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Author Topic: Sensor and Sensibility  (Read 18418 times)
Ray
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« Reply #40 on: August 17, 2006, 10:28:18 AM »
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Olivier,
Good attempt! But I can summarise your argument as follows. A large bucket that is half full holds as much water as a small bucket that is completely full.

A large sensor will always receive the same light as a small sensor if you allow the same amount of light through the lens. That's obvious but needs mentioning.

Your argument could also be used to claim that 35mm film delivers approximately the same image quality as 8x10" format. DoF at F8 on 35mm film is equivalent to f64 on 8x10" format. To get the best, finest grain, lowest noise, highest resolution result with 35mm color film, I'd use something like Royal Gold ISO 25 or Ektar 25.

If I were to use the same type of film with 8x10", there would be no question that the large format would produce vastly superior results. However, if I insist on letting the same amount of light through the lens at f64 (ie. same shutter speed), I'd either have to push process the film by a ridiculous amount or use an ISO 1600 film. Either way, results would be considerably degraded.
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Olivier_G
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« Reply #41 on: August 17, 2006, 12:14:13 PM »
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Ray,

Well...
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For equal FoV and DoF of the same scene under the same lighting conditions, the smaller sensor receives less light.
I used your explanation for comparing with same FoV and DoF and just added your own suggestion on using the same shutter speed in order to slightly correct that statement...

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A large sensor will always receive the same light as a small sensor if you allow the same amount of light through the lens. That's obvious but needs mentioning.
Thanks. Actually, I am pretty sure 99% of photographers don't consider my previous message as obvious... and just mix everything (Signal, noise, Sensor's size, lenses...) in that flawed and simple credo: "A bigger photosite is better" I can read everywhere.

By the way, I propose a much better one: "A larger lens is better"! (and really: it's quite accurate as the lens diameter for a given FoV will be rather well related to the amount of light reaching the sensor... whatever the sensor's size)


About your film comparison (who won? 25 ISO on 35mm or 1600 ISO on 8x10"? ):
My message was on the noise that plagues 95% of photographers: when you don't have enough photons to get a "good" picture. This is also the issue raised by Michael in 'Sensor & Sensibility' and other articles. And as I explained, this is related to the lenses you can use with the system... not to the sensor's size.

If you consider the optimal situation where the photographer has enough light (ie: tripod, etc...), the issue is quite different and is related to the limited Dynamic Range and impact in the shadows. I agree that sensor's size is quite important here...
But this represent 5% of photographers (ok: they are all on this board...   ) and, more important, unlimited Dynamic Range may become available soon, whatever the sensor's size... => that *real* advantage of larger sensors regarding Noise will vanish in the mid/long term.

Olivier
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jani
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« Reply #42 on: August 17, 2006, 01:33:12 PM »
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By the way, I propose a much better one: "A larger lens is better"! (and really: it's quite accurate as the lens diameter for a given FoV will be rather well related to the amount of light reaching the sensor... whatever the sensor's size)
To nitpick: no, it's not better, because it depends on other factors, too.

"All generalisations are bad, including this one."

BTW, I see that you didn't answer my point regarding the Nikon D1H and D1X.
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Jan
Olivier_G
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« Reply #43 on: August 17, 2006, 01:59:58 PM »
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To nitpick: no, it's not better, because it depends on other factors, too.
"All generalisations are bad, including this one."
...Ok: it's not better... it's less bad...
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BTW, I see that you didn't answer my point regarding the Nikon D1H and D1X.
Well, I read that the D1X has actually a tricky 11MP sensor...   ...not exactly the best situation to compare things.

Olivier
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jani
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« Reply #44 on: August 17, 2006, 03:34:20 PM »
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Well, I read that the D1X has actually a tricky 11MP sensor...   ...not exactly the best situation to compare things.
Wow, that was new information for me, too.

But that's the D1, not the D1H or D1X.

Too bad he doesn't mention what they did with the D1H and D1X.
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Jan
Ray
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« Reply #45 on: August 17, 2006, 07:34:46 PM »
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If you consider the optimal situation where the photographer has enough light (ie: tripod, etc...), the issue is quite different and is related to the limited Dynamic Range and impact in the shadows. I agree that sensor's size is quite important here...

