Digital Cameras: Does Pixel Size Matter?

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Hi,
I've quantified digital camera sensor performance, comparing
photon-noise-limited DSLRs and point and shoot cameras.
Good cameras are now photon noise limited and this sets
basic properties that depend on pixel size and how many photons
can be collected by the sensor. The photon limits and resulting
gain factors explain the major differences observed between the
large pixels of DSLRs and the small pixels found in point and
shoot cameras.

Article at:
http://clarkvision.com/imagedetail/does.pixel.size.matter

Roger
 
G

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Roger N. Clark (change username to rnclark) wrote:
> Hi,
> I've quantified digital camera sensor performance, comparing
> photon-noise-limited DSLRs and point and shoot cameras.
> Good cameras are now photon noise limited and this sets
> basic properties that depend on pixel size and how many photons
> can be collected by the sensor. The photon limits and resulting
> gain factors explain the major differences observed between the
> large pixels of DSLRs and the small pixels found in point and
> shoot cameras.
>
> Article at:
> http://clarkvision.com/imagedetail/does.pixel.size.matter
>
> Roger

Roger,

Many thanks for another stimulating view.

There are some typos:

- 1st paragraph: "dod not get"

- table 4 - shouldn't the S60 shadow total be 332?

Questions:

1 - ISO measurement

What it all says to me is that the ISO figures are grossly misleading. If
all cameras were required to give the same SNR at (say) 18% grey level,
then couldn't we just say that the S60 had ISO 100 sensitivity and the 10D
ISO 400 sensitivity? What are we doing wrong in stating ISO sensitivities
today?

2 - Number of pixels for a given sensor size.

I know I've raised this before but I'll do so again. If you have a given
sensor (let's think small sensor here - e.g. 8.8 x 6.6mm), then you could,
for example, have either 2MPixels at one size or 8MPixels at half the
linear size or a quarter of the area. Neglecting packing efficiency, you
could have a higher-resolution image with poorer SNR in each pixel, or a
lower resolution image with higher SNR per pixel. Which does the eye
prefer in typical viewing conditions? The image with the smaller pixels
will appear crisper. How is the different noise spectrum perceived by the
eye?

Cheers,
David
 
G

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David J Taylor wrote:
> Roger N. Clark (change username to rnclark) wrote:
>>Article at:
>>http://clarkvision.com/imagedetail/does.pixel.size.matter

>
> Roger,
>
> Many thanks for another stimulating view.
>
> There are some typos:
>
> - 1st paragraph: "dod not get"
>
> - table 4 - shouldn't the S60 shadow total be 332?

Thanks David,
You are correct. I fixed these.
>
> Questions:
>
> 1 - ISO measurement
>
> What it all says to me is that the ISO figures are grossly misleading. If
> all cameras were required to give the same SNR at (say) 18% grey level,
> then couldn't we just say that the S60 had ISO 100 sensitivity and the 10D
> ISO 400 sensitivity? What are we doing wrong in stating ISO sensitivities
> today?

This is a good question. I think the answer is partly that
ISO was originally defined as a density on film at a given
exposure time for a given light source intensity. If one takes
a percentage of full well capacity in electronic sensors,
as a substitute for density on film, then we see ISO definition
has to do with how many photons are collected as a percentage
of the total possible. That says nothing about noise, just as
with film the specification says nothing about noise
(or film grain).

Side note:
Because film has a much lower quantum efficiency than CCDs or
CMOS sensors, it collects many fewer photons, about 6 times
lower (at least). Scaling the numbers here,
we could compute the number of photons incident per
square micron at a given ISO, and f/ratio. It appears to me
that film too may be close to photon noise limited, or at
least a large part of the noise we see in film, which we call
grain is due to photon statistics, added to the random
distribution of film grains, which induces its own noise.
But I believe the photon noise is the larger of the two
components.
>
> 2 - Number of pixels for a given sensor size.
>
> I know I've raised this before but I'll do so again. If you have a given
> sensor (let's think small sensor here - e.g. 8.8 x 6.6mm), then you could,
> for example, have either 2MPixels at one size or 8MPixels at half the
> linear size or a quarter of the area. Neglecting packing efficiency, you
> could have a higher-resolution image with poorer SNR in each pixel, or a
> lower resolution image with higher SNR per pixel. Which does the eye
> prefer in typical viewing conditions? The image with the smaller pixels
> will appear crisper. How is the different noise spectrum perceived by the
> eye?

