Acutance - is there something real behind the hype?

[sanking]

Ralph,


My disagreement with you is based on my belief that acutance (which is really edge effects) is more important for LF than for 35mm and MF negatives. In fact, as Clay points out, very enhanced adjacency effects may actually be detrimental to image quality with small format negatives because by the time they are enlarged several times the lines become visually obnoxious. With LF film you almost always have sufficient resolutioin, and contrast is easy enough to control. However, if you have no acutance, or edge effects, the image is likely to not look very sharp. I have developed some very large negatives (12X20) with rotary agitation (continuous) that have a world of resolution and the contrast is right (in terms of macro contrast) but the fact that there is no internal micro-contrast makes them print flat. The bottom line is that reduced agitation to enhance adjacency effecs is more important for sharpness than with 35mm and MF film

BTW, I agree with you on the three components of sharpness, i.e. resolution, acutance (edge effects) and global contrast. However, there is another micro-contrast that is produced by acutance/edge effects that can be as important or more so than global contrast in making a print look sharp.

Sandy King

 

[keithwms]

Tonality isn't clearly defined in the analogue world simply because the analogue tone scale is not discretized.

If the (virtually) continuous analogue tone scale were discretized then we could readily quantify the tonality in terms of:

(1) bit depth: the number of tones recorded; obviously the more you have, the smoother the gradations and the less posterization (a.k.a. banding);

(2) resolution***, which plays a big role in tonal transitions. The usual example I give is to imagine the extreme case that grains (or pixels) can only be pure white or pure black. Then you'd need a lot of them to have smooth gradations and to avoid the 'bit-mapped' look. Actually, it is possible for a 2-bit image to look smooth, it just takes a lot of grains (or pixels). N.b. if we are more realistic and allow the grains (or pixels) to take on a range of intermediate tones, thus working at a more realistic, higher bit depth, then I would assert that you still need a certain critical number to prevent posterization from being evident to a standard eyeball viewing a standard enlargements from a standard... well, you get the idea.

(3) The tone curve itself- the toe, the knee, and the slope of the midtones.

So... because the analogue tone scale is (virtually) continuous, it is harder to define tonality and people resort to describing it in vague terms. Obviously, in the analogue world, most of the emphasis is placed on (3) because it is the most readily definable quality of a continuous tone image, and it is something you an measure and plot. But when people speak of 'format advantage' with regard to tonality, they are invoking mostly (2). When people speak of the characteristics of a developer at such and such dilution etc., or suggest using pyro developer to reduce the appearance of grain, they are invoking (1) and (2): remember that development is basically an amplification process, and at the level of sensitization of individual grains, the tone scale isn't so continuous. Also, when people feed their neg or print through an analogue-to-digital converter and post an image online, they certainly see the big roles played by (1) and (2).

So... that we don't have mathy ways to describe analogue tonality is perhaps a good thing. This is a consequence of the tone scale being (virtually) continuous, giving (virtually) limitless bit depth.

***I should probably cite MTF here rather than 'resolution,' but then we'd have to discuss frequency response and all that.

 

[sanking]

I
In other words, this reduced agitation technique and developer choice might be useful for sharper appearing contact prints, but is definitely a bad option for a photo you intend to enlarge.

I developed a roll of HP5+ in 120 size with reduced agitiation and got rather extreme adjacency effects. As I recall this was with Pyrocate-MC, which gives similar results to Pyrocat-P with reduced agitation. The subject was large black script on a white wall. The white lines aroudn the dark script are so exagerated that when people see the print I made they immediately assume that I used too much unsharp mask in Photoshop, when in fact I used absolutely none.

There is no question in my mind but that the degree of magnification is one of the most important considerations we must make in selecting a developer and method of agitation. With any of the Pyrocat versions stand or semi-stand development give such strong adjacency effects that the result may look weird with any significant magnification. I generally agitate MF B&W film every couple minutes for about 10 seconds when developing in Pyrocat-HD or -MC 1:1:100. This gives a nice pictorial look with good sharpness but without extreme adjancency effects.

Sandy King

 

[Keith Tapscott.]

I developed a roll of HP5+ in 120 size with reduced agitiation and got rather extreme adjacency effects. As I recall this was with Pyrocate-MC, which gives similar results to Pyrocat-P with reduced agitation. The subject was large black script on a white wall. The white lines aroudn the dark script are so exagerated that when people see the print I made they immediately assume that I used too much unsharp mask in Photoshop, when in fact I used absolutely none.

There is no question in my mind but that the degree of magnification is one of the most important considerations we must make in selecting a developer and method of agitation. With any of the Pyrocat versions stand or semi-stand development give such strong adjacency effects that the result may look weird with any significant magnification. I generally agitate MF B&W film every couple minutes for about 10 seconds when developing in Pyrocat-HD or -MC 1:1:100. This gives a nice pictorial look with good sharpness but without extreme adjancency effects.

