Sigma SD1 review

[Ray]

What exactly micro-contrast could mean, I have no real idea.

Really!  Well you've certainly come to the right website to be informed, haven't you! 

I'll repeat from Michael's definition of microcontrast:

the ability to take two small areas of slightly different luminance and distinguish the boundary of one from the other

Seems quite clear to me. What's the problem? Another way of describing this is to use the word 'sharpness'. There are various ways of increasing the sharpness of an image and different terms to describe such increases in sharpness. Increases in sharpness at the pixel level, with a pixel radius in Photoshop of 1 or 2 or 3 pixels or so, could reasonably be described as an increase in microcontrast.

Increases in sharpness using a pixel radius setting of 20-50 pixels, in Photoshop, could be described as an increase in local contrast.

Increases in sharpness using pixels of significantly greater radius than 50 are usually referred to as increases in global contrast, and are perhaps better done with the brightnesst/contrast control.

What's the problem? Images produced from sensors without an OLPF generally need less sharpening because they have greater microcontrast to begin with.

[Dave Millier]

Sounds no different from detail to me. What is detail/resolution if not the ability to distinguish one spot of an image from another. No detail = homegenous tone, detail=distinguishable tones. Fine detail=ability to distinguish ever small bits of tone.

Why invent a new term?
 

Really!  Well you've certainly come to the right website to be informed, haven't you!  

I'll repeat from Michael's definition of microcontrast:

Seems quite clear to me. What's the problem? Another way of describing this is to use the word 'sharpness'. There are various ways of increasing the sharpness of an image and different terms to describe such increases in sharpness. Increases in sharpness at the pixel level, with a pixel radius in Photoshop of 1 or 2 or 3 pixels or so, could reasonably be described as an increase in microcontrast.

Increases in sharpness using a pixel radius setting of 20-50 pixels, in Photoshop, could be described as an increase in local contrast.

Increases in sharpness using pixels of significantly greater radius than 50 are usually referred to as increases in global contrast, and are perhaps better done with the brightnesst/contrast control.

What's the problem? Images produced from sensors without an OLPF generally need less sharpening because they have greater microcontrast to begin with.

[Ray]

Sounds no different from detail to me. What is detail/resolution if not the ability to distinguish one spot of an image from another. No detail = homegenous tone, detail=distinguishable tones. Fine detail=ability to distinguish ever small bits of tone.

Why invent a new term?
 

It's not exactly new. It's been around for a long time. It sort of explains what the sharpening process is trying to achieve.

[michael]

A couple of comments.

Resolution and sharpness aren't the same thing. I thought that this was pretty well understood.

When talking about digital sharpness we are in fact discussing edge boundaries... the transitions between tonalities. That's what digital sharpening does – it modifies these boundaries.

One can have a high resolution image that needs sharpening and a low resolution image that doesn't. The factors that affect this are a lens' resolving power, its contrast (MTF) the scene's unique characteristics, the presence or absence of an AA filter, and so forth.

I don't know why anyone had difficulty finding a definition. A Google search turns up an article on this site by Mike Johnston from some years ago as the top hit. It reads in part...

What we're talking about is the ability of the lens to differentiate between smaller and smaller details of more and more nearly similar tonal value. This is also referred to as "microcontrast." The better contrast a lens has (and this has nothing to do with the light­dark range or distribution of tones in the final print or slide) means its ability to take two small areas of slightly different luminance and distinguish the boundary of one from the other

If you've ever used the "Clarity" slider in Lightroom or Camera Raw you are trying to enhance micro-contrast. This is also known as "local contrast enhancement". You can look that up as well.

Finally, as one of the characters in Alice in Wonderland said – "words mean what we want them to mean". The English language has some 1 million words, and is growing by many thousands a year. The reason it's growing is become we need to invent new words to describe things that either didn't esists or didn't need describing before. Now they do.

On a personal note, I think I was the first person to coin the phrase "pixel peeping" some years ago. It doesn't really matter whether I did or not. But the point is that one doesn't need an ISO definition to be created for this to have become an important, even necessary phrase in the digital photography milieu. Everyone now knows what it means. Just as many know what micro-contrast means.