Olivier,
From a purely technical point of view, we could say that image quality generally improves as the amount of light available to describe the scene increases. There are exceptions for esthetic reasons. For example, a very long exposure on a moonlit night (with a low-noise Canon DSLR) will produce a remarkably high quality image, almost as though it was taken in full daylight, but might destroy the mood of the scene.

The larger format camera (and it is assumed the size of the lens is going to match the size of the sensor) allows one to use more light when taking the shot. Of course, one can choose not to let more light through the lens for the sake of a faster shutter speed, whatever the size of the sensor, assuming one has manual control.

My impression is, when you compare a small format digital camera with a larger format under conditions of same DoF, same FoV, same shutter speed, the camera with the better or most technological innovation wins. The larger camera is generally more expensive, bigger, heavier and can therefore 'hold' more technological wizadry.

For example, an 8mp P&S such as the KM A200 with a sensor diagonal of 11mm has about a 2.5 stop advantage compared with a Canon 20D. To get the same DoF and shutter speed with both cameras, you would need to use ISO 600 on the 20D (if that were possible) in place of ISO 100, or ISO 300 in place of ISO 50 on the KM A200. My guess is, the 20D would still produce the better image in these circumstances, especially when you take into consideration the 'real' ISO values. I believe ISO on the 20D is conservative. ie. the ISO 400 setting is actually ISO 500.

It would be interesting to compare the Leaf Aptus 75 at ISO 800 with the 1Ds2 at ISO 400, after adjustments to get the real ISO values comparable. Since the Aptus 75 has double the number of pixels and higher resolution, the differences might not be as great as one might imagine. Image quality might be very close. If image quality were still worse from the Aptus, one could lay the blame on a lack of on-chip processing, which is much easier with the CMOS type sensor and a major advantage.

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By the way, I propose a much better one: "A larger lens is better"! (and really: it's quite accurate as the lens diameter for a given FoV will be rather well related to the amount of light reaching the sensor... whatever the sensor's size)

I can't see any logic here. Whatever the size of the lens, equal DoF and equal FoV means equal aperture diameter. The amount of light the lens lets through is then governed only by the shutter speed in these circumstances. For example, the diameter of a 50mm lens at f8 is 50/8=6.25mm. The diameter of an equivalent lens at f64 for 8x10 format is 400/64=6.25mm).

In summary, one could say that the inherent advantages of the small format camera are compactness, light weight and low price. But nothing much else.
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Olivier_G
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« Reply #46 on: August 18, 2006, 03:34:21 AM »
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The larger format camera (and it is assumed the size of the lens is going to match the size of the sensor)...
Why is it assumed that the smaller format must be equipped with a smaller/lesser lens? My point is that we should break those 'rules' to get a better picture.
What are the equivalents of 35mm f/1.4, 85mm f/1.2, 200mm f/1.8, 400mm f/2.8, 600mm f/4 or 1200mm f/5.6 in the Medium Format?

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The larger camera is generally more expensive, bigger, heavier and can therefore 'hold' more technological wizadry.
I rather think a manufacturer that has large volumes can put more technological wizardry in its products, as Canon does (its CMOS, noise, processing power, etc... is quite exciting) whereas medium backs are specialized products optimised for high quality photography: they use higher quality components and can avoid degradations required for other types of shooting (handheld low light, speed...) like the use of microlenses, the fast read-out speed, etc...


About different systems comparisons: by using the 'same FoV, DoF, Speed' equivalence (ie: same amount of light, different ISO), I compared samples from a lot of digicams/DSLR and found that the sensor used in the Fuji F30 (6MP 1/1.7") is quite efficient, the latest Sony 1/2.5" and 1/1.8" are close and the Canon CMOS used in APS format are also very good.
I believe the sensors used in the 1D2/1Ds2 (and even 5D) are not quite so light-efficient, Nikon/Sony DX as well as sensors used in '4/3' are behind. Kodak/Dalsa sensors for MF are probably even less light-efficient as it is not their priority (ultimate quality is).
This is not exactly a scientific test, but it gives some interesting trends: light efficiency is not related to sensor's size, but rather to the R&D/Production effort the manufacturer put into it (ie: the level of technology). There is no limit at 5 or 6.8 micron... as the excellent results from the 2.6 micron Fuji F30 shows (OK, This 1.75 micron CMOS sensor is probably pushing the envelope at this time).

