Acutance - is there something real behind the hype?

[keithwms]

Yeah, with no clue as how to proceed with a digital workflow from a film scan one will always screw up.

Hybrid.com or the LF forum is a better place to discuss the merits of digital workflow from film scans.

Sandy King

I certainly didn't mean to imply that someone will necessarily get poor results via the hybrid workflow. Rather, my point was... ach never mind Of course, very fine results can be obtained via hybrid methods.

 

[sanking]

I certainly didn't mean to imply that someone will necessarily get poor results via the hybrid workflow. Rather, my point was... ach never mind Of course, very fine results can be obtained via hybrid methods.

With apologies. I did misread the intent of your message to which I responded. No foul intended.

Sandy King

 

[RalphLambrecht]

Ralph, for example the late Forte Polywarmtone was extremely sharp, the Agfa MCC not. Graded papers are also supposed to be sharper than VC papers: don't ask me why!

Philippe

The first problem is that there is no universal measure for sharpness. There is a measure for resolution, but (apparent) sharpness is a combination of contrast and resolution. Contrast is further divided into overall contrast and micro-contrast or acutance.

Any paper's resolution (60+ lp/mm) is far better than what the eye can see (~7 lp/mm). Consequently, any apparent sharpness difference between papers most likely comes from paper contrast and surfaces structures. A negative printed on grade 3 will always appear sharper than the same negative printed on grade 2, and a matt paper will always look duller than a glossy print.

 

[RalphLambrecht]

John Sexton uses a Jobo rotary processor for his 4x5 film sheets developed in Jobo Expert-Drums and his prints look sharp to me.

John Sexton's prints are sharp as sharp can be, but he does not need acutance to get sharp images. He uses large-format film. Acutance is not important when using large format.

 

[RalphLambrecht]

1. The way we perceive sharpness in the print has a lot to do with tone transitions i.e. lower frequency effects, and less to do with high-frequency sharpness and lp/mm and MTF etc. which is normally chatted about when people discuss sharpness.

2. By the way, I find that, quite generally, traditional-grain films give more perceived sharpness in print than the t-grain films. Honestly, I find it very hard to get a snappy, sharp-looking print from tmax or delta unless I shoot and develop for higher contrast in the neg. This isn't really a flaw of the film or a technical lack of sharpness in the film, it's just that the tonal transitions are just too gentle and gradual (i.e. dreamy) for my taste.

I agree with 1. but isn't the effect you describe in 2. a high-frequency characteristic?

 

[RalphLambrecht]

Probably not with rotary processing, but L.F.A. Mason recommends continuous agitation for the first 10 seconds, then 5 seconds every minute/8 twists of the spiral with High Acutance developers. That's about normal for any developer for me.

Mason was head of Research at Ilford and would have been involved with Hyfin, which was the compnies High acutance developer

Ian.

If you agitate once a minute, you won't get full acutance. Full acutance needs standing development until 'the bromide streaks come home'.

 

[RalphLambrecht]

So I have usually stop down a lot.

This might be where you're losing your sharpness!

Try to limit yourself to f/8-11 and compare.

 

[sanking]

John Sexton's prints are sharp as sharp can be, but he does not need acutance to get sharp images. He uses large-format film. Acutance is not important when using large format.

I don't agree at alll. In IMHO acutance (combiantion of edge effects and contrast) is very important with LF and ULF film development.

With LF and ULF you are pretty much assured of resolution. Question is, will this detail be rendered sharply (which depends on edge effects and contrast).

Sandy King

 

[Usagi]

This might be where you're losing your sharpness!

Try to limit yourself to f/8-11 and compare.

That's what I am going to do for the test. Ilford HP5+, aperture for best resolution, scenery with lots of structural and textural details and then xtol, rodinal and pyrocat-hd, perhaps DiXactol also.

Ofcourse I don't have calibrated developing times for all developers and one film, so I have to guess good developing times and then hope that I can get somewhat equal prints in terms of tonality.

 

[RalphLambrecht]

I don't agree at alll. In IMHO acutance (combiantion of edge effects and contrast) is very important with LF and ULF film development.

With LF and ULF you are pretty much assured of resolution. Question is, will this detail be rendered sharply (which depends on edge effects and contrast).

Sandy King

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. Apparent sharpness has three components, resolution, acutance (edge effect) and overall contrast. In MTF terms, contrast, acutance and resolution performances are measured at different MTF limits.

a) Contrast is very important for all film formats and is measured at low MTF frequencies. You can get overall contrast without resolution or acutance.
b) Acutance is very important for 35mm film but less so for LF. It is measured at medium MTF frequencies. With an 8x10 contact print the edge effect is so small that it is invisible to the human eye.
c) Print resolution increases with film format and is measured at high MTF frequencies. The jump from 35mm to MF has a huge effect on print resolution. The jump from MF to LF is less dramatic, because the increase in resolution is beyond the eye's detection capability unless big enlargements are made. The lack of resolution with small formats is often compensated with acutance. It's not true resolution but the human brain doesn't seem to care. That's why acutance developers ( and digital sharpening techniques) are so successful.