This is a very good question, and I have been researching this
subject. It appears that to first order there is an even trade for
signal-to-noise and spatial resolution. So in your example,
the images would appear similar. But because it is
subjective, some people will prefer one, others the other,
but they would be judged close, especially compared to the
same images with equal signal-to-noise ratios. I've created
images like this and that is what I see and what others have
indicated. But it is a subjective test. I plan on creating a
series of images, trading spatial resolution and signal-to-noise
ratio and presenting that on a web page for people to judge for
themselves. But it will be a few months before I get this done.

Roger
 
G

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You do, however, still have to match your expected image scale to your pixel
size, hence the paucity of wider angle lenses for CCD sensors (Nyquist
criteria, etc, etc...).
________________
G. Paleologopoulos


"Roger N. Clark (change username to rnclark)" <username@qwest.net> wrote in
message news:4205AD5A.5080009@qwest.net...
>
> Hi,
> I've quantified digital camera sensor performance, comparing
> photon-noise-limited DSLRs and point and shoot cameras.
> Good cameras are now photon noise limited and this sets
> basic properties that depend on pixel size and how many photons
> can be collected by the sensor. The photon limits and resulting
> gain factors explain the major differences observed between the
> large pixels of DSLRs and the small pixels found in point and
> shoot cameras.
>
> Article at:
> http://clarkvision.com/imagedetail/does.pixel.size.matter
>
> Roger
>
 
G

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Guest
Archived from groups: rec.photo.digital (More info?)

g n p wrote:

> You do, however, still have to match your expected image scale to your pixel
> size, hence the paucity of wider angle lenses for CCD sensors (Nyquist
> criteria, etc, etc...).
> ________________
> G. Paleologopoulos

I don't see what this has to do with sensor noise versus
pixel size, which is all my page addresses.
Could you please clarify what you mean?

Roger
>
>
> "Roger N. Clark (change username to rnclark)" <username@qwest.net> wrote in
> message news:4205AD5A.5080009@qwest.net...
>
>>Hi,
>>I've quantified digital camera sensor performance, comparing
>>photon-noise-limited DSLRs and point and shoot cameras.
>>Good cameras are now photon noise limited and this sets
>>basic properties that depend on pixel size and how many photons
>>can be collected by the sensor. The photon limits and resulting
>>gain factors explain the major differences observed between the
>>large pixels of DSLRs and the small pixels found in point and
>>shoot cameras.
>>
>>Article at:
>>http://clarkvision.com/imagedetail/does.pixel.size.matter
>>
>>Roger
>>
>
>
>
 

Marvin

Distinguished
May 2, 2004
248
0
18,830
0
Archived from groups: rec.photo.digital (More info?)

Roger N. Clark (change username to rnclark) wrote:
> Hi,
> I've quantified digital camera sensor performance, comparing
> photon-noise-limited DSLRs and point and shoot cameras.
> Good cameras are now photon noise limited and this sets
> basic properties that depend on pixel size and how many photons
> can be collected by the sensor. The photon limits and resulting
> gain factors explain the major differences observed between the
> large pixels of DSLRs and the small pixels found in point and
> shoot cameras.
>
> Article at:
> http://clarkvision.com/imagedetail/does.pixel.size.matter
>
> Roger
>
The first figure on the Web page compares the amount of light photons that would be collected by two pixels of different
size, assuming that both were exposed to the same number of photons per unit area. That means that the total amount of light
over the whole sensor needs to be larger when the pixels are larger. That, in turn, means that the amount of light collected
by the lens must be larger in the second case. I haven't read the whole article carefully, but if the argument takes into
account the need for the lens to collect more photons, I missed it. Perhaps someone in the NG can point it out for me.

In the past, at least, larger pixels gernerated more thermal noise. The Web article assumes that the problems with dark
noise has been corrected, and that the read noise is too small to matter. If dark noise really is no problem, then one
should be able to take very long exposures with digicams, just as with film.
 
