Measuring Image Quality

[liopleurodon]

A few points.

As an enthusiast I don't measure image quality; I compare results. Using photos shot with two different cameras under controlled conditions (like DPR's studio scene) makes for a great way to compare bodies. I can look at an ISO12800 exposure and compare noise levels between two bodies for example. When doing these comparisons I give little attention to color or sharpness as color is malleable in post and sharpness is a function of your glass. So its also prudent to look at lens reviews wherever you can find them and compare results as well.

As far as sharpness/detail goes, so much more than resolution goes into it. It starts with the lens itself; it has its own resolving power (how much resolution it can render). Then that lens projects that light onto a sensor. The sensor either undersamples (less resolution than what the lens resolved) or oversamples (more resolution) it. For example my Sony 35/1.8 is capable of resolving much more than the 24MP my a7 III has. In this case switching to a 42MP a7R III would result in more detail. However there are lenses that won't even resolve the 24mp of my a7 III; mounting those lenses on an a7R III would result in zero extra detail. Then we also need to factor in whether or not the sensor has an AA (anti-aliasing) filter as those can reduce detail. Consideration must be given to ISO needed to expose correctly as well. At higher ISO there is obviously more noise, which can and will drown out fine detail.

 

[Keithpictures]

Do you shoot JPG, if so you are judging the IQ the camera algorithms can produce, or do you shoot RAW, in which case you would be judging the IQ your raw converter produces.

Hi, I do shoot JPG with my X100V during the day. At night I switch to RAW for more flexibility in post, to boost exposure without going to higher ISOs. Maybe that's a mistake..

With my GX9 I always shoot RAW, though I'd prefer not to. I just find no flexibility at all if I shoot JPG on m43.

In all the years (over 55 now) I have never considered what processor is in any of my cameras. NEVER. To me it's a non issue. My biggest concerns used to be the camera buffer size and the speed with which it could clear the buffer. But that is a problem I have NOT encountered anytime in the past 10-12 years, unless my memory card is way too slow :-D .

I don't even know what a processor does, really, but my assumption is that it handles the speed of many aspects (ie turn on time, shutter responsiveness), and somehow aids in the read-out of the sensor. I've also read/heard that it can aid heavily in IQ, especially in JPG engine, since it can add sharpness/noise reduction etc for a look I would edit for in PP anyway. Hard to trust all these, but I pay attention to reviews about it.

In your quest for IQ have you considered medium format? MF has more megapixels, and more DR than even FF.

Yes, the 50R and the X1D II look awesome, and I would consider if I had that kind of budget. Anything over 2000 is unlikely for me. I want the X-Pro3, but also wonder if any brand will release a rangefinder-style FF, and how it might compare regarding noise and DR. Those are what I care about, regarding IQ, especially noise.

Just out of curiosity, what is it that you normally shoot and how do you view your images? Do you print, and if so, how large? To me these factors are more important than any of the factors you listed.

I do print for photo exhibitions (I am a professional artist), ideally up to A1, but mostly A2 and A3. But mostly my work exists online. You can see my work on my instagram: https://www.instagram.com/keithpictures/

Today I would consider that any current camera that sports a 1" sensor, or larger, to be more than good enough for 99% of all photographic needs I might have. Even then, I still encounter many smaller sensored cameras producing excellent images.

My m43 photos at Yellowstone recently were hard to edit; the highlights and shadows crushed with minor color tweaks. I post-process quite a bit, and desire more latitude in my images.

 

[Keithpictures]

To get specific, I am shooting a Fuji X100V, which I love, and a Panasonic GX9 for zoom stuff. the m43 sensor disappoints me, specifically its DR and SNR. Too blotchy when pushing shadows. Too much noise in the sky.

I will upgrade, either to a Fuji ILC or something FF. But I wonder how much I'll benefit from something like a Panasonic S5 over a Fuji X-Pro3, and how to best understand the differences in sensor, besides size.

You probably should have started with that as the proposition.

True, my mistake. Oh well. I thought we could get a discussion about all the ways to measure it, since I'd be happy to hear others on the topic. Discussions of "doesn't matter" or "totally depends" are less helpful.

Several of us here are scratching our heads about the proposed idea of assigning numerical weights to various aspects of 'image quality' and eventually deriving what might be called a score after tabulating the result. If doing that works for you, fine ... but I don't think many people approach it that way.

DxOMark does it that way, sort of. Have you checked that source?