Michael

[kwalsh]

Micro-contrast as well as why "resolution" is a poor metric for a lens is covered by Zeiss in this document:

http://www.zeiss.com/C12567A8003B8B6F/EmbedTitelIntern/CLN_30_MTF_en/$File/CLN_MTF_Kurven_EN.pdf

The basic idea is that micro contrast is related to the steepness of the MTF curve as it approaches the resolution limit.  Resolution and micro-contrast are loosely correlated, but not directly correlated.  Hence the additional term.  As a weak analogy it is sort of like the difference between the MTF50 and MTF10 points.

Like most terms it is overused and misused extensively.  Michael has the basics of it right in his his MTF article.  Read the Zeiss article linked for a bit more technical description.

Can a camera affect micro-contrast?  Yes, most definitely, as the OLPF is not a sharp roll off filter it also attenuates the spatial frequencies associated with micro-contrast.  Take the OLPF out and you'll see not just increased resolution (and false resolution corrupted by aliasing) but also increased micro contrast (less susceptible to aliasing as these lower spatial frequencies are further from the Nyquist folding frequency).  Some more about how the camera is involved is covered in this second Zeiss article:

http://www.zeiss.com/C12567A8003B8B6F/EmbedTitelIntern/CLN_31_MTF_en/$File/CLN_MTF_Kurven_2_en.pdf

NOTE: Appears links don't parse correctly, you'll have to cut and paste

Ken

P.S. Cross posted with Michael, I agree again with what he says for the less technically inclined.

[Sareesh Sudhakaran]

Sounds no different from detail to me. What is detail/resolution if not the ability to distinguish one spot of an image from another. No detail = homegenous tone, detail=distinguishable tones. Fine detail=ability to distinguish ever small bits of tone.

May I take a guess? Contrast can be determined by an automated process with reference to the information already provided within the pixels, and nothing else. But to determine detail requires a further reference to a standard of what it (human or algorithm) wants to judge (or match) as detail.

In a photograph of a red rose, the algorithm for contrast will perform its task by analyzing the pixels within the file and comparing it to one standard. Even if we shoot a blue shark, it will do the same, referencing the same standard. To put it technically: Contrast detection is a limited system that does not require a feedback loop.

However, the details inherent in the (rose) photograph can only be ascertained (and judged, or matched) by knowing what a rose is, and what it looks like. If we zoom into the rose using an electron microscope, then we will find more detail. Yet this detail has nothing to do with the contrast analysis that has been assigned to the contrast detection algorithm. If we zoom out with a telescope, the details of the rose will be lost due to the limited resolving power of the sensor, yet the contrast algorithm will happily chug along, not knowing what the heck it is comparing. But it does its job well, and leaves the job of finding detail (and selecting the right tools for the job) to the eye of the beholder. Without this great subjective eye, there is no such thing as detail. Sometimes the same eye is content to find details in cells within the rose, or is content with the detail present in just a silhouette of the rose against the sky. This system requires a feedback loop - and the feedback mechanism can only be generated via an ever changing (and sometimes conflicting) library of past experience - both subjective and objective.

[Graeme Nattress]

To my mind, "Clarity" or other such sliders enhances large area contrast - it does nothing for small scale contrast. If you look at what the effect is - unsharp mask with a very very large radius and low amount, you can see it's a large area effect. It's enhancing the low frequency MTF, not the highest frequency MTF which traditional use of unsharp mask alters.

What it comes down to is we need an MTF plot rather than talking a single number for resolution. We need to be able to see how much contrast we get for all frequencies represented in the image and also to be able to see how much aliasing occurs and how much that is contributing to the MTF.

http://reduser.net/forum/showthread.php?58227-Leica-Lens-Resolution-test...&p=759177&viewfull=1#post759177 are not exactly MTF plots, but they're related and show pretty clearly what's going on.

Graeme

[hjulenissen]

What it comes down to is we need an MTF plot rather than talking a single number for resolution. We need to be able to see how much contrast we get for all frequencies represented in the image and also to be able to see how much aliasing occurs and how much that is contributing to the MTF.