G

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Marvin wrote:
> Roger N. Clark (change username to rnclark) wrote:
>
>> Hi,
>> I've quantified digital camera sensor performance, comparing
>> photon-noise-limited DSLRs and point and shoot cameras.
>> Good cameras are now photon noise limited and this sets
>> basic properties that depend on pixel size and how many photons
>> can be collected by the sensor. The photon limits and resulting
>> gain factors explain the major differences observed between the
>> large pixels of DSLRs and the small pixels found in point and
>> shoot cameras.
>>
>> Article at:
>> http://clarkvision.com/imagedetail/does.pixel.size.matter
>>
>> Roger
>>
> The first figure on the Web page compares the amount of light photons
> that would be collected by two pixels of different size, assuming that
> both were exposed to the same number of photons per unit area. That
> means that the total amount of light over the whole sensor needs to be
> larger when the pixels are larger. That, in turn, means that the amount
> of light collected by the lens must be larger in the second case. I
> haven't read the whole article carefully, but if the argument takes into
> account the need for the lens to collect more photons, I missed it.
> Perhaps someone in the NG can point it out for me.

As long as the f/ratio of the lenses between the two cameras
is the same, then the number of photons per square micron
will be the same. But since one camera has larger pixels,
each pixel collects more photons. This is why an f/8 lens
and ISO 100 is camera and sensor size independent.
I guess I should add some more explanation.

> In the past, at least, larger pixels gernerated more thermal noise. The
> Web article assumes that the problems with dark noise has been
> corrected, and that the read noise is too small to matter. If dark
> noise really is no problem, then one should be able to take very long
> exposures with digicams, just as with film.

People are doing 10 minute and longer exposures with DSLRs at room
temperature, and even 30+ minute exposures at high ISO.
Noise from dark current is still a factor, but much less than
it used to be. ISO 100 and long exposures could probably go for
more than an hour on the better cameras.

Try a DSLR astrophotography web search and you will see amazing photos.

Roger
 
G

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Guest
Archived from groups: rec.photo.digital (More info?)

Roger N. Clark (change username to rnclark) wrote:
[]
> This is a good question. I think the answer is partly that
> ISO was originally defined as a density on film at a given
> exposure time for a given light source intensity. If one takes
> a percentage of full well capacity in electronic sensors,
> as a substitute for density on film, then we see ISO definition
> has to do with how many photons are collected as a percentage
> of the total possible. That says nothing about noise, just as
> with film the specification says nothing about noise
> (or film grain).
>
> Side note:
> Because film has a much lower quantum efficiency than CCDs or
> CMOS sensors, it collects many fewer photons, about 6 times
> lower (at least). Scaling the numbers here,
> we could compute the number of photons incident per
> square micron at a given ISO, and f/ratio. It appears to me
> that film too may be close to photon noise limited, or at
> least a large part of the noise we see in film, which we call
> grain is due to photon statistics, added to the random
> distribution of film grains, which induces its own noise.
> But I believe the photon noise is the larger of the two
> components.

I would welcome a more sensible definition of ISO as applied to digital
cameras. Percentage well filling without specifying a noise level is a
plain nonsense. Perhaps then we could move on from the DLSR versus Point
& Shoot argument and simply say if you can be satisfied with a low ISO
camera go P&S and if you need high-ISO go DLSR?

>>
>> 2 - Number of pixels for a given sensor size.
>>
>> I know I've raised this before but I'll do so again. If you have a
>> given sensor (let's think small sensor here - e.g. 8.8 x 6.6mm),
>> then you could, for example, have either 2MPixels at one size or
>> 8MPixels at half the linear size or a quarter of the area. Neglecting
>> packing efficiency, you could have a higher-resolution
>> image with poorer SNR in each pixel, or a lower resolution image
>> with higher SNR per pixel. Which does the eye prefer in typical
>> viewing conditions? The image with the smaller pixels will appear
>> crisper. How is the different noise spectrum perceived by the eye?
>
> This is a very good question, and I have been researching this
> subject. It appears that to first order there is an even trade for
> signal-to-noise and spatial resolution. So in your example,
> the images would appear similar. But because it is
> subjective, some people will prefer one, others the other,
> but they would be judged close, especially compared to the
> same images with equal signal-to-noise ratios. I've created
> images like this and that is what I see and what others have
> indicated. But it is a subjective test. I plan on creating a
> series of images, trading spatial resolution and signal-to-noise
> ratio and presenting that on a web page for people to judge for
> themselves. But it will be a few months before I get this done.
>
> Roger

When I was involved with this professionally some years ago, we used a
weighted SNR based on the sensitivity of the eye & brain at various
spatial frequencies. Rather as in audio where there are various weighting
curves you can use to make the measured SNR tie up better with the
subjective SNR than an unweighted flat frequency response figure. This
attempts to remove at least some of the subjective element, but you are
right that different observers will have different preferences making a
weighted SNR only representative of a median of observers. Viewing
conditions would need to be standardised as well!

