Re: Pixel Density: when Moore is less

Started Jul 8, 2008 | Discussions
Eric Fossum
Eric Fossum Senior Member • Posts: 1,243
Re: Pixel Density: when Moore is less
1

Most image sensor specialists use pixel size or pitch as a key parameter for the sensor. I would urge DP Review to consider converting to this conventional metric rather than its inverted square (I assume), or provide both (even better).

If you want to be unconventional in a useful way, please consider reporting the active diagonal in mm instead of the 1/X inches system that dates from pre-solid-state image sensor days.

On the other hand, the use of pixel density has generated a lot of almost-amusing forum discussion so it certainly has that value, but I don't think that is what you intended.

I know I am not the first to suggest this, but I wanted to be heard. Keep up the good work,
Eric

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A California Non-Profit Public Benefit Company

and H. Fellow, Royal Photographic Society

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Phil Askey Veteran Member • Posts: 9,821
Re: Pixel Density: when Moore is less

Eric, I'm sure to you it's a simple amusement (and frankly it wasn't intended for brains such as yours), but to most others it provides if nothing else a new value with which to 'consider the difference'. As you will also know many of the things people have been 'asking for' (nay demanding) are simply unavailable (such as exact pixel pitch). We do provide sensor dimensions where available (as you will note from the camera database).

We are improving the glossary entry and intend to expand the information surrounding pixel density.

Eric Fossum wrote:

Most image sensor specialists use pixel size or pitch as a key
parameter for the sensor. I would urge DP Review to consider
converting to this conventional metric rather than its inverted
square (I assume), or provide both (even better).

If you want to be unconventional in a useful way, please consider
reporting the active diagonal in mm instead of the 1/X inches system
that dates from pre-solid-state image sensor days.

On the other hand, the use of pixel density has generated a lot of
almost-amusing forum discussion so it certainly has that value, but I
don't think that is what you intended.

I know I am not the first to suggest this, but I wanted to be heard.
Keep up the good work,
Eric

Eric R. Fossum, Ph.D.

President, ImageSensors, Inc.
A California Non-Profit Public Benefit Company

and H. Fellow, Royal Photographic Society

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Phil Askey
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nickleback Forum Pro • Posts: 11,111
Re: Pixel Density: when Moore is less

Phil Askey wrote:

As
you will also know many of the things people have been 'asking for'
(nay demanding) are simply unavailable (such as exact pixel pitch).

If you know pixel density, you know pixel pitch.

pixel pitch = 1/sqrt((pixel density per cm^2) (10000^2))

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Eric Fossum
OP Eric Fossum Senior Member • Posts: 1,243
Re: Pixel Density: when Moore is less

Thanks Phil. The amusement comes from the strong opinions people have about the value of the parameter "pixel density". Personally, I think pixel density or pixel pitch is very nice to know. I still think you can easily convert from one to the other in at least an approximate way (as has been discussed in 2 out of 3 postings here) but perhaps that ship has sailed.

best regards.
-Eric

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Ken Phillips Forum Pro • Posts: 16,361
I'd rather see a "normalized" number ...

... of course, then you gotta pick a camera as "1"! Perhaps the old 1Dinosaur could be 1.0. The 1D Mk. III would be 2.5-ish.
KP

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Phil Askey Veteran Member • Posts: 9,821
Re: Pixel Density: when Moore is less

I'd rather deliver a figure such as megapixels/cm^2 than trying to approximate pixel pitch (which has to be a much more precise figure).

Eric Fossum wrote:

Thanks Phil. The amusement comes from the strong opinions people
have about the value of the parameter "pixel density". Personally,
I think pixel density or pixel pitch is very nice to know. I still
think you can easily convert from one to the other in at least an
approximate way (as has been discussed in 2 out of 3 postings here)
but perhaps that ship has sailed.

best regards.
-Eric

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Roland Karlsson Forum Pro • Posts: 26,396
Re: Pixel Density: when Moore is less

Phil Askey wrote:

I'd rather deliver a figure such as megapixels/cm^2 than trying to
approximate pixel pitch (which has to be a much more precise figure).

Hi Phil - I understand what you are trying to accomplish with this measure, and I salute your good intentions.

But - I find it more confusing than helpful. Here are some reasons why.

1. Size matters

Modern cameras have say 8-16 Mpixels - thats a factor 2. The area of the sensors vary with a factor way more than 20 though.

So - pixel density is (with reasonable accuracy) a function of sensor area only, the number of pixels does not really affect it.

Therefore - sensor size is good enough.

But sensor size should be REAL sensor size and not some bogus inch based vidicon tube stuff.

2. The mythical optimal PD

There is a myth that there exist some kind of optimal pixel density (PD). It does not - which several posters here have shown. John Sheehy even clam to have shown that with todays sensors more pixels is always better. One special case of this myth is that lower PD always is better. It certainly is not; one pixel cameras are rather uninteresting for photography and to have VGA resolution in your mobile phone is no fun.

