High accutance developers and resolution...
Every once in a while, I'll come across a posting on APUG or photo.net along the lines of..
[tired old film/developer combo thread omitted]
"...and be sure to use a high accutance developer to maximize resolution"
The posters act as if fine-grain developers contained all manner of evil silver solvents that happily munch away at your negative, turning your grain to mush and obliterating fine detail.
I'm not a photo-chemist, but is it possible that the posters have got this backwards or are - at the very least - exaggerating?
From the very little I have read about adjacency effects (e.g. so-called "border" and "fringe" effects) it would seem that these might actualy result in diminished resolution in the immediate area of the image in which they occur. Applying an exaggerated unsharp mask to an image in PhotoShop also seems to suggest this could be true.
At the same time, I know that all the metallographs that were produced in my grad school lab work back in the mid-90s were developed using D-76c and I understand that this developer is very commonly-used in scientific work. Being from the D-76 family, I doubt it's considered a "high-accutance" developer...but I suspect that scientists researchers aren't keen on the loss of image resolution in their work.
I have not seen any scientific findings on the web regarding the benefits of accutance developers for resolution.
Does anybody any such findings that they can share?
Digital Photography is just "why-tech" not "high tech"..
Get yourself a copy of the Film Development Cookbook. It'll tell you everything you want to know.
Originally Posted by Bruce Watson
Then what's the point of having a discussion group if the best advice is to go read a book?
I have made a rather extensive series of posts on macro and micro edge effects and the contrast effects that also take place. You may want to look them up to further extend your understanding of this.
If something is the best advice, then why would the OP want less.
Originally Posted by alanrockwood
In the case of suggesting "The Film Developing Cookbook", the OP got a very good piece of advice. The book contains far more information than would be manageable to communicate within the constraints of a forum.
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As I understand it from what I have read, part of the sharpness increase of developers like Beutlers is that they do not develop as deeply into the grain of the film. They develop only the surface of the grain layer which results in less defraction of the light rays or less development of the diffusion of the light rays from going through the film grains. In fact I am going to just get my book and copy what it says:
By Gerald Koch
Any discussion of high acutance developers should begin with a definition of acutance. Unfortunately this term is not easily explained. However for the purpose of this article we can think of actuance as the scientific name for what the eye perceives as sharpness. Some years ago, when emulsions were grainier than they are today, developers containing very active silver halide solvents such as thiocyanate and phenylenediamines were popular. Their popularity was not universal since it was found that the reduction of the granularity was at the expense of sharpness. As far as the eye is concerned, it is the final critic in any photographic process, a sharp print is obtained by favoring acutance over grain.
The acutance of an emulsion is related to many variables notably it's thickness, grain size and contrast. Of these the first seems to be the most important. In order to understand this, let us consider a single ray of light from a point source striking two photographic emulsions, a thick one and a thin one. As the ray passes through each emulsion it is scattered by the halide grains. A similar scattering enables us to see the projectors beam of light in a smoky movie theatre. If we developed the two films and looked at each image and it's cross section they would appear as below
(at this point there is a drawing showing a beam of light going through a thin emulsion with no scatter and a beam of light going through a thick emulsion with a spreading out of the beam)
The result of scattering in both cases is to make the image of the point source larger than it should be if the emulsion were infinitely thin. Obviously, the thicker the emulsion the more the image is spread and the fuzzier are it's edges. This is why, all other things being equal, a print from a fast film cannot be as sharp as one from a slow film, In order for the fast emulsion to be fast it must contain more silver halide and therefore must be thicker.
Now we can improve on the already considerable sharpness of modern slow speed films by making them thinner. Physically we can't do this but chemically we can, buy using developer which works only on the surface of the emulsion. Such a developer must be very active to offset the loss of sensitivity resulting from using only a portion of the emulsion layer. But in order to prevent the excessive contrast the developer must be compensating. That is, it must stop working in the regions of the high exposure once a certain density is reached while still continuing to work in the regions of low exposure. A simple way to solve these two problems is to use a very dilute solution of a very active developer. Because of the high dilution the developer will be exhausted at the sites of high exposure, keeping the contrast at the proper level. Being very dilute it will have little effect on the silver halide in the interior of the emulsion. In addition high acutance developers usually contain no bromide; the bromide released during development being used to restrain the action in areas of high exposure, thus increasing the compensation.
Pioneer work on the technique was done by Willi Beutler working for Tetanal-Photowerk in Germany. His formula, originally published in the Leica News, together with some high acutance formulas appearing in the British Journal of Photography and in Mason, Photographic Processing Chemistry are given in the table below.
(at this point the article lists Beutler, FX-1 FX-13 and Mason and the formula for each using Metol, Sodium Sulfite, Potassium Iodide and Sodium Carbonate. The formulas all use just these chemicals though they vary quite a bit)
Yes the article was written by G. Koch and is included in
Originally Posted by dpurdy
Patrick Dignan's Classic B&W formulas.
The developers you've mentioned are very dilute, very active,
and metol based. I believe metol is the chosen agent due to it's
reduction in activity in the presence of bromide. That reduction
in activity aids in achieving additional compensation. Dan
Simple questions don't always have simple answers. In this case the answers are so complex that it literally takes a book to adequately explain them all.
Originally Posted by alanrockwood
If you think you can do better, have at it.
Rodinal is one developer that can show infectuous development. You can look "infectuous development" up in "The Theory of the Photographic Process." The symptoms will be seen if you photograph a resolution chart which has black and white lines of equal width and the negative shows black lines wider than clear ones. This is a distortion that in certain cases looks like increased acutance, but may actually decrease resolution. In an extreme case, the lines can merge.
Originally Posted by aldevo
My response may not be precisely what you're looking for, but hopefully it helps to clarify a couple of thinks.
To me, sharpness (or image clarity) has three components, resolution, acutance and contrast. With 'acutance' I mean 'edge contrast' and with 'contrast' I mean overall image contrast. Film development has a large influence on acutance and contrast but only a minor influence on resolution, which is mostly coming from a good lens combined with a fine-grain emulsion.
Certain film developers work better for acutance than others, but one very important aspect of acutance is the development technique. Optimum acutance requires standing development. Rotation and even 30s-interval inversion techniques destroy most of what a good acutance developer can do.
By the way, subjective image evaluations clearly show that there is an importance ranking for the three components of sharpness.
1. contrast (high)
2. acutance (high)
3. resolution (low)
A high-contrast image looks sharper than a soft one, and high acutance is often mistaken for resolution (that why unsharp masking works). Resolution is not necessarily seen as sharpness. Indeed, a high-contrast, low-resolution image is often perceived as being sharper than a low-contrast, high-resolution image. If you have a choice, go for acutance and contrast and forget resolution (it works for digital, it will work for you).
To prove the point about digital, I have not measured a on-chip resolution of more than 60 lp/mm on any digital camera, but good 35mm lenses combined with fine-grain film can easily achieve over 100 lp/mm. Still, you would be hard pressed to see the difference between the two on an 8x10 print. Why? At that magnification, the resolution of both is beyond human detection, but increasing the acutance of the digital image is as easy as a mouse click. The story quickly changes in favor of film at larger magnifications.