Sandy King

Sandy, to save people wading through the threads or doing a web search, could you post the various Pyrocat formulas in the staining film developers section of this site?
(there was a url link here which no longer exists)

 

[sanking]

Sandy, to save people wading through the threads or doing a web search, could you post the various Pyrocat formulas in the staining film developers section of this site?
(there was a url link here which no longer exists)

OK, I will try to put that information up today or tomorrow.

Sandy King

 

[Kirk Keyes]

And maybe a FAQ with the top ten questions that keep getting asked over and over and over! Perhaps put them all in s sticky thread in this forum.

 

[sanking]

And maybe a FAQ with the top ten questions that keep getting asked over and over and over! Perhaps put them all in s sticky thread in this forum.

OK, I just added information about the various formulas of Pyrocat and mixing directions. If folks want to send me questions they feel should be in a FAQ I will try to write that up as well. It would sure save me some time.

Sandy King

 

[RalphLambrecht]

Ralph,


My disagreement with you is based on my belief that acutance (which is really edge effects) is more important for LF than for 35mm and MF negatives. In fact, as Clay points out, very enhanced adjacency effects may actually be detrimental to image quality with small format negatives because by the time they are enlarged several times the lines become visually obnoxious. With LF film you almost always have sufficient resolutioin, and contrast is easy enough to control. However, if you have no acutance, or edge effects, the image is likely to not look very sharp. I have developed some very large negatives (12X20) with rotary agitation (continuous) that have a world of resolution and the contrast is right (in terms of macro contrast) but the fact that there is no internal miro-contrast makes them print flat. The bottom line is that reduced agitation to enhance adjacency effecs is more important for sharpness than with 35mm and MF film

BTW, I agree with you on the three components of sharpness, i.e. resolution, acutance (edge effects) and global contrast. However, there is another micro-contrast that if given by acutance/edge effects that can be as important or more so than global contrast in making a print look sharp.

Sandy King

Sandy

This might be worth a test. I'll get with Ian to get the best acutance development for 35mm, MF and LF and we'll compare the effect of acutance on all formats when making the same-size print. This will tell how much difference there is.

 

[Ian Grant]

Ralph, I have a formula that I suspect is very close to Ilford Hyfin. My experience with these High Acutance developers was that they give a Higher Acutance than Pyrocat HD both with similar agitation, but Pyrocat has better tonality and micro-contrast in fine details.

I've used Pyrocat with similar reduced agitation to Sandy King's agitate every 2 mins, 120 & 5x4, maybe there's an increase in edge effects but it isn't the exaggerated effects I've seen with other Acutance developers.

Perhaps it's fair to say Pyrocat is on a par with Acutol (in terms of Acutance), rather than the more extreme High Acutance developer like Acutol-S or Hyfin.

It's worth noting that the original High Acutance developers were designed to get the highest possible definition from slow, 100 ISO and less, 35mm films. Crawley made a point of describing what he meant by all the terms, like definition, acutance, etc in his series of articles in the BJP 1960/1.

Ian

 

[sanking]

Sandy

This might be worth a test. I'll get with Ian to get the best acutance development for 35mm, MF and LF and we'll compare the effect of acutance on all formats when making the same-size print. This will tell how much difference there is.

That would be interesting. I am basing my comments on observations over time but must confess that I have never conducted a good study to support my confusions. And at this point in time I am printing almost exclusively with digital negatives that I produce from film scans which complicates comparisons even more.

Sandy King

 

[Kirk Keyes]

Sandy

Your disagreement might be based on a difference in terminology. I see that you're mixing edge effect and contrast into acutance. I'm keeping them separate.

Richard Henry defines "acutance" in the 2nd Ed. of Controls in Black and White Photography as:

"Objective measurement of the abruptness of film or print response to an exposure made with a knife edge, to one side of which the film is exposed to light while the other side is blocked from light impingement. The exact mathematical determination of a number to express acutance, for the measurements made in this book, differs from those previously proposed. There is no a priori "correct" mathematical expression for this value."

Henry goes on to discuss the equations that have been used historically to measure accutance, and they all include a measurement of mean square of the gradient (i.e. contrast) across the knife edge transition.

The ability of a film and developer to change contrast is the major component of accutance.

 

[RalphLambrecht]

Richard Henry defines "acutance" in the 2nd Ed. of Controls in Black and White Photography as:

"Objective measurement of the abruptness of film or print response to an exposure made with a knife edge, to one side of which the film is exposed to light while the other side is blocked from light impingement. The exact mathematical determination of a number to express acutance, for the measurements made in this book, differs from those previously proposed. There is no a priori "correct" mathematical expression for this value."

