Acuity vs resolution vs micro contrast?

[Masukami]

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(admins, please move this post if it should be somewhere else) 

Acuity, resolution and micro contrast…

I read often these three terms and think I understand them.

But actually, upon reflection, I’m not sure I do. 

I’ve done some research and, as usual, there’s a lot of conflicting info out there.  

The best I can come up with is:

- resolution is the pixel density of a sensor / grain size of the film

- acuity is the ability of a lens to resolve image detail on the sensor / film, and

- micro contrast is something about distinguishing between different tones (unsure if this applies to both lens and sensor / film or just the lens).

After all my years of photography, I feel I should know this. But I don’t.
 

Could anyone please teach an old dog some new tricks and explain simply what these three terms actually mean?

 

 

[UliWer]

Since you posed your question in the subforum for lenses, please allow me to stick to the meaning of "resolution", "contrast" and "acuity" as qualities of a lens.

You may imagine a totally black line on a totally white background. You have maximum contrast between the line and its background - just because there is no light on the line and all the light you can get on the background. So first of all, contrast is about light. A lens which reveals the maximum contrast between the line and its background is certainly "better" than one which shows the line in dark grey and the background in light grey. It conveys more light and therefore more contrast. 

Now imagine not just one black line on white background, but 20, 40, 80, ...  pairs of black lines and white background in a space of one millimeter. You take a photo of these different pairings of lines.

With 20 black lines and whites on millimeter almost all lenses will show you the distinction of dark and bright - though perhaps you will see differences so that with some lenses the distinction between black and white is not so good as with others.

With 40 black lines and white on one millimeter many lenses will have problems to show what's actually on the photo. If you perhaps used a pre-aspherical Summilux fully opened, you probably would see just a greyish surface with almost all the distinction between black and white gone. The lens - fully opened - does not show enough contrast to resolve a microcontrast of 40 lines per millimeter. It's resolution is too low - or, which means the same, the lens does not reveal enough light to let you see the contrast between black and white - even though it is fully opened.  If you stopped it down to f/5.6 the picture would change: you would be able to see most of the distinctions. The light finds a better way through the optical system and so shows more contrast which results in better resolution. Though with 80 or more lines and whites on one millimeter, f/5.6 of a pre-asph Summilux would not help much, you would need an Apo-Summicron. 

Now, where is sharpness or acuity in your image? Just imagine that your lens does reveal the black line drawn with a pencil without the help of a ruler. May be your pencil is very sharp, hard and dark black - you will see a lot of contrast, but the line is not completely straight, there will be irregularities at the bounders between dark and bright. Some lenses reveal quite a good amount of light resulting in good resolution of micro contrast, but they lack sharpness because some optical faults show their bad effects in the optical system of the lens. Your lens may be rather contrasty but not very sharp.

Though in most cases the lack of sharpness also results in lack of contrast. Since the boundaries between black and white are not clear any more, you (and your lens) will have much more problems to see the distinction between both; your picture doesn't show enough resolution. So sharpness and contrast are not exactly the same but very closely related to achieve resolution. In practice  - certainly at 40 black and white lines on one millimeter - both terms mean the same: without sharpness there is not enough contrast; without contrast you cannot achieve enough sharpness.  

[spydrxx]

This is how I usually explain resolution and contrast

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[cboy]

@19:50

- key take away the better the lens performs the better the lens can transfer low contrast object to the image point

[pgk]

16 hours ago, Masukami said:

Acuity, resolution and micro contrast

Acuity is a subjective term which referes to the visual sharpness of an image.

Resolution can be measured and is usually measured using high contrast targets (test charts) and will give a figure for the smallest resolved detail.

Micro-contrast is discussed here: https://lenspire.zeiss.com/photo/en/article/micro-contrast-and-the-zeiss-pop-by-lloyd-chambers/ but could very roughly be said to be the acuity of fine detail (so is linked to resolution in a way).

The problem is that all are used in various ways to support contentions, but few ever really define the words that they use. If you want to look at an MTF chart (a very good starting point for lens performance) you will find that lenses considered to exhibit good characteristics of acuity, resolution and micro contrast will have higher graphical values at smaller spatial frequencies. That is to say that the differentiation of fine details is better than that from poorer lenses which will have lower graphical values as they approach their resolution limits (10% value is often considered the point at which contrats is too low to see visually). But this is only part of the story. Its a complex one and not always easy to figure out. Acuity can be manipulated (unsharp masking will do so) for example but cannot increase resolution (although visually it might look as though it does subjectively). So there is also some crossover but not always.