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By the way, I propose a much better one: "A larger lens is better"! (and really: it's quite accurate as the lens diameter for a given FoV will be rather well related to the amount of light reaching the sensor... whatever the sensor's size)
I can't see any logic here. Whatever the size of the lens, equal DoF and equal FoV means equal aperture diameter. The amount of light the lens lets through is then governed only by the shutter speed in these circumstances. For example, the diameter of a 50mm lens at f8 is 50/8=6.25mm. The diameter of an equivalent lens at f64 for 8x10 format is 400/64=6.25mm).
Ray:
- Do you agree that same FoV, DoF and shutter speed give the same image (or 'as close as could be') and same Signal/Noise whatever the sensor size?
- Do you agree that for a given FoV, when light is lacking (ie: camera or subject movements), you will be better off with a lens of large aperture diameter on APS rather than with a lens of smaller aperture diameter on 645 to get a picture with little noise/blur?

The logic is to replace the usual "a larger sensor has less noise" which is not true by "a larger lens has less noise" which is really more accurate in those situations.

Olivier
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Anon E. Mouse
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« Reply #47 on: August 18, 2006, 04:49:27 AM »
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The entrance pupil by itself has nothing to do with how much light is used in an exposure. What is important is the effective aperture, usually given in f-numbers - this is important as it is going to produce the intensity of the signal. That is the factor that the International Standards Organization uses to determine exposure for noise and saturation based ISO. The entrance pupil does not indicate the amount of light used in exposure without the distance to the image plane; what goes in does not matter unless the distribution (magnification) is taken into account. That is why effective aperture is the thing.

Physically big lenses don't mean anything except they are big lenses. And most likely expensive.

I am sorry if I misunderstood the point you folks are trying to make - it is hard to follow what you are talking about.
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Mark D Segal
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« Reply #48 on: August 18, 2006, 08:26:58 AM »
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Anon, I think you've hit the nails on the head: nail #1: indeed, the amount of light reaching the sensor depends on more than just the size of the lens - distance from the lens to the sensor is very critical; nail #2: indeed alot of this discussion is hard to follow, most likely because there may be a large number of non-sequiturs in it, possibly due to the comingling of issues and the elements relevant to one issue but perhaps not the other. One needs to distinguish carefully first of all what one is talking about: noise or resolutiion, or both, or be explicit about how they interact if they do, then sort out clearly in what ways lenses or the sensor is a more determinative factor in the one than in the other. Some such sorting out of the discussion by those who seem to understand it would help the rest of us.
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Mark D Segal (formerly MarkDS)
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Ray
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« Reply #49 on: August 18, 2006, 11:53:24 AM »
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Why is it assumed that the smaller format must be equipped with a smaller/lesser lens? My point is that we should break those 'rules' to get a better picture.
What are the equivalents of 35mm f/1.4, 85mm f/1.2, 200mm f/1.8, 400mm f/2.8, 600mm f/4 or 1200mm f/5.6 in the Medium Format?

Well, I must battle on   .

Olivier,
It's not essential that the smaller format be equipped with a smaller lens. It's just more sensible and more efficient. You wouldn't get far using a 35mm lens on a 2/3rds P&S camera (assuming you could fit it). The lenses used for P&S cameras need to be higher resolving than 35mm lenses because the pixels and sensors are so small.

Likewise, you could use your large format camera and lenses to take MF size images, but you would not get the best of results. It seems to be a fact of lens design that absolute resolution (lp/mm) is compromised in lenses that are optimised for a large image circle. A 400mm lens designed for 35mm format is generally much higher resolving than a 400mm lens designed for LF.

