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Author Topic: Sensor and Sensibility  (Read 17874 times)
Mark D Segal
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« on: August 09, 2006, 06:33:22 PM »
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Michael, many thanks for this very straightforward, clear and sensible discussion. And all the more so after what you have just been through.

I have only one observation to contribute. Toward the bottom you mention that one probably won't see much difference in image quality between a 16 and 22 MP sensor. Based on the underlying logic and assuming all else equal, no argument.

However, as your article is intended to provide factual guidance to those contemplating equipment options, it may be useful to caution that this isn't necessarily a reason not to buy a 22 MP camera (rumoured to be the next generation DSLR forthcoming some time soon) - because most of the time not all else is equal and there are other considerations.

For one thing, as we saw between the 1Ds and the 1Ds Mk2, when they bring out a new model, not only the pixel count changes, but so do other things which add-up to producing cleaner images - i.e. ancillary technical change in a number of variables at the same time all contributing to improved image quality.

For another, with 22MP, one has considerable added luxury for cropping images and still getting excellent resolution on large prints. For example a 1Ds Mk 2 at 16MP provides about 4900 pixels on the large dimension, for a print dimension of 16.3 inches at 300PPI. Bump up the sensor to 22 MP with the same aspect ratio and the 4900 PPI dimension changes to about 5800 PPI, or 19.3 inches printed at 300 DPI. This is about an 18% increase in "croppable real estate" on the large dimension, while roughly maintaining 1Ds Mk2 resolution.
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Mark D Segal (formerly MarkDS)
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« Reply #1 on: August 09, 2006, 06:58:36 PM »
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The only additional comment that I would make is that the article, like many other discussions, confuses pixel pitch with pixel size. Early sensors may have had a larger pixel pitch than current sensors, though due to design and production limitations, the fill factor was lower than current sensors and, therefore, the amount of light capturing area on the chip was less. Where we are seeing increasing pixel density on the chip then there is probably no reduction in the light capturing area compared with previous generations - i.e. the pixel pitch is decreasing but the pixel size (light capturing area) remains constant as the fill factor increases. We will probably see another couple of generations of improvement in pixel density as manufacturers move production to more advanced production processes with finer resolution in the lithography and improvements in fill factor.
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David Plummer    http://photo.tanzo.org/
michael
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« Reply #2 on: August 09, 2006, 08:13:45 PM »
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David,

It's even more complicated that that, as you likely know. Fill factor is an issue, but so is the use of microlenses, which some chips use and others don't, and which helps conversion efficiency.

I deliberately refrained from going into too much esoteric detail in an essay of just a few paragraphs, with a very specific intent.

Michael
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Ray
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« Reply #3 on: August 09, 2006, 08:36:49 PM »
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Just a few thoughts. When a new Canon model comes out, I usually check the dpreview noise and resolution charts, after they get around to publishing their in-depth review. It seems to be the case so far that Canon never introduces a new model that has higher pixel noise than a previous model. On the noise front, it seems Canon refuses to go backwards (with regards to DSLRs anyway).

Many people were disappointed the 30D had no more pixels than the 20D. I guess it's because Canon were simply unable at that stage to provide smaller pixels that would not also be noisier. But I'm confident they are working on it and that we'll see more pixels on the next generation of cropped format and full frame sensors and those individual pixels will be no noisier (and possibly even less noisy at high ISOs) than the current 30D and 1Ds2 pixels.

Another issue is the effect of noise on a given size print. Most of us do not restrict ourselves to making prints of a size that does not require either upsampling or downsampling of the image file. If we did, we'd be restricting ourselves to a 12x18" print for the 5D and a 9.5x14' print for the 30D. When we upsample (interpolate) an image to make a large print, we interpolate the noise also. The reverse is also true.

The consequence of this is, for any equal size prints, big or small, the sensor with the greater number of pixels will produce less visible noise, provided the smaller, more numerous pixels have, individually, at least equal noise to the larger, less numerous pixels on the other sensor.

