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Digital cameras have two characteristics: sensor size and pixels amount (in MP). I know the bigger sensor is and the more pixels amount is the better for the final image quality. But what characteristic has more image quality influence?

E.g. I've bought Canon EOS 90D with cropped APS-C sensor and 32MP. But later I've read older Canon EOS 6D Mark II which has Full Frame sensor but only 26MP has better image quality. Does it mean sensor size is more important for image quality than pixels amount?

So how to define and compare which camera will produce better image quality bases on it's characteristic (sensor size and pixels amount)?

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  • This question, as asked, can't be answered because you haven't defined image quality sufficiently. Further, different shooting scenarios would favor one or the other. You've asked a question like which vehicle is "better", a two seat sports car that will go 200 mph or a 24 foot box truck that can haul 20,000 lbs of cargo? It all depends on whether you want to get two people somewhere really fast or whether you need to move 20,000 pounds in an efficient manner. Both are better than the other for some use cases.
    – Michael C
    Feb 1 at 15:35
  • I came here with exactly the same question after looking at the 6D II and the 90D for a while, trying to decide which to get! Shame that none of the answers have specifically covered these models off as they are mentioned in the question.
    – 5Diraptor
    Aug 31 at 12:39
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Image quality is highly correlated with pixel-size. A larger pixel accumulates more light during an exposure of a certain duration. By having more light, there is less noise in the image because the pixel gets more signal from light and noise is relatively stable. So proportionate, which is what we perceive, images are less noisy.

Larger pixels also have a larger well which is the capacitor that accumulates the electrical charge that photons release when they hit the sensor surface. These deeper wells allow a pixel to represent more dynamic-range. It is applicable to scenes of high contrast, where smaller pixel reaches its maximum level faster and clips image highlights.

Now, technology has created a lot of optimizations for it such as BSI (Back-Side-Illumination) that allows silicon of the same size and number of pixels to effectively have larger pixels because more photons landing on the sensor get used, so pixel size alone is not an absolute measure, but it is far more accurate than resolution.

When comparing two cameras, what you can look at is the pixel-pitch which some sites list in specifications or you can estimate it yourself. Taking a full-frame sensor of 36x24mm and divide it by resolution (Say 26,000,000 for the 26 MP 6D Mark II) and compare it to 22x15 for Canon APS-C (or 24x16 for APS-C of other brands) and divide by the resolution (Say 32,000,000 for the Canon 90D). It is not necessary to use exact numbers because there are a lot of factors at play (which is why a 24 MP APS-C camera today gets better output the a 24 MP of 10 years ago) but this pixel-size calculation gives you good idea of relative quality.

Now if you were to take an image from a certain sensor and scale it down, you are effectively simulating larger pixels, there is more noise usually than if the sensor had a lower native resolution. So, if you want to compare at equal print sizes, you can divide by the resolution you intend to use for a given print size.

Camera makers know this which is why some - and Fujifilm had a particularly clever way of doing this - bin pixels to simulate larger pixels. They essentially group a number of pixels together, typically 4 (in a 2x2 grid), and output one image pixel for each 4 sensor pixels. This improves image quality at the expense of resolution.

Addendum: As someone pointed out. Lens has a great impact on image-quality. This comparison is purely from a sensor design but if a lens cannot deliver sufficient resolution, then output quality gets reduced but it does not affect noise or dynamic-range. Again there are subtitles since some lenses can have such strong vignetting that they effectively diminish light which makes noise apparent in peripheral areas.

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  • Larger pixels don't necessarily have deeper wells, they have higher full well capacity due to their larger surface area (not due to their depth).
    – Michael C
    Feb 1 at 15:38
  • The size/FWC/DR advantages of larger pixels are engineering considerations... they only exist in isolation. Feb 1 at 15:58
  • Yes, I mentioned this implying that there are other factors but pixel size dominates image-quality by far.
    – Itai
    Feb 1 at 16:12
  • That's splitting hairs, the point is that they can accumulate more charge.
    – Itai
    Feb 1 at 16:14
  • @MichaelC, "Well" is a metaphor. E.g., sometimes people talk about a minimum in a mathematical function as a "well." Feb 1 at 16:20
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I am not comparing the two models you are quoting. I will only make some general observations.

We need to define what do we mean by quality.

1. Noise

A more modern sensor will generally better low light performance. Noise is more noticeable in darker zones.

A bigger sensor is better for reducing the noise because it has more information from more photons "to average" on each sensor element (sensel).

2. Sharpness

Images produced by a lens has a limit of resolution. Imagine that one X lens projects a thin line but a bit blurry because it reached the limit of the lens. Let's say that this line falls on one line of big sensels, but on smaller sensels, it occupies one and a half sensel. The line will be a bit blurrier on the smaller sensor. Smaller sensor+more megapixels = smaller sensel.

3. Bokeh

Using one f 1.8 lens on a bigger frame will give you more bokeh than the same FOV equivalent lens on a crop frame.


On these three categories, a bigger sensor will produce overall a better quality image.

It is not that image quality does not improve using more Megapixels, but I think the improvement of Mpx is not linear, but a curve, where you benefit more when the pixel count starts, but at some point, you need to, let's say, double the pixel count to notice an improvement. But only before reaching the capabilities of the lens.