The equivalent MF lenses for 35mm f1.4, 85mm f1.2 and 200mm f1.8 would be approximately 70mm f2.8, 180mm f2.5 and 400mm f3.5, but you are probably not going to get a 400 f3.5, or an 800 f5.6 because they would simply be too heavy, expensive and cumbersome. Even the 1200/5.6 for 35mm has to be manufactured on demand and is ridiculously expensive and heavy.

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I rather think a manufacturer that has large volumes can put more technological wizardry in its products

I don't think so. Are there any laptops that have the same performance as the bigger and heavier workstations? Miniaturisation without quality compromise is terribly expensive.

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The logic is to replace the usual "a larger sensor has less noise" which is not true by "a larger lens has less noise" which is really more accurate in those situations.

I've never come across any noise specifications for lenses, so I don't know how to respond. Lens resolution is either limited by various types of aberration, such as coma, spherical aberration, chromatic aberration, etc.... or diffraction.
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Ray
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« Reply #50 on: August 18, 2006, 11:18:54 PM »
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Olivier,
On reflection, I guess what you mean by lens noise is the sum total of all the aberrations, effects of diffraction, opacity of the glass etc, that reduce contrast and resolution.

If this is what you mean, then I still can't see the logic behind your assertion that a larger lens (a lens designed to throw a larger image circle) has inherently less noise than a smaller lens.

What seems to occur, as I understand, is that lenses with a smaller image circle tend to be diffraction limited at smaller f stop #s. Zuiko lenses for the 4/3ds format tend to be higher resolving than equivalent 35mm lenses, roughly in inverse proportion to the diagonal of the sensors the lenses are designed for, but very roughly.

Theoretically, if you compare lenses for different formats at DoF-equivalent apertures and FOV-equivalent focal lengths, and at those apertures both lenses are diffraction limited, then both lenses are capable of equal total image resolution. (Or in your jargon, have equal noise).

To use an extreme example, the main reason an 8x12" LF camera with 400mm lens at f64 produces better image quality than a 50mm lens at f8 on 35mm format is due to the limitation of the film. Provided the 50mm lens is fully diffraction limited at f8 and the 400mm lens is fully diffraction limited at f64, both lenses deliver essentially the same quality image. Do you not agree with this principle?
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Olivier_G
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« Reply #51 on: August 19, 2006, 03:56:40 AM »
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I will first reply to the previous messages. However, I agree with Anon and Mark that the discussion is hard to follow. Therefore I will try to sum-up things faithfully in my next message, focusing only on the issue of noise (Signal/Noise) as described in 'Pixel Size' and 'The Laws of Physics' of Michael article.

Anon,
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The entrance pupil does not indicate the amount of light used in exposure without the distance to the image plane; what goes in does not matter unless the distribution (magnification) is taken into account. That is why effective aperture is the thing.
In the discussion, we always compared for the same Field of View, which means the focal lengths are known (or at least their ratio involved) -> we have a relationship between the entrance pupil and the effective aperture. To say it differently: for a same Field of View and speed shutter, the entrance pupil diameter determines the amount of light reaching the sensor.


Ray,
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It's not essential that the smaller format be equipped with a smaller lens. It's just more sensible and more efficient. You wouldn't get far using a 35mm lens on a 2/3rds P&S camera (assuming you could fit it).
If I apply this logic to 645 vs 35mm, it would be "unsensible and unefficient" to use a 400mm f/2.8 because its too large for this format, based on what is available for 645.

I will not enter the resolving power of lenses discussion, because I am not that interested in resolution and I want to focus on noise.

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On reflection, I guess what you mean by lens noise is the sum total of all the aberrations, effects of diffraction, opacity of the glass etc, that reduce contrast and resolution.
Sorry, I should have said "a system with a larger lens has less noise" (and added: for a given FoV, same shutter speed, etc...). I was not talking about optical limitations of lenses...