Exactly what I am thinking. We seem to be inventing all kinds of indirect, flawed descriptions of the real MTF, some subjective and and some measured ones. Are there any relevant (for this context) moments that are not contained within the MTF?

Reminds me of how to describe a lowpass filter. Do you say "-3dB@10kHz", "12dB/octave Butterworth, fc=100Hz", "a really fat, booming sound", or a detailed sufficient description to capture its behaviour (pole-zero representation, or similar)?

-h

[michael]

Not all photographers have the technical background to understand MTF. That's why alternative descriptors are necessary.

I don't care what the PH of a wine is, how much tanin it contains in PPM, or the spectral frequency of its colour under a standard light source. I just want to know how it tastes!! And failing the ability to actually taste it myself, if someone describes it as "ruby coloured, with a rich nose and and a strong finish" I have garnered some information which a chemical assay wouldn't provide.

Michael

[Graeme Nattress]

Michael, that's a good argument to provide for a well written description and visuals that elucidate points about images (as you do), but it's not an argument to avoid showing MTF plots. With whisky tasting, flavour graphs are often provided to help explain the taste profile visually, just as an MTF plot shows the contrast profile over frequency visually.

Graeme

[Dave Millier]

Maybe it depends on which part of the world you were brought up in, which terms are used?

When I learned photography formally, we used a well known starter text by Michael Langford called "Basic Photography". Langford described "sharpness" as the subjective experience of two factors: resolution and acutance. Acutance sounds like what you call micro-contrast; it's edge contrast, the rate of roll off between tones. Our vision responds much more to acutance than resolution, such that a high acutance shot of relatively modest resolution looks subjectively sharper than a high resolution, lower acutance shot - even though the high res shot has more real detail. This property was exploited by film developers like Rodinal which enhanced edge sharpness.

I have no problem with the concept of acutance, my objection is to inventing new terms for old ones or inventing new terms to describe something that doesn't exist.  When a new term is coined, it isn't always obvious whether it is a synonym or BS.... ;-)

A couple of comments.

Resolution and sharpness aren't the same thing. I thought that this was pretty well understood.

When talking about digital sharpness we are in fact discussing edge boundaries... the transitions between tonalities. That's what digital sharpening does – it modifies these boundaries.

One can have a high resolution image that needs sharpening and a low resolution image that doesn't. The factors that affect this are a lens' resolving power, its contrast (MTF) the scene's unique characteristics, the presence or absence of an AA filter, and so forth.

I don't know why anyone had difficulty finding a definition. A Google search turns up an article on this site by Mike Johnston from some years ago as the top hit. It reads in part...

What we're talking about is the ability of the lens to differentiate between smaller and smaller details of more and more nearly similar tonal value. This is also referred to as "microcontrast." The better contrast a lens has (and this has nothing to do with the light­dark range or distribution of tones in the final print or slide) means its ability to take two small areas of slightly different luminance and distinguish the boundary of one from the other

If you've ever used the "Clarity" slider in Lightroom or Camera Raw you are trying to enhance micro-contrast. This is also known as "local contrast enhancement". You can look that up as well.

Finally, as one of the characters in Alice in Wonderland said – "words mean what we want them to mean". The English language has some 1 million words, and is growing by many thousands a year. The reason it's growing is become we need to invent new words to describe things that either didn't esists or didn't need describing before. Now they do.

On a personal note, I think I was the first person to coin the phrase "pixel peeping" some years ago. It doesn't really matter whether I did or not. But the point is that one doesn't need an ISO definition to be created for this to have become an important, even necessary phrase in the digital photography milieu. Everyone now knows what it means. Just as many know what micro-contrast means.

Michael

[Dave Millier]

Isn't this kind of commentary exactly the problem that hifi reviewers invented?

It's all very well to say "Speaker A's high frequencies are more open an airy than Speaker B's" but what does it mean? How can any reader know for sure that their interpretation of what it might mean is what the author intended or what another reader would think? You can't really as there is no common reference point.

What happens is everyone develops their own ideas of what the phrase means, can talk to other people using the same phrase and even appear to have a common language and agree on things - whilst having no idea whether they are talking about the same thing. Intrinsically this kind of undefined casual speech seems to protect itself from scrutiny by being inately vague enough to sound plausible to everyone whilst meaning nothing.

