The Sleeping Giant's Sea Lion

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On Snapsort I found this explanation:

True resolution

Manufacturers advertise high megapixels to sell their cameras, Snapsort calculates the true resolution of each camera based on the physical limitations of the size of it's sensor.

However, I would like a better explanation.

For example, I have a compact camera that has 16.1 MP, and true resolution is 9.7 MP. Why is that? What are the differences?

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5 Answers 5

up vote 5 down vote accepted

Let me start by saying that the term "true resolution" has no set meaning. It is a term that Snapsort uses to try and simplify the meaningful detail a camera can capture.

Resolution, at its most basic, is the level of granularity of detail that a camera can capture. You could have a 200 mega pixel camera, but if the image was out of focus and you only had a giant brown blob, the meaningful resolution would be pretty much nothing because you can't make out any level of detail.

A large number of factors impact how much detail you can capture, the quality, speed and clarity of the lens, the positions within the frame that you are looking at (the center is generally higher detail than the outside), the size of the sensor (and consequently the diffraction limit), the level of noise on the sensor, even atmospheric conditions can impact the total level of actual meaningful detail that can be captured by the camera.

"True Resolution" is simply Snapsort's attempt to generalize that down in to an easily consumable number, but as it is a gross over-simplification of a complex topic, it is also next to useless. For example, one cheap lens may have a super sharp center but fall apart near the corners. It would end up with a low total resolution because of the average, however another lens that is generally uniform but fairly low quality might end up being marked as higher "total resolution".

The problem is, if you are taking a portrait with the subject in the center for example, that you may not care about edge sharpness since all the detail you want will be in the center. Thus, the "lower resolution" lens would actually be the better choice.

Snapsort is a good source of basic stat comparison's between cameras, but a large amount of their information is overly simplified and thus useless. Don't put a lot of stock in their comparisons as they are not particularly trustworthy or reliable.

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Thank you, AJ! So in short what I should take into account: the actual resolution, like 21MP, or the ''true resolution''? –  Morpho Apr 2 at 20:30
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@Morpho - that isn't a one or the other answer. The "true resolution" that Snapsort makes is an oversimplification and is near meaningless, however the concepts that are behind it are extremely important if you are trying to get maximum quality. Extra megapixels won't hurt you, but it might not always be of much benefit either unless you have better optics. –  AJ Henderson Apr 2 at 20:50
    
OK, to put it differently: when my compact camera has actual resolution of 16 MP and Snapsort says that its ''true resolution'' is 9.7 MP, how should I address this camera? A camera that has 16 MP or a camera that has a 9.7 MP? I hope you understand my confusion here. –  Morpho Apr 2 at 20:54
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But so is the 9.7. You are still capturing 16 million pixels, but some of them won't be sharply defined. –  AJ Henderson Apr 2 at 20:57
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Because it isn't actually 9.7mp of data. It is still more than 9.7mp, but there is some blurriness and lack of detail, but if you only care about the center of the shot for example, then it might as well be 16mp because you don't care about the detail you are losing. –  AJ Henderson Apr 2 at 20:58

"True resolution" is a term that this particular site (snapsort.com) has made up in an attempt to account for the fact that pixel size and density play a factor as well. You can check out their whole page about it here. There is no industry standard term called 'true resolution'.

They're calculating it based on the size of the Airy disk, given a maximum of four pixels per Airy disk at f/5.6 . The concept being that a greater density of pixels beyond four per Airy disk doesn't yield any further detail. So, given the physical size of the sensor and the focal lengths possible on the lenses, they seem to be trying to calculate a separate measure from actual megapixels - more like 'effective' megapixels for measuring the ability of the sensor to capture detail.

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Thank you, rfusca! However, I am looking for the simplest answer. What is the main difference between resolution like 21 MP and true resolution? What I need to pay attention at when purchasing a camera? –  Morpho Apr 2 at 18:13
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@Morpho - None of snapsorts numbers should be the only factor in a buying decision. And generally these days, on the list of things to look for in a new camera, super high megapixels shouldn't be near the top for most people. We could debate the methodology behind snapsort's 'true resolution' and say "they're not taking this or that into account" but overall, they're probably in the realm of a good measure for this. So, if you're looking at two cameras and everything else is equal, but its a different 'true resolution', get the higher 'true resolution'. –  rfusca Apr 2 at 18:18
    
The listed resolution of '21 megapixels' references to the physical pixel count on your sensor. The 'true resolution' that Snapsort offers, trying to account for the fact that all 21 megapixels can't be capturing detail because they're too closely packed together. –  rfusca Apr 2 at 18:22

This is a contentious issue, and I'm afraid that you won't find a single, universally-accepted definition anywhere. The website in question is using a relatively simple calculation that doesn't really cover all of the variables involved (and rfusca's answer addresses that).

The "most correct" answer (if this was one of those confusing multiple-choice questions with several answers that are partly right) looks at the modulation transfer function (MTF); that is, what size details at what contrast level can the sensor record and translate into pixels. That is, the answer is come by experimentally by taking test pictures (or projecting images directly onto the sensor) and determining what size a pattern needs to be before it's rendered at an acceptable level of contrast and detail.