Right. I appreciate that a site like DxO exists. I'm not often this kind of person, and don't even understand Ev stops properly, and quantifying shadow/highlight recovery. But that's my aim here.

Unfortunately DxO doesn't have any modern Fuji cameras for some reason (my main interest), nor my m43 camera (GX9) to compare things against...

https://www.dxomark.com/Cameras/

 

[Dennis]

But then I think of my dream camera, the Leica Q2, and I wonder how much better it would do, when I fail to properly expose my shot. Can it save me even more than my X100V does? Would a FF camera be more prudent if I can avoid noise at higher ISOs, and achieve great DR? This is why I started the thread. I suppose I should have stated my interests more specifically, but it's fun to come up with equations IMHO

So that's interesting, because you're not looking at system cameras, so you can compare the lenses and sensors specifically knowing that these are the only lenses and sensors that are relevant. And the lens on the FF camera is fast enough that you exploit the large sensor (so long as shallower DOF isn't a problem).

I can see where that A or B comparison makes sense to do. I don't know how I'd go about doing it, short of reading lots of reviews, because as I mentioned, I struggle to relate the numeric measurements on sites like dxomark to real world implications. And so I don't know how I'd convert the Q2's "better" to monetary worth. I think a lot of subjectivity would come into play (ergonomics, enjoyment, etc.)

I wish you luck with your endeavor!

- Dennis
--
Gallery at http://kingofthebeasts.smugmug.com

 

[Keithpictures]

My starting point: sensor + processor + glass = IQ
I will ignore "glass" for this post, since it's a whole thing on its own. I'd happily accept DXO Mark scoring; unfortunately they are far from thorough in terms of lenses reviewed.

Focusing on the bits inside the camera, I separate sensor from processor, but I'm not even sure that makes sense...

It doesn't.

Care to elaborate why not? I've heard of Panasonic (for instance) using the same sensor as a previous camera, but getting more out of it with an updated processor. The Venus Engine is a separate electronic than the Sony IMXxxx. And perhaps resolution/microns/SNR might stay the same, but other aspects might change. Or is your point that the changes are simply not related to IQ?

I break down sensor like this:
Sensor size (best measured in area mm2), Resolution, Microns, Signal-to-Noise Ratio, Dynamic Range

Color accuracy.

I value Fuji film sim colors the most, esp casting sky blue more teal/desaturated. That's my look. I can achieve this in post if there's enough latitude (color gamut?), and often with my m43 there's not.

Ratio of sensor cell count to linear resolution.

I haven't heard of this. I'm interested in learning more! Does pixel size play any role here?

Characteristics of noise (pattern, speckling, etc).

THIS. I've grown to disdain m43 noise. I often over-correct in LR to smooth out noise, particularly in the sky.

Acceptance angle (which helps control vignetting and lateral color smear).

Never heard of this..

Sensor size is straightforward enough. I give "medium format" sensors a 12, "full-frame" sensors a 10, APS-C sensors 7.5, M43 sensors a 6, "one-inch" sensors a 4.5, and 1/2.3" sensors a 1.5... Debatable,

No, it's not "debatable", because everyone bakes their own concept of what size is "best" into the process of setting the "10" size.

What I mean is that my rubric for giving a 10 to FF, a 6 to m43 and a 1.5 to tiny sensors is debatable. But I take it as evident that larger sensors should presumably offer an IQ advantage, and it's a fair enough place to start. I'm happy to be proven otherwise. And I do suspect that, given the myriad of categories by which to measure quality, FF might not be advantageous at all.

but good enough.

Resolution I break down similarly. It's an easy stat to find and score accordingly.

No, it's not. There are dozens of methods.

For resolution? Surely 6240x4160 has a clear advantage over 5184x3888. At least in print size, if not detail. How is that wrong?

The big problem is, either you present the user with a few dozen pages of numbers of your measurements, or you mix them down to a simpler number or group of numbers. And every mix has its proponents and detractors, which are largely based on what one shoots. A portrait photographer doesn't care that you can see every pore on a 15ft print of a face: they care that there's no weird artifacts in the hair. The landscape photographer doesn't care about failures of observer metamerism because all their subjects are drawn only from one color domain.

"One size fits all" fits none.

Well I take this point for sure. But most of us can agree about how ugly an image can degrade when shadows fail to recover. No one likes splotchiness, or noise artifacts. I suppose I'm curious mostly about these things. The "quality" of the degradation might be subjective, but the stops/slider amount at which degradation becomes apparent should be measurable - that's how I'm thinking about it.