4. Some other considerations

Dynamic range, processing of the image on the camera´s chip, color accuracy, etc.

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Both sensor size and the number/size of the photosites¹ that sensor contains can have an effect on the images taken using that sensor.

You seem to be wondering if your 32MP APS-C camera is "better" than a 26MP FF camera. The answer is that neither is inherently better than the other for all conceivable use cases.

Which is "better" for taking a specific photograph depends entirely upon many other variables you haven't identified.

  • What is the lens resolution? Is it higher or lower than the sensor(s) being considered?
  • What is the dynamic range of the scene being recorded? Are there very bright and very dark parts in the scene, or is most of the scene closer to the same brightness level?
  • What are the applicable limits to exposure time and aperture that may limit the total amount of light collected? Is the light too dim to use base ISO and the largest aperture available and still use an exposure time short enough to freeze subjects in motion?
  • How large and heavy can the camera and lens be before it begins to affect one's ability to get the shot at all? Is the subject a 20 mile hike from the nearest road?

If you're working in low light and need to limit exposure time to prevent motion blur, then larger sensels are usually more important.

If you're taking tripod mounted landscape photos using multiple exposures to capture the entire dynamic range of the scene and want to create very large viewing sizes, then more pixels are probably more important to you.

If you've got a near unlimited lens budget so that the higher cost of longer focal length lenses with faster apertures is not a factor, then a larger sensor with larger pixels is probably your best choice. But if you've got a tight lens budget and desire to get as much "reach" out of the lenses you can afford, then higher pixel density on a smaller (and cheaper) sensor may be the better choice, even for the same use case of photographing birds in flight.

There are many other conceivable use cases in which a larger sensor with larger photosites may be more beneficial. There are also many use cases in which a smaller sensor with more dense resolution can work to one's advantage.

Cameras are tools. Just like many other kinds of tools, which one is "best" depends upon the job one desires to do using them.

A 24 oz. framing hammer would be practically useless for driving railroad spikes into crossties. An 8-pound sledge hammer is better for that. On the other hand, an 8 pound sledge hammer would be practically useless for framing a house. That's what the framing hammer was created to do well. In order to pick the right tool for the right job, one must first know what the job is.

¹ Technically, sensors don't have "pixels", they have photosites a/k/a sensels a/k/a pixel wells. A pixel is a discrete unit of a digital image when it is stored or displayed.

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Pretty much every aspect of image quality is determined by the amount of light received/recorded for the image. And it is primarily a light/area consideration; larger sensors have a larger area, and receive/record more light for any given composition.

The only aspect pixel size really relates to is resolution; how much you can enlarge/magnify that light/area and how much detail is recorded. There are two sides to this; if the lens cannot deliver the resolution then the sensor cannot record it. A larger sensor is also capable of recording more lens resolution, and conversely it is less demanding of lens resolution. With modern sensors it is most common for image IQ to degrade due to a lack of light, or a lack of lens resolution (technique), well before sensor resolution becomes the limit... If you are not looking at a pixelated image; then the individual sensor pixel considerations are not the limiting factor.

I would generally put IQ considerations in order of:

  1. Technique
  2. Lens
  3. Sensor Size/Design
  4. Sensor Resolution

But there are times where 2-3 might be reversed... what actually matters most to you may change for a given situation. I would suggest you don't get too concerned about it; the difference between APS and FF are not huge, and to actually see the benefits of the larger sensor is expensive (see 1-2 above).

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    That is untrue. Image quality can be seen even if you don't see a pixlated image. If you see a pixelated image, you need more resolution but that doesn't affect image-quality
    – Itai
    Feb 1 at 16:31
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    Just because one can't see a pixilated image does not mean one might not be seeing other artifacts that are the result of smaller or larger sensels. The result of lower dynamic range due to the smaller size of sensels, for instance, won't result in a pixilated image, but it can still have a visible impact on the image in shooting scenarios that demand high dynamic range..
    – Michael C
    Feb 1 at 16:39
  • With modern sensor design there is no appreciable reduction in DR capability due to smaller pixels on the same size sensor... the light/area is simply divided up between more pixels. The pixels are smaller, they get less so they don't clip, and total light/area recorded/received is the same. Actually, DR is increasing in modern sensors mainly due to the smaller pixel sizes due to their lower gain requirement (extending DR farther into darks) . However I would agree that color accuracy is a potential difference; I included that as detail (color detail). Feb 1 at 16:44
  • @Itai- I did not say IQ cannot be seen w/o seeing a pixelated image. But at any lower output resolution you are not seeing per-pixel considerations/characteristics. E.g. you are simply seeing some scaled view of the total noise recorded. Feb 1 at 16:53
  • It's the opposite Steven. Larger pixels show more dynamic-range because even though you can cluster smaller ones together, the added circuitry introduces more noise and loss of photos plus potential electronic leaks between capacitors. The cameras with the most dynamic range at a certain level of technology are those with larger pixels. Since DR is a proportion, highly noise reduces it even if the capacitance is the same. The way Fujifilm tacked this years ago is by creating a sensor with two different photosite sizes and blending those or one can read interleaved photosites earlier.
    – Itai
    Feb 2 at 16:34

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