 

[RalphLambrecht]

That's what I am going to do for the test. Ilford HP5+, aperture for best resolution, scenery with lots of structural and textural details and then xtol, rodinal and pyrocat-hd, perhaps DiXactol also.

Ofcourse I don't have calibrated developing times for all developers and one film, so I have to guess good developing times and then hope that I can get somewhat equal prints in terms of tonality.

A proper test takes 5 rolls of film for each unknown film/developer combination. Shoot each roll at box speed and develop them at 4,5.5, 8, 11 and 16 minutes. This way, you'll be able to compare apples with apples and have all the data you'll ever need.

 

[gainer]

Some make the mistake of using a blue filter with a grain focusser to focus the enlarger. The makers of the best grain focussers used to supply such a filter. The problem lies with the human eye. It is at its sharpest with green light, worst with blue or red. Further, its focal length is longer in red light and shorter in blue. I showed in a paper for Photo Techniques some years ago that there is a mean shift of focus as well as a greater scatter when the focussing light is not white or green. Most enlarging lenses these days have little or no focus shift over the useful range of printing materials, so nothing is gained by not using white light.

 

[keithwms]

2. By the way, I find that, quite generally, traditional-grain films give more perceived sharpness in print than the t-grain films. Honestly, I find it very hard to get a snappy, sharp-looking print from tmax or delta unless I shoot and develop for higher contrast in the neg. This isn't really a flaw of the film or a technical lack of sharpness in the film, it's just that the tonal transitions are just too gentle and gradual (i.e. dreamy) for my taste.

I agree with 1. but isn't the effect you describe in 2. a high-frequency characteristic?

Well, yes and no, I think. Obviously the high and lower frequency detail is all convolved together in how we perceive the image. But my half-baked reasoning (which could well be flawed; I haven't put much thought into it) is that the ultimate resolution of the traditional grain and t-grained films is just about even.... certainly moot for the smallish enlargements that I do. Yet my results from the traditional grained films almost always look sharper. That being the case, I conclude that the apparent lack of sharpness that I see in tmax and the deltas has more to do with separation of tones, which I think of as a lower frequency aspect- something you see even in broad gradients and areas of a print that don't even necessarily have high frequency detail.

The edge 'bite' of the traditional grained films is, I suspect, mostly coming from the traditional grain structure- the edginess comes across as acutance. Perhaps the grain structure interacts with the fast tonal gradients (e.g. at edges) to create more apparent sharpness. What led me to this thought was that the really low res digicams can produce images that looked very sharp, at a glance, even though the overall level of detail isn't so good. But what was happening, I suppose, is that the edges were grained and that tends to make them appear sharper. And people often don't miss the fine detail that isn't there; what they do miss is edge definition, when it is lacking.

This requires further thought- it is an issue quite separate from the clinical definition of sharpness and more to do with how we perceive the level of detail.

Do you have a different take?

 

[gainer]

Acutance has scale effects due to the fact that it has no scale effect. Nonsense? No, an edge effect depends on a local gradient. A black line of a certain true width will be seen as wider or narrower according to the developing agent and other factors by the same amount, not proportion, and that amount will be a greater proportion of a small negative than of a large negative of the same subject. Just something to keep firmly in mind, especially if you use both small and large format.

 

[Ian Grant]

If you agitate once a minute, you won't get full acutance. Full acutance needs standing development until 'the bromide streaks come home'.

Depends on your definition of full acutance.

Developers like Hyfi, Definol & Acutol-S were designed to give good noticeable acutance with that amount of agitation, if you cut the agitation significantly you would over exaggerate the acutance and eventually get extreme Mackie lines etc. I tried all three of those developers and they did give high acutance but at a cost, grain and tonality were affected and with 35mm that gave a distinct decrease in overall image quality, with 120 or larger formats this was far less noticeable.

Of course if a higher degree of acutance is desired then cutting the agitation or using stand development gives it, but the practice is more common with large format negatives for contact printing.

Ian

 

[Keith Tapscott.]

Depends on your definition of full acutance.

Developers like Hyfin, Definol were designed to give good noticeable acutance with that amount of agitation, if you cut the agitation significantly you would over exaggerate the acutance and eventually get extreme Mackie lines etc.

Ian

The only developer I know of that type that is still available from the more well known photo-chemical manufacturers, is Tetenal Neofin Blue. It`s still a great developer for slow conventional B&W films of around ISO 25-50.
I don`t understand why it isn`t more popular.
Try it with Pan F Plus, Adox/Efke 25 or Rollei Pan 25.
Tetenal`s MSDS show it to be an M.Q. developer.

 

[Keith Tapscott.]

If you agitate once a minute, you won't get full acutance. Full acutance needs standing development until 'the bromide streaks come home'.

