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A new sensor that can be upgraded is now an obligation

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..If you have a full frame sensor then the Circle of Confusion can be taken as 31 micron again. In that case having a pixel density of about 1Mp/cm2 would be more than good enough & you would get less noise/higher ISO.

Would you mind to explain me the link between CoC and noise please. Thanks.

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The CoC is just some arbitrary number that was decided to be 'good enough' for getting an acceptably sharp photographic print from a film of a certain size. So with a MF film the CoC is bigger than with a 135 or 110 film, as the last need to be enlarged more.

 

For a 135 film the industry standard is 31 micron CoC, purely by definition. Many nowadays find this too lenient as enlargements are cheaper and pixel peeping is more popular than ever. But that is the way it is - like it or not.

 

Anyway what it boils down to is that if you have a larger sensor then on paper you need THE SAME number of pixels to get the same resolution enlargement. So if it remains 10MP that give the same 'sharpness' of the sensor. The secret however is that you can scale up the surface area of the pixels in this process, so you will collect more photons for a given aperture, illumination, and shutter speed. More photons => less noise, because the measurement of the light intensity becomes more reliable if you have a larger no. of counts.

 

You are using a bigger bucket to catch the stream of photons.

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...More photons => less noise, because the measurement of the light intensity becomes more reliable if you have a larger no. of counts...

Yes i know this thanks but that's the link between noise and CoC i don't understand. I mean, a FF sensor generates the same CoC whatever MPs it has i.e. 0.03mm for the 12.7 MP sensor of the Canon 5D for instance but also 0.03mm for the 24.6 MP sensor of the Sony A900 as well. In other words, the CoC does not depend upon the pixel count but only the size of the sensor, so that there is no link at all between CoC and digital noise AFAIK. Am i missing something here?

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For whatever reason people want a "full frame look" which you will achieve with a FF sensor.

 

If you have a full frame sensor then the CoC is given and does not depend on anything, I think/hope you will agree with me on that.

 

So then it boils down to deciding what no.of pixels you need to reach that level of resolution on the sensor. On the M8 the pixels are 6.8 micron so roughly 3 pixels for one CoC of 21 micron.

 

On a FF Mx (x>8.2) you need to resolve a Circle of Confusion of about 31 micron, so using again 3 pixels they could be as big as 10 micron. The surface per pixel would then be (10/6.8)^2 times higher which is about 2.5 so you get that number of photons extra per pixel.

 

So you are right that the sensor dimensions and resulting CoC has nothing to do with noise per-se but it gives you more design freedom to trade sensitivity vs. resolution.

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Yes i know this thanks but that's the link between noise and CoC i don't understand. I mean, a FF sensor generates the same CoC whatever MPs it has i.e. 0.03mm for the 12.7 MP sensor of the Canon 5D for instance but also 0.03mm for the 24.6 MP sensor of the Sony A900 as well. In other words, the CoC does not depend upon the pixel count but only the size of the sensor, so that there is no link at all between CoC and digital noise AFAIK. Am i missing something here?

 

 

Yep- the Nyquist frequency;)

 

If the resolution of the lens is less than the resolution of the sensor - a very real possibility and a near-certainty in the case of 20Mp+ sensors in 24x36- the lens will use more than one pixel for every image "point". That will even out the noise.

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the lens will use more than one pixel for every image "point". That will even out the noise.
It will primarily waste precious disk space. There is no reason physical, esthetic or otherwise to exceed the CoC value to any major extent in standard photography. Even a prehistoric lens can probably handle the optical resolution to get a focal point smaller than a 20 micron CoC, its primarily about the lack of distortions, abberations, flare resistance, contrast & micro contrast etc. etc.

 

Thats what we are paying for.

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...If you have a full frame sensor then the CoC is given and does not depend on anything, I think/hope you will agree with me on that...

I'm afraid i can't sorry. A FF sensor has a 0.03mm CoC because it is full frame and the CoC of 24x36 film is 0.03mm as well. If the film CoC had been 0.02mm, then the FF digital CoC would have been 0.02mm as well. AFAIK the CoC depends upon the size of the film or the sensor, not the pixel count of the sensor or ISO of the film. This way, given a 'classic' CoC of 0.03.mm for FF, the M8 has a CoC of 0.03 : 1.33 = 0.023mm. The result will remain the same if the M9 has a 50 MP sensor of the same size as that of the M8. But i'm no techie at all so i may be wrong of course.

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The COC is indeed a source of confusion.. It is an arbitrary value, based on the characteristics of an early 20th century film and the small prints that were usual back then.

Nowadays with sharply defining sensors, thin, highly resolving films and large standard prints, it is totally obsolete. For normal daily use 0.02 is on the large side for 24x36 capture media and even a COC of 0.015 is defensible. For the M8 I would use 0.01.

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...So you are right that the sensor dimensions and resulting CoC has nothing to do with noise per-se but it gives you more design freedom to trade sensitivity vs. resolution.

Sure it gives more freedom but we agree that digital noise does not depend upon CoC right?

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The COC is indeed a source of confusion.. It is an arbitrary value, based on the characteristics of an early 20th century film and the small prints that were usual back then....

I beg to disagree Jaap. The DoF markings of our lenses are based on what you call those arbitrary values, i.e. a 0.03mm CoC for 24x36 lenses. As long as those markings will work, they will stay valid IMHO. For me they worked with film and they still work with digital.

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... The circle of confusion... is not governed by the size of the pixels ... but by the CROP FACTOR....

Hence by the size of the sensor. I had not read this part of your post sorry so i agree of course.

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There is just another important sensor parameter to be regarded,

The size of a pixel is not equal to the size of the photodiode.

The ratio between the two is called "fill factor".

Quite some effort has been put lately in increasing this fill factor a much as possible.

An 1D MkII has a pixel size of 8,2*8,2 micron, against 7,2*7,2 for the 1D MkIII, but the photodiode is just as large because of a higher fill factor.

This another step forward to reduce noise.

 

Hans

 

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