 

[Keithpictures]

But then I think of my dream camera, the Leica Q2, and I wonder how much better it would do, when I fail to properly expose my shot. Can it save me even more than my X100V does? Would a FF camera be more prudent if I can avoid noise at higher ISOs, and achieve great DR? This is why I started the thread. I suppose I should have stated my interests more specifically, but it's fun to come up with equations IMHO

So that's interesting, because you're not looking at system cameras, so you can compare the lenses and sensors specifically knowing that these are the only lenses and sensors that are relevant. And the lens on the FF camera is fast enough that you exploit the large sensor (so long as shallower DOF isn't a problem).

I can see where that A or B comparison makes sense to do. I don't know how I'd go about doing it, short of reading lots of reviews, because as I mentioned, I struggle to relate the numeric measurements on sites like dxomark to real world implications. And so I don't know how I'd convert the Q2's "better" to monetary worth. I think a lot of subjectivity would come into play (ergonomics, enjoyment, etc.)

I wish you luck with your endeavor!

- Dennis
--
Gallery at http://kingofthebeasts.smugmug.com

Thanks Dennis. I won't be getting a Q2 anytime soon. My trickier technical issue there would be to compare its digital zoom at 75mm to the 16-80mm Fuji lens at the same AoV. Tricky stuff! I'll be curious to compare noise levels/patterns, sharpness, PDR in these cases. It's difficult to research that, but pretty fun... I'll keep at it.

 

[yardcoyote]

I shoot both Olympus MFT and Fuji and Pentax crop, and yes, there are major differences in image quality related to sensor size. And yes, those differences are less noticable in good light and good shooting conditions and more noticable in more difficult situations.

I carry my Oly EM10 ii ( a simple, basic camera that was a fairly small investment secondhand when equipped with a few basic secondhand and refurbished lenses) when small size and light weight are most important. It's always a much better camera than my phone camera, which I would be relying on otherwise, and with the 60mm macro on board the Oly is a delight for garden photography. (Which usually takes place outdoors in at least decent light.)

Otherwise it's APS-C for me. Would FF be "better"? In many ways, yes. I'd get improved performance in low light and better dynamic range, My lenses would finally behave "normally" as I remember them from film. But so what? I can't afford to replace all my gear with something much more expensive and I don't want to carry around large, heavy cameras and lenses. I do have loose plans to eventually add a compact FF mirrorless body with a 40-45 mm small slow lens, because a dedicate street/ walkaround package is the one place I really think the larger sensor will help me.

No saying you will make the same choices of course, but there's no math needed except the financial kind.

--
Instagram: @yardcoyote

 

[Mark S Abeln]

I'd say "at worst" would be open mockery and ridicule, which some of this thread has bordered on. As if it's ridiculous to ascertain IQ between systems. Unhelpful can at least be quickly forgotten about, but derailing the conversation toward handling/composition/other photo aspects is worse than unhelpful. I can do all that on my own, you know?

Frustrating, isn’t it?

Obviously, people here have radically different points of view. Different assumptions often lead to different conclusions.

On the one hand, some think that art is a a product of inspiration or inborn talent, and if you don’t have it, there is nothing you can do to get it; but with enough talent, any camera can be used to get good photography, and you may not even need a camera at all Others do think that you can learn the art of photography and that better technology can help.

My current issue is that my m43 files degrade quite quickly, when trying to push/pull blacks/whites. My APS-C files do WAY BETTER at this, even if shooting JPG! So I am quite intent on at least abandoning m43 in favor of Fuji (unless m43 comes out with something game-changing soon; I do like their lenses).

But then I think of my dream camera, the Leica Q2, and I wonder how much better it would do, when I fail to properly expose my shot. Can it save me even more than my X100V does? Would a FF camera be more prudent if I can avoid noise at higher ISOs, and achieve great DR? This is why I started the thread. I suppose I should have stated my interests more specifically, but it's fun to come up with equations IMHO

My full frame D750 was getting repaired, and I found using my old D7000 quite frustrating by comparison. Oddly enough, I find my even older D200 more pleasant to use than the newer D7000: it feels better in my hands and gets more predictable results.



So yes, for a lot of people I’d recommend getting a FF and never look back. But ergonomics matters a lot as well: how does it feel in your hands and how pleasant is it to use? Can you get good results with less effort?