With a typical Bayer-pattern sensor (or with similar colour-array sensors), this number can never be the same as the number of sensor elements. Since each sensor element records only one colour, its neighbours need to be consulted for colour information before any one pixel's value can be determined. At best, you can expect a "true" resolution that is approximately 1/sqrt(2) of the number of sensor elements/pixels. (The obvious exception here is a multishot studio camera like the Hasselblad 50MS back, which has a Bayer filter but takes four sequential images, each shifted by one pixel, so that every pixel in the image has its own complete colour information recorded along with the luminance info.)

There is also the antialiasing (optical low-pass) filter to consider, when there is one in place. Its job is to deliberately blur the image by a controlled amount in controlled directions to prevent image artifacts (like moiré patterns) from appearing in the image when the size and pattern of the details approach the size and pattern of the sensor elements. That is to say that the amount of detail you are able to record is deliberately limited to less than the bare sensor can theoretically record (the Nyquist limit) by some amount in order to prevent false details from appearing in the output image. This overlaps somewhat with the resolution you're losing due to the colour array filter, so the effect is less than cumulative. (That is, you can't just multiply the colour filter array loss by the optical low-pass filter loss and come up with a number.)

At best, a Bayer-pattern sensor will only have about a 70% data-to-pixel ratio. Monochrome sensors, whether manufactured that way or as the result of an aftermarket modification, as well as Foveon-type sensors, when not "choked" by an optical low-pass filter, approach 100%. (At exactly 100%, you can never be sure whether you're seeing the real data or an aliasing artifact. That's a fundamental problem with discrete data; all you can do about it is hope - or ensure - that the data you are recording is "bigger" than the buckets you are recording it in. And that's why very high-resolution sensors can get away without optical low-pass filters in most instances - you are rarely recording anything that has a repeating pattern small enough to cause a problem on the one hand, and the lens itself will lend a certain amount of low-pass filtering to anything that isn't very sharply in focus.)

There are other things that will influence the amount of real detail you can record as well, such as the inherent noise of the sensor and reading circuitry. Since the "effective megapixels" depends on you actually being able to see details in the image, anything that can't be easily distinguished from noise doesn't really count. With a very noisy sensor, it may take the cumulative data of several neighbouring pixels before you can objectively determine what constitutes image information. That's not a horrible thing, necessarily; Nokia is using a tiny 41MP sensor in some of its devices to produce 5MP images, which allows it to have an effective "digital zoom" while embracing all of the data losses.

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I' just looking for a more basic answer. Is it possible to have a simpler answer? –  Morpho Apr 2 at 17:53
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@Morpho - No, it isn't. –  user2719 Apr 2 at 17:53
    
How is it possible to explain this to students? –  Morpho Apr 2 at 17:56
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@rfusca - No, the question asked "what is true resolution", not "what does Snapsort mean by 'true resolution'". And since your answer to that question is apparently not meeting the "simplest answer" criterion, simply answering the Snapsort question is apparently not doing the job either. In any case, I quit. –  user2719 Apr 2 at 18:17
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@rfusca - you can't really get in to the snapsort numbers in detail without understanding what they are talking about though. I agree that it is possible to skim the details and explain the gist of it, but it isn't a complete answer to someone else who may want a technical answer. Ideally, having both an in-depth answer like Stan's as well as a spectrum of different levels of detail in other answers works out for the best in terms of anyone who comes to this question being able to walk away with a good answer. –  AJ Henderson Apr 2 at 20:25

Let's start with resolution, which is not the same as the sensor pixel density which is usually called resolution.

To get the actual resolution of a camera and lens combination, you would photograph a resolution chart, and use the resulting image to determine how many lines can be reproduced. You would measure the resolution for primary horizontal, vertical and diagonal lines.

Ref: Camera testing resolution charts explained

It sounds like that website does some calculation based on the sensor size and sensor type, to determine what the maximum usable pixel density could possibly be with a theoritical ideal lens. The "true resolution" would be lower than the sensor pixel density if the sensor has more pixels than it can actually make use of.

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Could you please explain it to me in more basic lines? –  Morpho Apr 2 at 17:49

They are simplifying a much more complex world. your true resolution will be a product of lens and sensor. but the lens resolution (lp/mm) will depend on its settings as well as its quality. so to do those conversions they need to make a lot of assumptions. To spread light on the complexity of resolution:

First of all, MP is not a measure of resolution. It starts in the real world a line pairs (black and white adjacent lines), and the get projected and squeezed onto the sensor plane. a dot in the real world wont end up on a dot, but a circle, which is wavelength dependent, too. your sensor samples these in a certain digital resolution. The result of that depends on the sensor size and pixel count in each direction. the resolution is typically close to (not always equal) eachother on the X and Y axis. The lens should also be close to eachother in each direction. that means a 4000x3000 sensor 36*24mm will have the same resolution in X and Y, but not diagonally! Lets say a lens with 120lp/mm projects those lines so they are perfectly aligned on that 4000x3000 sensor. Then you get a perfect picture - but only if it is a monochrome camera! If the lines are not aligned, you get moire. So the producer adds a optical blur filter. then the neat image gets screwed up. now you need to increase the resolution of the lens, or move closer for better magnification and lose half the image or project on a larger sensor to spread those "Airy disks" apart. add bayer pattern interpolation to the mix and you need to double or quadruple your resolution ( on each axis, not the MP which will be 4x - 16x as high).

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