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Theoretically, if you compare lenses for different formats at DoF-equivalent apertures and FOV-equivalent focal lengths, and at those apertures both lenses are diffraction limited, then both lenses are capable of equal total image resolution. (Or in your jargon, have equal noise).
To use an extreme example, the main reason an 8x12" LF camera with 400mm lens at f64 produces better image quality than a 50mm lens at f8 on 35mm format is due to the limitation of the film. Provided the 50mm lens is fully diffraction limited at f8 and the 400mm lens is fully diffraction limited at f64, both lenses deliver essentially the same quality image. Do you not agree with this principle?
I agree.

Ray, I think we agree on the principles involved and just have some misunderstandings on the conclusions... maybe because we don't have exactly the same point of view on how they should be used. I would like to solve this with my next 'summary' message.

Olivier
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Anon E. Mouse
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« Reply #52 on: August 19, 2006, 07:29:36 AM »
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In the discussion, we always compared for the same Field of View, which means the focal lengths are known (or at least their ratio involved) -> we have a relationship between the entrance pupil and the effective aperture. To say it differently: for a same Field of View and speed shutter, the entrance pupil diameter determines the amount of light reaching the sensor.

Why bother? The effective aperture says it all. Entrance pupil is only used to describe object space. Using a convoluted reasoning to related it to the effective aperture is messy, confusing, and unnecessary.

But if you are saying a 20mm entrance pupil on a normal lens gives the same illumination at the image plane regardless of the focal length of the lens, then the statement is false. The "light gathering area" of a lens does not determine the illumination at the image plane.
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Olivier_G
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« Reply #53 on: August 19, 2006, 07:54:07 AM »
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But if you are saying a 20mm entrance pupil on a normal lens gives the same illumination at the image plane regardless of the focal length of the lens, then the statement is false. The "light gathering area" of a lens does not determine the illumination at the image plane.
I am not talking about illumination at the image plane. I am considering the amount of photons reaching the sensor (still same entrance pupil, same FoV, same exposure time) in order to determinate the Signal/Noise Ratio in each photosite.

Don't you agree with this:
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Let's consider formats A and B where B has a 2.0 cropping factor:
I put a 300mm f/4.0 on A, used at f/4.0, 1/500s and 400 ISO and a 150mm f/2.0 on B, used at f/2.0, 1/500s and 100 ISO.
=> FoV, DoF, Shutter speed are the same and I use the same Exposure Value as (1/500s & f/4.0 & 400 ISO) = (1/500s & f/2.0 & 100 ISO).
With the same number of pixels, each photosite of A is 4x larger compared to photosites of B. But to keep the same DoF, the lens on B is opened more by 2 stops (4x more photons per area) => photosites of B gets exactly the same amount of photons as photosites of A.

Added
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Why Bother? The effective aperture says it all.
Effective aperture alone says nothing about Signal/Noise... just as Sensor's size alone says nothing.
However, when you combine both of them, you get something meaningful regarding noise. For a given FoV, this combination is the Entrance pupil diameter.

Olivier
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Anon E. Mouse
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« Reply #54 on: August 19, 2006, 08:18:03 AM »
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I am not talking about illumination at the image plane. I am considering the amount of photons reaching the sensor (still same entrance pupil, same FoV, same exposure time) in order to determinate the Signal/Noise Ratio in each photosite.

Then you are confused. Signal/Noise ratio is always is reference to exposure. Exposure is in reference to the intensity of the light reaching the image plane. While the energy of an Airy disk does not change with focal length with a given entrance pupil, the intensity of the Airy disk does. Sensors react to the intensity of light.

I would suggest going to the ISO web site and get a copy of the standard for determining the ISO of an imaging sensor - you can't really separate the photosite from the sensor as a whole. They and every manufacturer uses exposure to determine the signal/noise ratio of a given sensor. The standard simply requires the effective aperture and transmittance of the optics used. This will make your job a lot easier. Forget about entrance pupil, it won't give you anything.