Cheers,
David
 
G

Guest

Guest
Archived from groups: rec.photo.digital (More info?)

David J Taylor wrote:

> I would welcome a more sensible definition of ISO as applied to digital
> cameras. Percentage well filling without specifying a noise level is a
> plain nonsense. Perhaps then we could move on from the DLSR versus Point
> & Shoot argument and simply say if you can be satisfied with a low ISO
> camera go P&S and if you need high-ISO go DLSR?

I would like to see the manufacturers specify the actual sensor
size instead of the "type." How many people really know
what a 4/3" sensor is?

Then if the manufacturers would specify the full well capacity
and read noise, consumers would learn quickly that you want
full well high, and read noise low, and people would have the
information needed to make an informed decision.

Roger
 
G

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Archived from groups: rec.photo.digital (More info?)

On Sun, 06 Feb 2005 08:54:39 -0700, "Roger N. Clark (change username
to rnclark)" <username@qwest.net> wrote:

>David J Taylor wrote:
>
>> I would welcome a more sensible definition of ISO as applied to digital
>> cameras. Percentage well filling without specifying a noise level is a
>> plain nonsense. Perhaps then we could move on from the DLSR versus Point
>> & Shoot argument and simply say if you can be satisfied with a low ISO
>> camera go P&S and if you need high-ISO go DLSR?
>
>I would like to see the manufacturers specify the actual sensor
>size instead of the "type." How many people really know
>what a 4/3" sensor is?
>
>Then if the manufacturers would specify the full well capacity
>and read noise, consumers would learn quickly that you want
>full well high, and read noise low, and people would have the
>information needed to make an informed decision.


Roger, I agree entirely and with your
explanation as well -- the current
nomenclature is designed specifically
to confuse and obfuscate.


rafe b.
http://www.terrapinphoto.com
 
G

Guest

Guest
Archived from groups: rec.photo.digital (More info?)

Roger N. Clark (change username to rnclark) wrote:
> David J Taylor wrote:
>
>> I would welcome a more sensible definition of ISO as applied to
>> digital cameras. Percentage well filling without specifying a noise
>> level is a plain nonsense. Perhaps then we could move on from the
>> DLSR versus Point & Shoot argument and simply say if you can be
>> satisfied with a low ISO camera go P&S and if you need high-ISO go
>> DLSR?
>
> I would like to see the manufacturers specify the actual sensor
> size instead of the "type." How many people really know
> what a 4/3" sensor is?
>
> Then if the manufacturers would specify the full well capacity
> and read noise, consumers would learn quickly that you want
> full well high, and read noise low, and people would have the
> information needed to make an informed decision.
>
> Roger

I agree that you want that in the specification somewhere, but those
aren't convenient measures for everyday use and, to be honest, is the
actual sensor size as its effect on picture quality. I was thinking more
like this: if picture quality was rated on a scale of 1 - 10 (say) and
actually meaning the weighted SNR, you could make comparisons like:

Picture quality 8:
DLSR has ISO 400
P&S 1 has ISO 80
P&S 2 cannot achieve PQ 8 at full resolution

Picture quality 5:
DLSR has ISO 1600
P&S 1 has ISO 200
P&S 2 has ISO 100

Cheers,
David
 
G

Guest

Guest
Archived from groups: rec.photo.digital (More info?)

Your analysis was excellent.
My comment was sort of an aside, a footnote to your page, to not mislead the
reader into thinking that large pixels are a general cure-all, and that a
balance must be found between spatial resolution and noise.
Stuff we into CCD astrophotography have beaten to death many, many years
ago!
________________
G. Paleologopoulos



"Roger N. Clark (change username to rnclark)" <username@qwest.net> wrote in
message news:4206370E.9020707@qwest.net...
>
> g n p wrote:
>
>> You do, however, still have to match your expected image scale to your
>> pixel size, hence the paucity of wider angle lenses for CCD sensors
>> (Nyquist criteria, etc, etc...).
>> ________________
>> G. Paleologopoulos
>
> I don't see what this has to do with sensor noise versus
> pixel size, which is all my page addresses.
> Could you please clarify what you mean?
>
> Roger
 
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