I don’t do much long focal length photography, so weight and size aren’t that important to me.

 

[Keithpictures]

We're in similar positions.

It's true that this quality-measurement math is not necessary, really. But I was curious to add it into my brain nonetheless, if there were helpful ways of thinking about it.

I'm also averse to big, heavy lenses. And I'd prefer not to change lenses at all: I love my X100V, and comparing it to a Leica Q2 is pretty straightforward. Like you, I won't be making the financial plunge anyway, so it's a bit of a moot point.

But in upgrading my Panasonic GX9 (for when a travel zoom really is needed), I'm eyeing Fuji, but curious about the FFs coming out. None are rangefinder-style, and I'm not in a rush. But hypothetically, if Panasonic did make their S-line GX camera, I'd be curious to throw the new 20-60 f3.5-5.6 on it, and compare that to a Fuji outfit. Wouldn't have to be huge, and the benefits of the sensor might outweigh the costs.

Again, direct comparisons like this make the research easier, as does defining my use cases. (Travel to world wonders, for instance.) But I also just thought there might be very specific measurements for full-frame SNR vs APS-C vs m43, and how much improvement I might get between systems regarding DR, when I fail to expose properly.

After this thread, I will say I'm ready to focus more on editing my photos

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[Keithpictures]

I'd say "at worst" would be open mockery and ridicule, which some of this thread has bordered on. As if it's ridiculous to ascertain IQ between systems. Unhelpful can at least be quickly forgotten about, but derailing the conversation toward handling/composition/other photo aspects is worse than unhelpful. I can do all that on my own, you know?

Frustrating, isn’t it?

Obviously, people here have radically different points of view. Different assumptions often lead to different conclusions.

On the one hand, some think that art is a a product of inspiration or inborn talent, and if you don’t have it, there is nothing you can do to get it; but with enough talent, any camera can be used to get good photography, and you may not even need a camera at all Others do think that you can learn the art of photography and that better technology can help.

Thanks for your words here, Mark.

My full frame D750 was getting repaired, and I found using my old D7000 quite frustrating by comparison. Oddly enough, I find my even older D200 more pleasant to use than the newer D7000: it feels better in my hands and gets more predictable results.

So yes, for a lot of people I’d recommend getting a FF and never look back. But ergonomics matters a lot as well: how does it feel in your hands and how pleasant is it to use? Can you get good results with less effort?

I don’t do much long focal length photography, so weight and size aren’t that important to me.

I actually shot with the Nikon D200 myself, way back in my bourgeoning years. I then immediately upgraded to the 5D II. I barely remember the D200, but it certainly didn't feel fun to carry around New York; I still opted for my Contax G2, when I still shot film. I wish I discovered mirrorless cameras a lot sooner - only many years later did I learn I could have gone with cameras similar to my current ones way sooner.

Live and learn, I guess. I do really prioritize the user experience, even over IQ. But it's still worth learning the science and developing technical skills as well.

 

[yardcoyote]

Not so similar, since I travel with the same primes I use at home and wouldn't drive across town to see a famous sight. Lots of other things, sure, but nothing I could buy already photographed on a postcard. I'll be in the parking lot taking a picture of a moped or a spider.

Be that as it may, I think you will be seeing rangefinder style (EVF in the corner) FF mirrorless cameras on the market fairly soon, as well as more simple "box" style compacts with just rear screens. These may have popup EVFs or accessory EVFs as an add on.

 

[57even]

However, some shots are on the margin of what is technically possible and need all the help they can get from the photographer and the gear.

Modern cameras allow us to push the boundaries of what's feasible, but this is not a random process.

 

[MHshooter]

Anytime a smaller sensor allows you to put more pixels on a specific subject, it beats a larger sensor for resolution. Other aspects of image quality notwithstanding. However, the highest megapixel 35mm sensors are almost at the point where they match m4/3rds sensors for resolution per area with a given lens.

Wow, I've never thought about how a m43 sensor could be advantageous over a FF one in any way, besides print size. I've never heard this idea of resolution per area..

m4/3rds pixel density relative to 35mm is 80mp.

How do you derive this number?

