then with Ps I took crops of textures and compared histograms, the difference is more than remarkable, it's huge. Many times the tonal distribution of the texture was doubled in width, the histogram shows it quantitatively.

IMO it's not only what agitation control alone does, it's also what it allows to do with compatible techniques.

This seems like a lot of words that are very delicately trying to sidestep a lack of experience and properly rigorous testing. I can also see your methodology producing a lot of horrible to print negatives, requiring a high grade to set the shadows sensibly, with some painfully tricky dodging - and that's before dealing with getting the highlights where they need to go. Masking should not need to be routine - especially with the sort of thing you propose photographing.

By excessive use of N-, all you are doing is making your life harder than it needs to be. All you want is to straighten out the film curve just enough to get as much of the highlights as needed on a decently linear portion of the curve - knowing that you also need to still be able to print the shadows hard enough that they still look 'right', thus too much underdevelopment is equally to be avoided.

And if you're using an Epson flatbed as a comparator, all you're doing is comparing its sensor noise/ aliasing response. Not much use as an analytical tool. Better to make a few prints in the darkroom and see if by slightly tweaking the overall contrast in printing you can match the results of the differences in negative processing. I can think of many negatives which after persuading on to paper a grade or more harder than 'correct' look dramatically crisper and sharper - all without wasted efforts at the developing stage.

 

[138S]

This seems like a lot of words that are very delicately trying to sidestep a lack of experience and properly rigorous testing.

This seems a personal disacreditation and I'm not to debate in that way, best regards.

 

[138S]

There is likely some visible - and noticeable - difference between regular development with full strength developer and stand development in dilute developer.

There are many sensitometric calibrations posted around that show the effect quantitatively.

Anyway, if one has doubts, Way Beyond Monochorome (Pag, 207-208) and The Film Development Cookbook explains it.

We cannot generalize what reduced agitation does, it depends on the film/developer/dilution/processing combo, but there is no doubt that controlling agitation can be a powerful resource for when it's nedeed.

But is the difference between Kodak's and Ilford's D-76 versions really noticeable?

I was a D-76 user but the times I've used ID-11 I saw no difference, IIRC, as I've been using Xtol last 7 years. Probably D-76 vs ID-11 have some difference as developer gets seasoned...