Plenty has been written about lenses (web searches will reveal just how much) and their characteristics. Subject matter and lighting play a significant role too and this is all too often ignored.

[adan]

For me, acuity/accutance and micro-contrast are more or less synonyms. They both derive from the "point-spread function."

They are also more about edge definition, as opposed to fine-detail definition.

I've had a few older Leica lenses that are a bit weak on resolution (lpmm) but still have an interesting ability to define tonal separation (e.g, the black, gray and white of an eye: pupil, iris, "white" of the eye.)

The 1980 90mm Summicron at f/2.0 falls into this category - won't win prizes for bringing out fine textures of skin or hair or cloth, but, boy, can it make eyes "pop!" and look right back at me! Even if slightly OOF.

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Edited February 2, 2022 by adan

[Jon Warwick]

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On 2/2/2022 at 1:10 PM, adan said:

For me, acuity/accutance and micro-contrast are more or less synonyms. They both derive from the "point-spread function."

They are also more about edge definition, as opposed to fine-detail definition.

I've had a few older Leica lenses that are a bit weak on resolution (lpmm) but still have an interesting ability to define tonal separation (e.g, the black, gray and white of an eye: pupil, iris, "white" of the eye.)

The 1980 90mm Summicron at f/2.0 falls into this category - won't win prizes for bringing out fine textures of skin or hair or cloth, but, boy, can it make eyes "pop!" and look right back at me! Even if slightly OOF.

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So to clarify ….and I agree your (excellent) image here really pops …. when I look at some of my prints with M Leica lenses, especially on film, I see “shape” to the subject. Nothing flat at all. On other lenses / systems, however, the subject looks more one dimensional and “flatter”. What causes the “shape” that I perceive? ….micro-contrast of the lens, or is it more the underlying film or sensor’s ability to differentiate between shades of the same colour?

[adan]

4 hours ago, Jon Warwick said:

What causes the “shape” that I perceive? ….micro-contrast of the lens, or is it more the underlying film or sensor’s ability to differentiate between shades of the same colour?

In the context of lenses, I would say that it is the LENS'S underlying ability to differentiate between tones. What I call its macro-contrast.

In painting terms, it is a form of chiaroscuro, or the use of light (chiaro) and shade (scuro) to make a flat shape look 3-dimensional.

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Although that can be tricky. Some lenses with high macro-contrast just blow highlights out of the water, and/or clip shadows. Other lenses manage to have strong macro-contrast in the mid-tones, but produce shadows and "whites" that seem to go on forever.

It is interesting to compare Medieval painting, which tends to be flat and 2D, with Renaissance painting. In medieval times, a face (or cloth, or field, or the shadows between fingers) was mostly one color/tone, with perhaps a few lines drawn in to sort-of define a wrinkle or a fold, or some experiments with cross-hatching. Didn't matter how sharp or contrasty the lines were, they still looked cartoonish. Once chiaroscuro, or broader areas of strong tonal change, rather than just lines, was (re)discovered by Renaissance painters, painting took on much more 3D realism.

https://www.medievalists.net/2014/10/memento-mori-medieval-images-death/

https://drawpaintacademy.com/chiaroscuro/

But there are other lens factors - the structure of the bokeh or blurs and how they separate planes in a picture; the transitions between sharp and unsharp with depth; the quote-perspective-unquote (big things are closer, small things are farther away). The brain uses all of those to perceive 3D depth even in a 2D image. And of course the lighting itself is going to play a role, whatever the lens does with it.

I just picked up a 50mm Elmar-M (the 1994 redesign of the 1950s lens). It is not exactly an APO/ASPH when it comes to resolution/clarity (although pretty decent) - and just 4 silly little elements. But boy, can it "paint" and "sculpt" 3D forms (eye-sockets, eyeballs, pudgy fingers, wrinkles ) nicely. Without falling off the ends of the histogram.

I haven't used a 50mm regularly for 40 years - that may be about to change.

[Jon Warwick]

Hi, thank you for your reply, which was absolutely fascinating (and made it much clearer to me what some of the drivers are for this 3D “look”). Thanks again!