As far as your example, it has nothing to do with the number of photons, but the intensity of light at the image plane. So no, I don't agree with it.
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Olivier_G
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« Reply #55 on: August 19, 2006, 09:58:48 AM »
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Anon, I'll do my best to answer:
- "Then you are confused" -> No, I'm not.
- "Signal/Noise ratio is always in reference to Exposure" -> Not necessarily (but I hear 'standard' coming...)
- "The intensity of the Airy disk changes with focal length for a given entrance pupil" -> As you advised, I consider my sensor as a whole where it becomes a non issue (even more if the airy disk is within a photosite dimension)
- "I would suggest going to the ISO web site" -> I see ISO 12232 as the 'Program mode': I have it but don't consider it very highly (and the use by manufacturers seems to support this: do you think Canon's ISO Ratings of the 5D and Ixus 800 IS are coherently based on ISO 12232?). But this ISO standard is really a different issue and does not help.
- "As far as your example, it has nothing to do with the number of photons" -> there are 'intensity of light at the image plane' and 'surface indications', I suppose it should be enough.

Olivier
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Olivier_G
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« Reply #56 on: August 19, 2006, 10:21:43 AM »
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I'll try to make now a summary about Signal/Noise (aka: "noise") when dealing with not enough photons, as in Michael's article (ie: when you don't have enough light because of shutter speed issue: camera/subject blur, etc...). I will not talk about Signal/Noise in the shadow area due to insufficient Dynamic Range to record highlights correctly (ie: "expose to the right") nor other issues like Resolution...

Basics
1. Signal/Noise Ratio in the sensor will determine noise in the picture.
2. Processing will modify this noise (Noise Reduction, Sharpness, etc...)
3. Noise can be classified as: Read Noise (from amplifier), Dark Current Noise (from silicon imperfections) and Shot Noise (quantum nature of light. Shot Noise=square-root of Signal)
4. Read Noise and Dark Current Noise generated in the sensor will depend on sensor's design (CMOS/CCD, architecture, size, quality, rate...) as well as temperature (including heat generated during exposure), etc...
(note that technology improvements tend to reduce those 2 Noises whereas Shot Noise is inherent of light and can't be avoided)
5. The Signal is the amount of light effectively converted in the sensor
6. There is a difference between the light arriving on the sensor and the light effectively converted based on: sensor Quantum Efficiency (+depends on light wavelength & ray angle), Aperture ratio (ie: conversion area / total area = photodiode/photosite), use of microlenses (focus light on photodiode), color filtering absorption, etc...
(CCD Full Frame Transfer used in Medium Formats have very high aperture ratio, whereas smaller sensors now use very efficient microlenses)

Some approximations for discussion purpose:
A. From [2]: assume the same processing (neutral)
B. From [4]: assume the same sensor technology/generation

Note: I'll have a break here, as I have to be very careful about how to continue the explanation.
If you have any issue or want to add/correct some points, please let me know: I'll update this message later.


Olivier
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Anon E. Mouse
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« Reply #57 on: August 19, 2006, 11:56:56 AM »
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Anon, I'll do my best to answer:
- "Then you are confused" -> No, I'm not.
- "Signal/Noise ratio is always in reference to Exposure" -> Not necessarily (but I hear 'standard' coming...)
- "The intensity of the Airy disk changes with focal length for a given entrance pupil" -> As you advised, I consider my sensor as a whole where it becomes a non issue (even more if the airy disk is within a photosite dimension)
- "I would suggest going to the ISO web site" -> I see ISO 12232 as the 'Program mode': I have it but don't consider it very highly (and the use by manufacturers seems to support this: do you think Canon's ISO Ratings of the 5D and Ixus 800 IS are coherently based on ISO 12232?). But this ISO standard is really a different issue and does not help.
- "As far as your example, it has nothing to do with the number of photons" -> there are 'intensity of light at the image plane' and 'surface indications', I suppose it should be enough.

Olivier
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I read this, but I have no idea what you are talking about. Your "photon" based hypothosis does not work. It is a useful metaphor to help explain pixel "buckets" and how photon "balls" fill them, but it falls short on an actual model of what is going on. A wave based explination is far better.