I'm also of course confusing pixel size with pixel pitch, which I think is forgivable because Canon never seems to make public the actual pixel (photodiode) size. But, as DiaAzul mentions, there's probably scope for improvement here by reducing the space on the sensor taken up with on-chip processing, whilst keeping the actual size of the photodiode the same, even though the pixel pitch has increased. However, it's not clear to me to what extent there might be benefit here. As I understand it, the microlens directs the incoming light onto the photodiode, whatever its size, so the photons are not wasted by falling unproductively on processing transistors. Nevertheless, a physically larger photodiode should translate to a greater full-well capacity and greater dynamic range. It's worth noting that, according to some facts in a Canon brochure on the D30 (not 30D), that Michael referred to in a thread a few years ago, the actual pixel size of the 10 micron-pixel-pitch D30 is only 5.25 microns. (I know! I've got a good memory, for some things   ).
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boku
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« Reply #4 on: August 09, 2006, 09:23:04 PM »
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David,

It's even more complicated that that, as you likely know. Fill factor is an issue, but so is the use of microlenses, which some chips use and others don't, and which helps conversion efficiency.

I deliberately refrained from going into too much esoteric detail in an essay of just a few paragraphs, with a very specific intent.

Michael
[a href=\"index.php?act=findpost&pid=72917\"][{POST_SNAPBACK}][/a]

I was sort of coming to the same conclusion. As the inter-site walls become thin, the microlense geometry overlaps and increases purple fringing. That's my theory.
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Bob Kulon

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Ray
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« Reply #5 on: August 09, 2006, 09:57:43 PM »
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One point that Michael made in his article, a point which is often made in the interests of realism and downplaying of unrealistic expectations, is the fact that the laws of Physics are 'inviolate'. In other words, that's where we come to a dead end.

This stance has always struck me as curious because the laws of Physcics are also the laws of man, and what man creates he can often break or modify, according to his imagination and ability.

I'm not suggesting that Maxwell's equations, Einsteins theories of Relativity or the current theories of Quantum Mechanics are necessarily wrong for our current purposes. They are probably inaccurate, as we'll probably eventually find out.

As technology progresses, new ways of 'circumventing' rather than 'violating' the so-called laws of physics are discovered, which is rather exciting, don't you think?

There's even promising research underway to circumvent the 'laws of diffraction' by producing materials with a negative refractive index. Refer to the following link here
« Last Edit: August 09, 2006, 09:59:33 PM by Ray » Logged
michael
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« Reply #6 on: August 10, 2006, 02:09:17 AM »
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Ray,

Good point.

My intention though is not to work from a base assumption that any inherent limitation can be overcome, but rather to suggest that people not have unrealistic expectations.

Remember, man can not fly.

Michael
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John Sheehy
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« Reply #7 on: August 10, 2006, 07:41:54 AM »
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This stance has always struck me as curious because the laws of Physcics are also the laws of man, and what man creates he can often break or modify, according to his imagination and ability.
[a href=\"index.php?act=findpost&pid=72926\"][{POST_SNAPBACK}][/a]

There may be hope with quantum efficiency, readout noise, etc, but digital cameras are counting photons, and without increasing subject lighting, there is a finite limit to how many you can capture per unit of focal plane area.  Shot noise is not some foreign disturbance to digital photography; it is a facet of its very substance.

However, shot noise is not the most immediate obstacle to greater DR with current sensors; sloppy readout noise at low ISOs is.  Shot noise affects mainly the highlights at low ISOs, and highlights and midtones at high ISOs.
« Last Edit: August 10, 2006, 01:49:47 PM by John Sheehy » Logged
Tim Gray
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« Reply #8 on: August 10, 2006, 08:04:42 AM »
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At the risk of adding one more variable into the mix, I'd be interested in some thoughts on how print size impacts perception of micro detail.

Michael indicates that I should expect to see the difference between a 1d2 and 1ds2 (8 vs 16 mpx) and consequently, obviously a difference between a 1d2 and a 22mpx sensor.