It's interesting to hear people say that micro-contrast as a term has a long heritage. That was unknown to me. Perhaps the term I have always believed to be a common technical term (actutance) isn't? Or perhaps it is in UK only? Geoffrey Crawley the late reviewer and inventor of Paterson developers like Actutol certainly used it in his writings....

Not all photographers have the technical background to understand MTF. That's why alternative descriptors are necessary.

I don't care what the PH of a wine is, how much tanin it contains in PPM, or the spectral frequency of its colour under a standard light source. I just want to know how it tastes!! And failing the ability to actually taste it myself, if someone describes it as "ruby coloured, with a rich nose and and a strong finish" I have garnered some information which a chemical assay wouldn't provide.

Michael

[michael]

Accutance is a very good word, and one that photographers in the darkroom days were well familiar with.

But in the digital era it seems to have passed out of favour. Not sure why. Edge contrast, micro contrast, edge sharpening, all are in vogue now and have slightly different meanings. I almost never hear the word accutance anymore except in chemical darkroom discussions.

Tri-X in Rodinal 30:1. Now that had high accutance.

Michael

[hjulenissen]

Not all photographers have the technical background to understand MTF. That's why alternative descriptors are necessary.

I do respect the difficulties of conveying a complex subjective matter that is so intermingled with technical stuff (incidentally, most of my professional and hobby interests tends to include both domains).

My point is that such alternative descriptors cause more confusion than enlightenment. For the less technically inclined, stating that "this lense enable me to make images that look really good to me, here, have a sample" is (to me) much better than claiming that "this lense have a magical oompti-doomph, and its spaciousness is only rivalled by its 3-dimensionality that really makes your images clarify their statement".

As my example earlier showed, the area of high fidelity has been so infiltrated by meaningless, pretentious words that I cannot have a meaningful discussion with most hifi salespersons or enthusiasts. They reject science, curiousness and humility and have created a quasi-religion. This makes me sad, as I am equally interested in music, music capture and music playback as I am in images.

I don't care what the PH of a wine is, how much tanin it contains in PPM, or the spectral frequency of its colour under a standard light source. I just want to know how it tastes!! And failing the ability to actually taste it myself, if someone describes it as "ruby coloured, with a rich nose and and a strong finish" I have garnered some information which a chemical assay wouldn't provide.

Even though I generally like wine analogies, I am not sure that it is relevant here. MTF can be measured with finite time and resources, and can be understood by finite time and resources. We are talking about simlified measures that contain no information beyond MTF.

Your wine examples, on the other hand, seems to show examples of what is difficult to measure/interpret. Much like "what is a good composition".

-h

[Ray]

My point is that such alternative descriptors cause more confusion than enlightenment. For the less technically inclined, stating that "this lense enable me to make images that look really good to me, here, have a sample" is (to me) much better than claiming that "this lense have a magical oompti-doomph, and its spaciousness is only rivalled by its 3-dimensionality that really makes your images clarify their statement".

I can't believe you are serious here. You seem to be feigning confusion on the issue.

Surely everyone who knows just a little about photography knows what contrast means. And even the least technical amongst us surely knows that micro means small, or very small.

The meaning of microcontrast should therefore be intuitive. It's also a term that seems particularly appropriate for the digital era. The term acutance implies a sharp edge. It's difficult to imagine how a pixel could have a sharp edge. The smallest degree of detail in a digital image is defined as the pixel. You can't have a representation of detail in a digital image which is smaller than a pixel, and therefore you can't have a pixel with a sharp edge.

But you can have a small cluster of pixels in one image, each differing in RGB values to a greater extent than the pixels of the same size of cluster, representing the same detail, in another image. In this context, it seems appropriate to describe such differences between the two images as differences in microcontrast rather than acutance.

Applying such concepts to the Foveon versus Bayer Array situation, taking as an example a cluster of 4 adjacent pixels, the cluster of 4 in the BayerArray will consist of two green, one red and one blue pixel, ie, a total of just 4 separate values, the other values being interpolated during the demosaicing process.

However, the cluster of 4 pixels on the Foveon sensor, representing the same image detail, will consist of 12 separate values of red, green and blue, each of which has been recorded in reality, at the time the shutter was open.