The trouble with that if you are using 35mm, is that you run the risk of getting streaks in areas such as clear sky in the image near the sprocket holes. Agitation at one-minute intervals is a good compromise. You could also try semi-stand to help prevent uneven development.
As for acutance, simply diluting a standard type developer helps to produce edge-effects.

 

[RalphLambrecht]

Depends on your definition of full acutance.

Developers like Hyfi, Definol & Acutol-S were designed to give good noticeable acutance with that amount of agitation, if you cut the agitation significantly you would over exaggerate the acutance and eventually get extreme Mackie lines etc. I tried all three of those developers and they did give high acutance but at a cost, grain and tonality were affected and with 35mm that gave a distinct decrease in overall image quality, with 120 or larger formats this was far less noticeable.

Of course if a higher degree of acutance is desired then cutting the agitation or using stand development gives it, but the practice is more common with large format negatives for contact printing.

Ian

So, you agree that the more you agitate, the less acutance you get?

 

[RalphLambrecht]

Do you have a different take?

No, I think you are correct in keeping resolution and sharpness apart. It is actually possible to have a lower-resolution image with high contrast looking sharper than a higher-resolution image with low contrast. The eye prefers contrast over resolution when it comes to sharpness.

IMHO, the best approach is to realize that sharpness is not a measurable characteristic but that it is made of resolution, acutance and contrast, which are measurable. It's like heat and temperature. 'Heat' means different things to different people, it is subjective, but 'temperature' can be measured and objectively compared.

 

[RalphLambrecht]

Acutance has scale effects due to the fact that it has no scale effect. Nonsense? No, an edge effect depends on a local gradient. A black line of a certain true width will be seen as wider or narrower according to the developing agent and other factors by the same amount, not proportion, and that amount will be a greater proportion of a small negative than of a large negative of the same subject. Just something to keep firmly in mind, especially if you use both small and large format.

Makes sense, but from the way you posted it, it's not clear what message this response relates to. Could you elaborate on the impact of the edge effect with different formats, please.

 

[dancqu]

'Acutance' is an overblown term, not as overblown
as 'tonality' but close. In any case 'acutance' is something
I do my best to avoid - I don't want the perception of sharpness
- I want real resolution.

Tonality = Gradation, true or false?

At last a definition. Acutance is "the perception of sharpness"
Does a high acutance follow from a high resolution? Dan

 

[Ian Grant]

So, you agree that the more you agitate, the less acutance you get?

I put it the other way round, correct development and agitation gives the degree of acutance that the manufacturers formulated a particular developer for. Mason also states that too little agitation with acutance developers can cause streamers. Crawley recommends similar agitation to Mason for his own High Acutance formula for FX-1.

So yes decreasing the agitation will exaggerate the acutance, but this in turn can sometimes be detrimental. The developers I mentioned earlier all gave exceptionally good high acutance and edge effects with their recommended agitation.

In addition certain developer types will give high acutance even with more vigorous agitation due to the developing agents tanning the emulsion, this is particularly true of Pyrogallol & Pyrocatechin. This is why some can give increased acutance compared to standard developers even when used in rotary processor's, although it won't be quite as prounced as when normal agitation is used.

So yes I agree but with a different starting point & perspective. Normal (not over zealous) agitation gives excellent edge effects with acutance developers.

Ian

 

[clay]

I can give you an example from a few years ago. I was trying Sandy's Pyrocat-P ( with p-aminophenol ) with reduced agitation, intentionally trying to induce edge effects. But I was shooting 35mm, and I got the edge effects alright! But the problem was this: because a Mackie line is dependent on the grain size, not the format, the degree to which you enlarge the negative is an important factor in whether the line will give a perception of edge sharpness or not.

When I made a contact sheet, the negatives from this test roll looked noticeably sharper due to edge effects. But when I did a 5x enlargement of the images, the Mackie lines pretty much just made the photos look stupid. Backlit edges in the photos had a line surrounding them that looked like someone with a Sharpie and a ruler had drawn on the photo.

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.

Makes sense, but from the way you posted it, it's not clear what message this response relates to. Could you elaborate on the impact of the edge effect with different formats, please.

 

[Nicholas Lindan]

Tonality = gradation true or false?

"Tonality" as used on APUG mirrors the word "Taste."

Of itself it means nothing. To say something "taste's great" only means the speaker likes it - until you know the likes or dislikes of the speaker the statement means less than nothing. To say a film has "great tonality" is meaningless on its own.

I would toss "Gradation" in the same pile. If someone said "Wow, this hill has great slope..." then immediately up pop the questions "Slope for what? And what is a great slope: a stable angle of repose? nearly vertical and full of moguls?"

These words describe attributes, they do not indicate quality per se.

 

[RalphLambrecht]

Tonality = Gradation, true or false?

At last a definition. Acutance is "the perception of sharpness"
Does a high acutance follow from a high resolution? Dan

Not sure that there is a 'true' definition, but I understand 'tonality' as the range of tones and 'gradation' as the change from one tone to another.