And equally clearly, I won't see any difference printed 5x7 or 8x12 or (probably?) 12x18.  My prints from the mpx dvd don't demonstrate clear differences at 17x25 even between the 5d and p45 (at least to my aging eyes).  I'm wondering, based on the best interpolation techniques available today, at what point up-rezzing begins destroy detail apparent in print to a skilled observer?
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Ray
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« Reply #9 on: August 10, 2006, 09:17:43 AM »
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However, shot noise is not the most immediate obstacle to greater DR with current sensors; sloppy readout noise at low ISOs is  [a href=\"index.php?act=findpost&pid=72948\"][{POST_SNAPBACK}][/a]

That may be true. Supposing a sensor manufacturer discovers a way of cutting read-out noise by 75%.  We could have 4 small pixels occupying the space of one large pixel, say 4x3 micron pixels in place of one 6 micron pixel. The total read noise from the 4x3 micron pixels would be the same as the read noise from the one 6 micron pixel. The shot noise would be the same because the area the light falls on is the same. However, the resolution would be higher because of the greater pixel count.

Add to that a few other noise cancellation breaks-through, cancellation of thermal noise and perhaps a little photon multiplying and we could have something really special. I'm getting excited already   .
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Quentin
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« Reply #10 on: August 10, 2006, 10:09:31 AM »
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Possibly not much difference between 16mp and 22mp in 35mm format, but it will be intriguing to see if there are differences, and if so, how much, between 22mp and 22mp in 35mm and 645 format.  That will help tell us whether the smaller format has really reached its limits.
« Last Edit: August 10, 2006, 10:10:11 AM by Quentin » Logged

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John Camp
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« Reply #11 on: August 10, 2006, 12:01:27 PM »
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Here's a question for those of you who are more technically apt:

It seems that sensor size is being squeezed between the image that a lens can relay to the sensor, and the limit (on the small end) of how small pixels can be made. In other words, couldn't really have, say, a 36mp sensor using Canon 35mm lenses because the pixels would have to be so small that you'd get unacceeptable noise, or the sensor qould have to be so large that you wouldn't get complete coverage. So if 5 micron pixels, say, wind up being the smallest acceptable, how big a chip could you squeeze into a camera that uses 35mm lenses? Also, do 35mm lenses have design aspects that means that they are most effective (or only effective) when they are locked into one (horizontal) format? Or would it be possible to build a squarish chip (say, 6x7 format) into a 35mm camera, using existing lenses?

JC
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Ray Maxwell
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« Reply #12 on: August 10, 2006, 12:17:41 PM »
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The biggest difference between the full frame 35mm chips and the 645 chips can be illustrated by my image at:

http://www.photo.net/photodb/photo?photo_id=4251879

I have two cameras.  One is a Canon 5D and the other is a Hasselbad with a Leaf Valeo 22 digital back.  For this picture I had shot a number of hand held test shots with the 5D in the morning light.  After studying the results of the hand held shots, I chose the composition that I liked.  The next morning I then set up my tripod with the Hasselbald SWC 38mm Biogon with the Leaf Valeo 22 and made this shot.

Notice the detail in the bright, directly lit highlights outside and then the shadow detail in the walls of the room in deep shadow.  The Canon 5D could not handle this dynamic range.  The MF back captures 11 to 12 f/ stops of dynamic range.  The chips with smaller pixels cannot do this.

While it is possible to do multiple shots with the 5D and use the HDR feature in Photoshop, it is not possible to capture this dynamic range in one click.

Ray M.
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Hank
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« Reply #13 on: August 10, 2006, 01:33:25 PM »
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Bravo, Michael.

Nuff sed.
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John Sheehy
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« Reply #14 on: August 10, 2006, 01:49:15 PM »
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However, shot noise is not the most immediate obstacle to greater DR with current sensors; sloppy readout noise at low ISOs is  Shot noise affects mainly the highlights at high ISOs.
[a href=\"index.php?act=findpost&pid=72948\"][{POST_SNAPBACK}][/a]

Wow, that paragraph got really mangled; I must have deleted part of the sentence.  Let me try again:

However, shot noise is not the most immediate obstacle to greater DR with current sensors; sloppy readout noise at low ISOs is.  Shot noise affects mainly the highlights at low ISOs, and highlights and midtones at high ISOs.
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oldcsar
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« Reply #15 on: August 10, 2006, 02:04:49 PM »
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I've read some articles from people who have done astrophotography with IR-modified cameras. What they suggest, is that a cold night can reduce apparent noise in their long exposures. I really don't know the science behind it, but the rule of thumb is that the colder the camera, the more controlled the noise is... the hotter the camera, the more noise.