It would be interesting to compare an image from the Canon 50D after its AA filter had been removed, with the same scene shot with the SD1. Both cameras are 15mp and have a similar size sensor, although the SD1's sensor is very slightly larger I believe.

[hjulenissen]

I can't believe you are serious here. You seem to be feigning confusion on the issue.

Surely everyone who knows just a little about photography knows what contrast means. And even the least technical amongst us surely knows that micro means small, or very small.

I am not trying to be difficult, but I think that you guys are inventing a vague pseudo-term for something that allready has a better name. How do you measure micro-contrast? What scale differentiates it from global contrast, regional contrast, acutance, sharpness, MTF50? (measured in micrometers or relative to sensor size?). If it cannot be measured, what agreement exists on which images has lots of it, and which have little? Where on the MTF-curve do you see the micro-contrast?

The term acutance implies a sharp edge. It's difficult to imagine how a pixel could have a sharp edge.

The entire system would allow sharp edges to be recorded with some precision in placement and intensity differential. A perfect discrete (sampled) system would only be able to pass frequencies up to (excluding) one half the sampling frequency without aliasing.

Neither a digital camera nor a film camera would record edges with infinite acutance. In my world, that is the system response to a step-function. Or we can measure the system response to an impulse, a frequency sweep or whatever stimuli makes sense.

-h

[Ray]

I am not trying to be difficult, but I think that you guys are inventing a vague pseudo-term for something that allready has a better name. How do you measure micro-contrast?

With absolutely precise accuracy. One pixel has a value of R=120, G=180, B=240, and the adjacent pixel also has a value of R=120, G=180, B=240. Result? Microcontrast zero between these two pixels.

However, if the adjacent pixel has a value of R=320, G=140, B=89, then we have a specific and clearly defined degree of microcontrast betwee the two pixels. What's your probledm?  

I should mention of course, as an afterthought, that the visual perception of such differences in microconstrast, may not correlate with the RGB readings for all individuals. We all have an individulaistic interpretations of color. Some of us are officially color blind, or can't distinguish between reds and greens. Some of us are so mildly color blind that we haven't recognised the problem, or don't see it as a problem.

There many degrees of color blindness in between, for which there is no name.

[Eric Myrvaagnes]

There many degrees of color blindness in between, for which there is no name.

I suppose a degree at one end might be called "micro-colorblind?"

[Ray]

I suppose a degree at one end might be called "micro-colorblind?"

Don't be silly, Eric. 

One great advantage of the computer age for the photographer is that one can enlarge any small detail in an image to whatever size one wants.

Here's a random crop of 4 pixels from one of my images. It's been significantly enlarged in Photoshop using nearest neighbour interpolation. It's come up quite well; worthy of any Foveon sensor.

Following is a crop of the same four pixels, but interpolated using bicubic smoother.  Oops! No amount of sharpening could fix that.

Now I'm tempted to suggest that the differences between these two versions is conceptually similar to the differences between the Bayer array and the Foveon sensor, but I stress the word conceptually, and I admit I might be drawing a long bow. 

[ErikKaffehr]

Hi,

To my understanding "the funnel" represents MTF and MTF is shrinking with distance due to target having higher and higher frequency.

The RED goes much further to the right than the Alexa, is this due to higher sensor resolution, different OLP filtering or both? What would the plot look like without OLP?

Best regards
Erik

To my mind, "Clarity" or other such sliders enhances large area contrast - it does nothing for small scale contrast. If you look at what the effect is - unsharp mask with a very very large radius and low amount, you can see it's a large area effect. It's enhancing the low frequency MTF, not the highest frequency MTF which traditional use of unsharp mask alters.

What it comes down to is we need an MTF plot rather than talking a single number for resolution. We need to be able to see how much contrast we get for all frequencies represented in the image and also to be able to see how much aliasing occurs and how much that is contributing to the MTF.

http://reduser.net/forum/showthread.php?58227-Leica-Lens-Resolution-test...&p=759177&viewfull=1#post759177 are not exactly MTF plots, but they're related and show pretty clearly what's going on.

Graeme