Henry goes on to discuss the equations that have been used historically to measure accutance, and they all include a measurement of mean square of the gradient (i.e. contrast) across the knife edge transition.

The ability of a film and developer to change contrast is the major component of accutance.

Kirk

Thanks for that. This is almost identical to Stroebel's description of acutance in the Focal Encyclopedia of Photography 3rd Ed.

 

[dancqu]

The ability of a film and developer to change
contrast is the major component of accutance.

Expressing it that way, I believe the word
resolution might be substituted for the word
acutance. Is not acutance no more than
resolution by another method? I am
inclined to see it that way. Dan

 

[Ian Grant]

No you can't substitute Definition for Acutance,

Expressing it that way, I believe the word
resolution might be substituted for the word
acutance. Is not acutance no more than
resolution by another method? I am
inclined to see it that way. Dan

No you can't substitute Definition for Acutance, they are quite different, it's possible to have high acutance with poor definition

Geoffrey Crawley published an extended article in the BJP Dec/Jan 1960/61 looking at film developers in depth. Crawley later became editor of the BJP and designed the Paterson range of B&W chemistry.

Here's Crawley's own summary of what he wrote, from his précis of the original 7 part article:

Notes on Present-day Monochrome Emulsions and their Development

"Sharpness" - the overall impression of a print or projected image, measured scientifically as "acutanc", seen from a normal viewing distance.

"Definition" - the extent to which fine detail is recognisably rendered in a print, etc. When acutance of fine detail is good then definition is good.

"Acutance" - the contrast at the edges of significant detail, a scientific measurement of the density gradient at that point.

"Resolving Power" - the scientific measurement of the actual fineness of detail recorded by lens, film, or developer or any combination of these three.

The key point is acutance of fine detail. Crawley went on to formulate FX-16 which was designed to exploit the grain of 400 ISO and faster B&W films while giving high contour sharpness, this is a high acutance developer with poor definition of fine detail.

Ian

 

[Steve Sherman]

Acutance, micro contrast and edge effects are perceptions, that is to my way of thinking the lens film and chemistry combinations are not capable of producing any more sharpness than we have become accustomed to with modern lens, films and pyro developers.

That said, how these three elements are manipulated together can effect a perception of higher acutance or sharpness. From my experience the Reduced Agitation technique can dramatically alter the perception of sharpness and acutance. Further the process can control scenes of little or extreme contrast like no other wet process known. For a full discussion and example see this link here

(there was a url link here which no longer exists)

My methods and examples deal strictly with the contact print and while I have never made extreme enlargements from these processed negatives I would agree with Clay's observations that the image can easily be destroyed from to great an edge effect or increase of micro contrast.

Cheers

 

[rst]

Sandy

This might be worth a test. I'll get with Ian to get the best acutance development for 35mm, MF and LF and we'll compare the effect of acutance on all formats when making the same-size print. This will tell how much difference there is.

I do not want to disturb the interesting conversation of you Sandy and Ian, but I am with Sandy on the acutance development being dependent on the factor of enlargement. Once you pass a certain point you will see the edge effects to get very prominent. I like HP5 developed in diluted perceptol for MF. This combination produces quite a bit of acutance and it is good for my usual print size. However, if I use 35mm and go for 12x16 (which is anyway too much for this combination, but probably not for a grain masking developer like pyro) I can see the edge effects as distracting halos of light around dark edges.

Cheers
Ruediger

 

[Kevin Caulfield]

Acutance, micro contrast and edge effects are perceptions, that is to my way of thinking the lens film and chemistry combinations are not capable of producing any more sharpness than we have become accustomed to with modern lens, films and pyro developers.

Acutance is a real, physical, measurable thing, not a "perception", just as described above by Crawley as a density gradient.

 

[Ian Grant]

"Acutance, micro contrast and edge effects are perceptions"

They are more than perceptions because the Acutance can be measured, and it is the edge effect. in fact the acutance can be measured in two ways - density & distance. From your contact printing point of view there are two element not three.

It is quite possible to produce higher definition than the "modern lens, films and pyro developers" combination, but you won't see it with contact printing and it's not necessarily the best overall balance of tonality and grain alongside the sharpness, if you're enlarging.

Acutance is a real, physical, measurable thing, not a "perception", just as described above by Crawley as a density gradient.

It is perceived as well, and Steve is talking about a visual impact that higher acutance can give particularlyto a contact print

Ian

 

[RalphLambrecht]

I do not want to disturb the interesting conversation of you Sandy and Ian, but I am with Sandy on the acutance development being dependent on the factor of enlargement. Once you pass a certain point you will see the edge effects to get very prominent. I like HP5 developed in diluted perceptol for MF. This combination produces quite a bit of acutance and it is good for my usual print size. However, if I use 35mm and go for 12x16 (which is anyway too much for this combination, but probably not for a grain masking developer like pyro) I can see the edge effects as distracting halos of light around dark edges.