Maybe the next important development in noise reduction is finding a way to produce circuitry which makes less heat, or devise additional systems which cool the camera's internals.
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EricV
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« Reply #16 on: August 10, 2006, 02:14:26 PM »
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One important point not mentioned in the article is the relationship between pixel size and lens resolution.  Increasing the pixel count of a sensor of a given size by making smaller pixels provides little improvement in image quality once the pixels become smaller than the lens resolution.  At this point, image quality is dominated by the lens rather than the sensor.  This consideration has nothing to do with noise.

In this situation, a sensor with larger pixels will provide a higher resolution image at the same pixel count.  (Of course the sensor will be larger, so a different lens will be required to give the same angular coverage, but let's assume both lenses have similar resolution.)

Even a perfect lens is limited by diffraction, which is a function of the lens aperture.  A lens stopped down to f/4 for example will have a diffraction-limited blur of around 5 microns at the image plane.  It makes a significant difference whether this image is digitized with pixels of size 2 microns or 5 microns or 10 microns, even if sensor noise is negligible.
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John Sheehy
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« Reply #17 on: August 10, 2006, 02:27:32 PM »
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I've read some articles from people who have done astrophotography with IR-modified cameras. What they suggest, is that a cold night can reduce apparent noise in their long exposures. I really don't know the science behind it, but the rule of thumb is that the colder the camera, the more controlled the noise is... the hotter the camera, the more noise.
[a href=\"index.php?act=findpost&pid=72988\"][{POST_SNAPBACK}][/a]

Noise due to heat is not applicable to short exposures, at least with some cameras.  Neither of my Canon DSLRs show any difference in noise in temperature extremes until the exposures get to be multiple seconds in length.  The noise from long exposures is mostly stray, bright pixels, which repeat from frame to frame.
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boku
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« Reply #18 on: August 10, 2006, 02:53:49 PM »
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Noise due to heat is not applicable to short exposures, at least with some cameras. Neither of my Canon DSLRs show any difference in noise in temperature extremes until the exposures get to be multiple seconds in length. The noise from long exposures is mostly stray, bright pixels, which repeat from frame to frame.
[a href=\"index.php?act=findpost&pid=72992\"][{POST_SNAPBACK}][/a]

Even at short exposures, I have found that a 120 deg F camera will always have much more image noise (mainly chromatic) in the shadows than the same camera at 20 deg F. It is noticable.
« Last Edit: August 10, 2006, 02:54:34 PM by boku » Logged

Bob Kulon

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« Reply #19 on: August 10, 2006, 06:28:52 PM »
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Noise due to heat is not applicable to short exposures, at least with some cameras.  Neither of my Canon DSLRs show any difference in noise in temperature extremes until the exposures get to be multiple seconds in length.  The noise from long exposures is mostly stray, bright pixels, which repeat from frame to frame.
[a href=\"index.php?act=findpost&pid=72992\"][{POST_SNAPBACK}][/a]

That's not quite true - noise is directly related to the temperature of the sensor. This is thermally generated noise associated with the random re-combination/splitting of electrons from the material. The effect that you mention with respect to longer exposures and repeatability from frame to frame is more associated with variations in the actual pixel construction during fabrication. One of the hardest parts of sensor production is ensuring that all pixels are fabricated within a certain given tolerance (i.e. +/- 5% of required specification). Higher quality and larger sensors will typically have greater uniformity across the sensor and greater immunity from variations (i.e. there is an argument that MF sensors have perhaps +/-2% variance from pixel to pixel in terms of electrical characteristics whereas a point and shoot may be +/-20%). This variance across the sensor will render an image of lower perceived quality even though there is the same (big assumption) thermally generated noise and will be particularly evident in longer exposures.

You can also add in that all of the electronics will be calibrated for a given temperature (range) and operating outside of that range will give imprecise results - e.g. if you have a colour profile for room temperature than it will be slightly off at extremes of hot and cold.
« Last Edit: August 10, 2006, 06:29:58 PM by DiaAzul » Logged

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