As an illustration, you believe if the Airy disk is contained in the photo site, you are recieving an equal amount of photons with a given entrance pupil regardless of focal length. But because you are thinking of light as a particle, then all exposures should be based on entrance pupil and time as all the photons going through the entrance pupil would end up in the pixel "bucket." 150 years of photography says that is not true. If you think of light as a wave, then you see the intensity of the light changes with a change in the focal length. And this IS how photography works, which is why exposure is based on effective aperture and time (as well as why an Airy disk is formed). (I noticed in your example that the two situations use different ISOs. This looks like a trick to make your model work.)

Another problem of trying to determine S/N ratio by entrance pupil is no one bases exposures by entrance pupil. So what would be the point of having an entrance-pupil S/N ratio? You would still need some conversion factor to obtain a usable exposure with a format not used as the "standard", which would make the entrance-pupil S/N ratio a little useless. Not a very practical approach.

Now if you are saying your model for determining S/N ratio has nothing to do with exposures, then what is the point of the model? The people who use the cameras are interested in the S/N of their exposures which will be in reference to an effective aperture and time.

I have no idea why you are not simply using effective aperture. While there are a small group of amateur astrophotographers who may agree with you, there are no models using entrance pupil to determine what is going on at the image plane to support your assertions (BTW, numerical aperture in microscopy is not an entrance pupil, but an effective pupil like the f-number system). All the ones I know contradict what you are saying. The fact that photography works does not really help your case.
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Ray
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« Reply #58 on: August 19, 2006, 08:19:33 PM »
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Olivier,
Why not look at it this way.

Sensors really have only one ISO, which depends on such factors as the quantum efficiency of the photodiodes. This base ISO is usually around 100, but can range from 50 to 200 on different cameras. Different ISO settings on a digital camera are actually 'push processing' instructions. The sensitivity of the sensor remains unchanged.

For comparison purposes, let's pretend the cameras don't have increased ISO options. They just have the one, base ISO setting.

To simplify matters with regard to resolution differences, let's compare an 8mp Sony F828 (or the latest 8mp P&S) with a Canon 20D.

I think we are already in agreement with the principle that f4 on the Sony F828 (or similar P%S) will give us the same DoF as f8 on the 20D for images with the same FoV.

Let's consider a situation where the exposure on the F828 at f4 and ISO 100 gives us the required shutter speed we need to freeze subject movement. (I'm ignoring the fact that base ISO on the Sony is actually ISO 64 and that the base ISO of 100 on the 20D is actually ISO 125).

In these circumstances, if I want the shot without blurring and with good DoF, I have no option but to underexpose the shot from the 20D by 2.5 stops, which is quite a significant underexposure.

How would the two images compare? I'd say they'd be pretty much on a par. However, if you want to argue this point, show me some real world comparisons.

Now we know those 'push processing' instructions really do serve a purpose. Let's consider what really happens in this situation. We don't underexpose the 20D shot at ISO 100 just because we need a fast shutter speed, if we know what we are doing. We bump up the ISO setting to ISO 600 (ignoring the fact that the 20D doesn't have any intermediate ISO settings).

If we then compare the two shots, you will find that the 20D provides better image quality and lower noise. In both cases the sensors have received the same amount of light but the superior processing technology of the 20D (amplification of the analogue signal before A/D conversion, for example) has dramatically improved the situation.

Once again. the bottom line is: the camera with the smaller sensor has the advantage of being lighter, more compact and cheaper, but nothing else.
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Ray
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« Reply #59 on: August 22, 2006, 11:58:00 PM »
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Once again. the bottom line is: the camera with the smaller sensor has the advantage of being lighter, more compact and cheaper, but nothing else.
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I was sort of hoping someone might disprove this point. Recently checking out the specs of the Panasonic LX1 P&S, I came across some amazing noise performance results for the FinePix F30. This camera appears to have a fully usable, low noise ISO 800 setting. The 20D is said to be usable at ISO 1600, but to get the same DoF as the small-sensor FinePix F30 at the same shutter speed, you'd have to use ISO 3200 on the 20D.

The $64,000 question is thus, 'How does the 6mp F30 image at ISO 800 compare with the 8mp 20D image at ISO 3200?'

Where are such comparisons? Why are reviewers not getting to the crux of the matter?
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