Cheers
Ruediger

I don't doubt that they get prominent with magnification, but to me that also verifies the reverse. They must be less prominent with lack of magnification. Consequently, and that was my point, acutance has more of an effect with small negatives and is less of an issue with large-format. I originally worded it as 'more important for 35mm' and 'less important for LF', and I think that was the termonology which found objection.

 

[RalphLambrecht]

"Acutance, micro contrast and edge effects are perceptions"

They are more than perceptions because the Acutance can be measured, and it is the edge effect. in fact the acutance can be measured in two ways - density & distance. From your contact printing point of view there are two element not three.

It is quite possible to produce higher definition than the "modern lens, films and pyro developers" combination, but you won't see it with contact printing and it's not necessarily the best overall balance of tonality and grain alongside the sharpness, if you're enlarging.



It is perceived as well, and Steve is talking about a visual impact that higher acutance can give particularlyto a contact print

Ian

Yes, acutance is density & distance, which can be expressed as a gradient. The steeper the gradient, the more edge contrast, edge effect or acutance. As I said before, sharpness is perception, but its three components (contrast, acutance and resolution) can be measured.

 

[Thomas Bertilsson]

I'll steer back toward the OP a little and tell my story. In the past I've used both Rodinal and Pyrocat-HD/MC. They are fantastic developers, and I like them a lot. Much of my film processing was done with reduced agitation, and some of the sharpest negatives I've ever produced were with my Hasselblad 150mm Sonnar, Plus-X film, and dilute Rodinal (I believe it was 1+200 or 1+150) with 10m agitation intervals. Those negs make great 11x14 prints that are as sharp as I've ever been able to produce.

About the time of November last year I made a couple of acquaintances. I was challenged with what I thought I knew, and have since adopted a completely new approach to photography. I'm using replenished developers, Xtol and Edwal 12 to be precise, and I've started using Tmax films, both 100 and 400.
Edge effects or not, I am by far producing the sharpest looking prints of my life, with the best impact and definition I've seen from my own work flow. I can't explain it in technical terms, but it leads to my point: Your developer is just a small piece of the puzzle. How you use it is just as important. Especially Xtol gives me some amazing resolution and sharpness when replenished, more so than straight or dilute.

I also object a little bit to the notion that not using a tripod is futile resolution wise. Much of the time, if you have a decent shutter speed, I vow that you have ENOUGH resolution to make sharp prints that are satisfying to look at. It doesn't have to be perfect the whole time. I could not have made half my shots if I used a tripod all the time, and more often than not I feel like walking with crutches if I am forced to use one, due to the inflexibility I have in dynamic scenes composition wise. With static scenes it's obviously OK.

 

[dpurdy]

Thomas your thoughts about xtol are a bit confusing to me. It is one of my main developers. There is not actually a replenisher made for it. You either extend the times as it ages or you dump some and replace with fresh. I do the extended times method and have done so with 1-2 and 1-1 and now I use it straight. It might make some logic if it got a bit sharper when you dilute it but I don't see how extending it's life would affect sharpness.
Dennis

 

[Thomas Bertilsson]

Dennis,

I don't know exactly how it happens technically, but I have a feeling that the byproducts from processing film (bromide, for example) helps in this manner.

You're right, there is no dedicated replenisher. You basically start with stock Xtol, and add 70-100ml fresh stock solution to the bottle of working solution, per roll you process, before you dump your processing solution back into your storage bottle. See Kodak publication J-13 for more info.

You can keep doing this perpetually, and you will see a change in sharpness, grain, and especially the highlights. Please don't ask me to explain it, though, as I have absolutely no knowledge of how developers actually work. I just know what I see in my prints.

- Thomas

 

[Ian Grant]

Xtol's unique in that the replenisher is just fresh devloper.

But Thomas is right, replenished developers do give very good sharpness/definition and better acutance than if used fresh or dilute. Commercial photographers using deep tanks knew that and it gave them a advantage. In this case it's partly the side effects of the build up of Bromide & Iodide in the developer.

I switched the other way to Thomas from Rodinal & Xtol to Pyrocat HD and I can see a gain in definition and tonality, but by how much is only really going to be seen in big enlargements.

Personally I think Rodinal, Xtol & Pyrocat HD are three of the best developers available. But a comment about Pyrocat HD, I enlarge my 10x8 negatives and the definition is stunning even with my late 30's coated Dagor.

Ian

 

[dpurdy]

I have done the extended times method for several years doing custom film processing for people. I have never tried the method of adding fresh. But I did figure it out once that it mathematically works out the same as amount of rolls per amount of developer. Next time I have to dump my 10 liter tank I am going to try out your method. I just realized that will actually save me time... no more extending times.
Dennis