The highest resolution 35mm sensor has 64mp, and 4 times the area. Using the same focal length lens, the m4/3rds sensor has a resolution edge in some circumstances, particularly if you have to crop the 35mm field. If for example you are using a 400mm telephoto and you shoot the same subject with the two cameras. With the m4/3rds camera, you fill the frame, and there are 20mp in it. With the 35mm, you fill 1/4 of the frame with the same image, and your pixel count is 16mp so slightly less than 20mp and slightly less resolution. Once 35mm reaches 80mp which should happen soon, then m4/3rds will no longer have the edge under those circumstances. It will still "fill a frame" with a lens 1/2 the focal length of a 35mm lens that does the same, but that is only a consideration if maximum resolution isn't a major consideration and extra lens size and weight do not matter.

Huh. So the smaller sensor can offer more detail, when the MP is greater..? So MP is important for detail, but by area in particular, not necessarily overall. A 20mp crop sensor has more detail than a 20mp FF...

But my m43 pictures have way too much noise for my liking, and poor DR; when I lift the shadows on my 20mp m43 sensor, they crush and look bad. I guess DR is unrelated to pixel density...

I also suspected that pixel size might matter at some point; m43 users aren't clamoring for MP greater than 20, and I thought this was partly because the pixels would get too small. Not sure what happens in that case...

If pixel size still mattered, there would be no 40-65mp cameras at all.

 

[MHshooter]

Well I take this point for sure. But most of us can agree about how ugly an image can degrade when shadows fail to recover. No one likes splotchiness, or noise artifacts. I suppose I'm curious mostly about these things. The "quality" of the degradation might be subjective, but the stops/slider amount at which degradation becomes apparent should be measurable - that's how I'm thinking about it.

In a print or on a screen? Whichever one, how large and viewed normally from what distance? There hasn't been a sensor made in the last 10 years where it's easy to tell them apart with a 16x20 inch print, that's how good they've gotten, whether by active in camera processing, or something literally hard-wired in the processor to give more "pleasing" images. Most of the people who talk incessantly about quality are still using 27 inch HD computer screens to view images, which is a massive JOKE when compared to sensor output today. Unless looking at tiny sections of images at 100% is their idea of "viewing."

 

[Eddie Rizk]

However, some shots are on the margin of what is technically possible and need all the help they can get from the photographer and the gear.

Modern cameras allow us to push the boundaries of what's feasible, but this is not a random process.

True.

In my case, in my number two genera, events without flash, I push up against the low light limits of every camera I have ever had every time I shoot an event at night.

In my number one genera, architecture/landscapes, sometimes known as real estate, My newest camera enables me to shoot some entire properties with no HDR, bumping up against the DR limits of the camera.

My new fun genera, macro, pushes up against the low light and focusing limits of the camera.

I successfully took all of those types of pictures with lower quality cameras, but I get more keepers and do less processing with my new camera.

 

[57even]

However, some shots are on the margin of what is technically possible and need all the help they can get from the photographer and the gear.

Modern cameras allow us to push the boundaries of what's feasible, but this is not a random process.

True.

In my case, in my number two genera, events without flash, I push up against the low light limits of every camera I have ever had every time I shoot an event at night.

In my number one genera, architecture/landscapes, sometimes known as real estate, My newest camera enables me to shoot some entire properties with no HDR, bumping up against the DR limits of the camera.

My new fun genera, macro, pushes up against the low light and focusing limits of the camera.

I successfully took all of those types of pictures with lower quality cameras, but I get more keepers and do less processing with my new camera.

You do a lot of technically challenging photography, but you make an interesting point.

The end result is a combination of the technical performance of the camera/lens, but also how the image is exposed and processed.

The limit isn't the camera, it's camera+photographer.

I like smaller formats as a do a lot of walking and travelling. I therefore take a lot of care to get the most from the smaller sensor. I would rather do that than stagger around under an overstuffed rucksack like I did in the past.

 

[Eddie Rizk]

However, some shots are on the margin of what is technically possible and need all the help they can get from the photographer and the gear.

Modern cameras allow us to push the boundaries of what's feasible, but this is not a random process.

True.

In my case, in my number two genera, events without flash, I push up against the low light limits of every camera I have ever had every time I shoot an event at night.

In my number one genera, architecture/landscapes, sometimes known as real estate, My newest camera enables me to shoot some entire properties with no HDR, bumping up against the DR limits of the camera.

My new fun genera, macro, pushes up against the low light and focusing limits of the camera.

I successfully took all of those types of pictures with lower quality cameras, but I get more keepers and do less processing with my new camera.

You do a lot of technically challenging photography, but you make an interesting point.

The end result is a combination of the technical performance of the camera/lens, but also how the image is exposed and processed.