3. Technology differences

Lots have been said here - I will only say Foveon. Is it 4.6 or 14 MP? The 14 MP value will give it very high pixel density - which (according to the myth) is bad.

Oh - I will also say that the future may be here tomorrow. Detectors with (more or less) only shot noise. Then more pixels is always better.

4. Understandability

Whatever you say regarding this being a simplification - I dont think it is. It is a rather hard figure to visualize. In what way do it affect my images?

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Roland

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Eric Fossum
OP Eric Fossum Senior Member • Posts: 1,243
Re: Pixel Density: when Moore is less

Phil, you could consider putting up a table on your pixel density page that compares density with pixel pitch, exactly. That might be helpful.

I know I should drop this, but the engineer in me needs to point out that if the error of the density is, say, within 10%, then the error of the derived pixel pitch is within half of that or 5% due to the square root relationship. That is, your pitch estimate would have less error than your pixel density estimate.

So, for example, if the pixel was estimated at 3.2 megapixels/cm^2, the estimated pixel pitch would be 5.6 um x 5.6 um. A + - 5% error spread would put the pixel pitch range between 5.3 and 5.9 um.

ok, I am done.
cheers,
Eric

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Victor Engel Forum Pro • Posts: 17,968
Re: Pixel Density: when Moore is less

But if you're comparing cameras with sensors of the same size, density really is a major thing that differs. Consider, for example, the wide array of APS-C cameras. They all have essentially the same size sensor. Someone having a decision to make may then prefer to distinguish the available options by density.

Roland Karlsson wrote:

1. Size matters

Modern cameras have say 8-16 Mpixels - thats a factor 2. The area of
the sensors vary with a factor way more than 20 though.

So - pixel density is (with reasonable accuracy) a function of sensor
area only, the number of pixels does not really affect it.

Therefore - sensor size is good enough.

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simpy Veteran Member • Posts: 3,090
Re: Pixel Density: when Moore is less

Victor Engel wrote:

But if you're comparing cameras with sensors of the same size,
density really is a major thing that differs. Consider, for example,
the wide array of APS-C cameras. They all have essentially the same
size sensor. Someone having a decision to make may then prefer to
distinguish the available options by density.

In that case, aren't you better off directly comparing the MP count instead of inferring it from the pixel density?

In whatever way you look at it, for the consumer the most important numbers are sensor size and MP count. If you're the engineering type, you may want to know the pixel pitch as well, but then you can calculate it as well.

Simon

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Victor Engel Forum Pro • Posts: 17,968
Sure

simpy wrote:

In that case, aren't you better off directly comparing the MP count
instead of inferring it from the pixel density?

I'm not sure about better off. The two would be equivalent.

In whatever way you look at it, for the consumer the most important
numbers are sensor size and MP count. If you're the engineering type,
you may want to know the pixel pitch as well, but then you can
calculate it as well.

I'd also like to know the light gathering efficiency. Do the microlenses gather light from the spaces between sensor elements, for example? Recent Canon cameras have been doing this to get better image performance.
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igb Senior Member • Posts: 2,637
Re: Pixel Density: when Moore is less

Victor Engel wrote:

But if you're comparing cameras with sensors of the same size,
density really is a major thing that differs. Consider, for example,
the wide array of APS-C cameras. They all have essentially the same
size sensor. Someone having a decision to make may then prefer to
distinguish the available options by density.

The question is why would he want to since it seems that pixel density has no real impact on IQ.

In that respect maybe metrics like #photon captured per area unit or other to quantify QE or sensor saturation levels would be more educational and equally differentiating from DPR's marketing standpoint.

Regards

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Victor Engel Forum Pro • Posts: 17,968
Re: Pixel Density: when Moore is less

igb wrote:

Victor Engel wrote:

But if you're comparing cameras with sensors of the same size,
density really is a major thing that differs. Consider, for example,
the wide array of APS-C cameras. They all have essentially the same
size sensor. Someone having a decision to make may then prefer to
distinguish the available options by density.

The question is why would he want to since it seems that pixel
density has no real impact on IQ.

Of course it does. BIF shooters harp about it all the time.
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Phil Askey Veteran Member • Posts: 9,821
Re: Pixel Density: when Moore is less

Understood, however the point here really is that a 'megapixel' is something people understand and so is a square centimeter. People (other than us geeks) just don't think in micrometers.

Eric Fossum wrote:

Phil, you could consider putting up a table on your pixel density
page that compares density with pixel pitch, exactly. That might be
helpful.

I know I should drop this, but the engineer in me needs to point out
that if the error of the density is, say, within 10%, then the error
of the derived pixel pitch is within half of that or 5% due to the
square root relationship. That is, your pitch estimate would have
less error than your pixel density estimate.

So, for example, if the pixel was estimated at 3.2 megapixels/cm^2,
the estimated pixel pitch would be 5.6 um x 5.6 um. A + - 5% error
spread would put the pixel pitch range between 5.3 and 5.9 um.

ok, I am done.
cheers,
Eric

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CSMR Contributing Member • Posts: 604
Re: Pixel Density: when Moore is less

Yes!