The limit isn't the camera, it's camera+photographer.

I like smaller formats as a do a lot of walking and travelling. I therefore take a lot of care to get the most from the smaller sensor. I would rather do that than stagger around under an overstuffed rucksack like I did in the past.

That's an important factor. I like my system, when I'm traveling by car, but I hate it, when I'm traveling by plane or just traveling around the city by Uber.

As the small sensors get better, I think more about trading my system in for an EF-M system or a Fuji system. I'm not nearly prosperous enough to get a second system for that right now. According to B&H, I could actually get enough money for all of my equipment used to buy every M thing that I could possibly want new. Fuji is another matter entirely.

I think seriously about doing that whenever I travel by plane or Uber or take a long hike. Then I get a picture that I thought was impossible to take and wonder exactly how much I would be giving up.

My real estate photo clients were satisfied with what I could do with the 60D and the EF-S 10-22. But they went nuts over my pictures once I got the 6D and TSE 17. I don't know if they would be happy to go back, even though they wouldn't be going back all the way. The R didn't add much to my IQ, but it cut my time on property by a third to a half and cut my processing time by probably 20% or so. R keepers take more processing for people, but the R adds a lot to my keeper rate.

 

[57even]

However, some shots are on the margin of what is technically possible and need all the help they can get from the photographer and the gear.

Modern cameras allow us to push the boundaries of what's feasible, but this is not a random process.

True.

In my case, in my number two genera, events without flash, I push up against the low light limits of every camera I have ever had every time I shoot an event at night.

In my number one genera, architecture/landscapes, sometimes known as real estate, My newest camera enables me to shoot some entire properties with no HDR, bumping up against the DR limits of the camera.

My new fun genera, macro, pushes up against the low light and focusing limits of the camera.

I successfully took all of those types of pictures with lower quality cameras, but I get more keepers and do less processing with my new camera.

You do a lot of technically challenging photography, but you make an interesting point.

The end result is a combination of the technical performance of the camera/lens, but also how the image is exposed and processed.

The limit isn't the camera, it's camera+photographer.

I like smaller formats as a do a lot of walking and travelling. I therefore take a lot of care to get the most from the smaller sensor. I would rather do that than stagger around under an overstuffed rucksack like I did in the past.

That's an important factor. I like my system, when I'm traveling by car, but I hate it, when I'm traveling by plane or just traveling around the city by Uber.

Yep. Or hiking the Alps for six days... ;-)

As the small sensors get better, I think more about trading my system in for an EF-M system or a Fuji system. I'm not nearly prosperous enough to get a second system for that right now. According to B&H, I could actually get enough money for all of my equipment used to buy every M thing that I could possibly want new. Fuji is another matter entirely.

I had a Fuji XE2 and a D800. When the Xpro2 came out I compared some test prints against the D800. Wasn't really a whole lot of difference, so I traded the D800 and all my Nikon lenses for a Xpro2, some new lenses and a trip to Portugal.

I think seriously about doing that whenever I travel by plane or Uber or take a long hike. Then I get a picture that I thought was impossible to take and wonder exactly how much I would be giving up.

My real estate photo clients were satisfied with what I could do with the 60D and the EF-S 10-22. But they went nuts over my pictures once I got the 6D and TSE 17. I don't know if they would be happy to go back, even though they wouldn't be going back all the way. The R didn't add much to my IQ, but it cut my time on property by a third to a half and cut my processing time by probably 20% or so. R keepers take more processing for people, but the R adds a lot to my keeper rate.

I hear you. If I had clients, I would not have sold the D800.

Having said that, pretty impressed with the DR on the Xpro2, and with macro you get the extra magnification when using adapted macro lenses.

 

[TonyBeach_61]

My basic question is: when might a smaller (APS-C) sensor be superior to a larger (FF) sensor? Because surely it can be the case.

Pixel density might be an advantage, and related to that the most important determinant will be the lens or lenses being used.

 

[MHshooter]

My basic question is: when might a smaller (APS-C) sensor be superior to a larger (FF) sensor? Because surely it can be the case.

Pixel density might be an advantage, and related to that the most important determinant will be the lens or lenses being used.

well since APS has been stalled below 30mp for, well EVER, FF is pretty much just as good now, using part of a FF sensor is the same as an APS. Only m4/3rds has a higher resolution as a "crop mode" camera these days and that advantage is small and likely time-limited (once an 80mp FF is released).