Roland Karlsson Forum Pro • Posts: 26,396
Re: Pixel Density: when Moore is less

Phil Askey wrote:

Understood, however the point here really is that a 'megapixel' is
something people understand and so is a square centimeter. People
(other than us geeks) just don't think in micrometers.

OK - I must admit that you have a point.

Maybe our cousins on the other side of the Atlantic may feel somewhat confused with regard to the square cm though.

But thats not the most important issue for me.

The most important issue is that I dont agree with what I believe to be the motivation for this measure. I believe the reason for this measure is "political". I think DPReview wants to convince the makers and/or buyers of digital cameras that the megapixel race is wrong; that low pixel density is an advantage.

There is some truth in this - but the sensor size is much more important.

You could give the area in square cm. That would be cool.

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Roland

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BJL Veteran Member • Posts: 8,901
How about Nyquist frequency, as a rough resolution measure?

There is yet another equivalent measure that I prefer: the resolution limit given by the Nyquist frequency in l/mm, already used in lens reviews. This contains the same information as the other options, being:
sqrt(pixel density) 20 (converting from cm to mm along the way)
or
1
(2*pixel pitch)

This could be dubbed the "telephoto reach factor", since for example a doubling of Nyquist with same focal length increases the reach about as much as a doubling of focal length would after cropping to images of equal detail (with sharp enough lenses.)

It has the virtues Phil seems to like of being a big number, with a "bigger is better" sense, at least for some of us.

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Phil Askey Veteran Member • Posts: 9,821
Re: Pixel Density: when Moore is less

Yes, you're right at least half of the reason for doing this is to 'expose' or 'put pressure' on the manufacturers to think about the terrible tradeoffs that the continuing megapixel race (mostly in compact cameras) are having on image quality. The other half (and to be fair the initial idea) is to provide people with some method of reference, and if nothing else to inspire some thought and conversation about the issue (I think we've achieved that).

Roland Karlsson wrote:

Maybe our cousins on the other side of the Atlantic may feel somewhat
confused with regard to the square cm though.

But thats not the most important issue for me.

The most important issue is that I dont agree with what I believe to
be the motivation for this measure. I believe the reason for this
measure is "political". I think DPReview wants to convince the makers
and/or buyers of digital cameras that the megapixel race is wrong;
that low pixel density is an advantage.

There is some truth in this - but the sensor size is much more
important.

You could give the area in square cm. That would be cool.

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Roland

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Phil Askey
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Roland Karlsson Forum Pro • Posts: 26,396
Re: Pixel Density: when Moore is less

Phil Askey wrote:

Yes, you're right at least half of the reason for doing this is to
'expose' or 'put pressure' on the manufacturers to think about the
terrible tradeoffs that the continuing megapixel race (mostly in
compact cameras) are having on image quality. The other half (and to
be fair the initial idea) is to provide people with some method of
reference, and if nothing else to inspire some thought and
conversation about the issue (I think we've achieved that).

hehe

But Phil - DO you think pixel density REALLY is a good and useful measure?

Be honest now

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Roland

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iso rivolta Regular Member • Posts: 492
Pixel aperture - hard to find

The effective pixel area would be the good thing to know, but it is practical impossible to find data about. Yes, area, as Fuji CCD's have non-rectangular pixels and there are also sensors with non-square pixels.

Many P&S cameras have unknown sensor manufacturers and most of the sensors with known manufacturers have no model name available. Even if you can find the manufacturer and the specific sensor model, often you cannot find the data about the exact dimensions of the sensor. The dimensions for a certain "standard" type (like 1/1.7") are way off from manufacturer to manufacturer and generally from sensor to sensor generation too.

Of course, if you know the sensor's active area dimensions does not mean at all that you know the effective pixel area, but only the pixel pitch (size) and the pixel's maximum theoretical area. More important is the photodiode area or the effective area - given by the pixel aperture(s) (especially for the sensors using a microlens array).

And even if you can find those pixel dimensions like in Sony's promotional data sheets, are these figures to be trusted ? They report constancy or even improvements in the pixel aperture with each generation of denser CCD sensors.

I would say, leave it like this, i.e. a rough pixel density calculated using the "standard" dimensions for sensor types (the ones in the glossary). If the pixel pitch would be reported in µm, it would give the impression of knowing the exact dimensions of the respective sensor and the pixel shape. The unconventional "pixel density" does not suppose that.

This mode of reporting could also represent a protest regarding camera and sensor manufacturers disdainfully lack of transparency. What is that "unknown manufacturer" ? In the old days everybody could know that a certain SLR was using a Copal or Seiko shutter. Maybe there is the moment for a "stripdown report" like in the PopPhoto magazine from the 70's when Norman Goldberg was taking apart cameras, showing their insides and commenting on the quality of manufacture.

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