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Author Topic: A 1/1.7 CDD sensor vs. a full frame CMOS sensor  (Read 37063 times)

ondebanks

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Re: A 1/1.7 CDD sensor vs. a full frame CMOS sensor
« Reply #20 on: July 22, 2012, 06:38:42 am »

How can this be true though? How can total chip area affect the image and if it is true we'll see a drop in IQ when we crop an image by switching off / masking off an area of the sensor. I see no drop in IQ when I crop an image and I wouldn't expect a FF camera with a built in crop mode to give lower IQ in that mode.


Of course you see no drop in pixel IQ when you crop an image - they're the same pixels. But it's not the same picture. To get the same picture (same composition and angular coverage) from that cropped subset of pixels, you'd have to mount a shorter focal-length lens. Now you have a clear and obvious drop in image IQ - lower magnification, lower angular resolution, lower photon count per object within the photo (something which covered say 9 pixels in the original now only covers 4 pixels, if you employed a 1.5x crop factor).

Personally, until someone demomstrates to me is person that chip size in itself affects IQ I probably wont belive it :)

I hope you believe it now? I think you just had to disentangle pixel IQ from overall image/picture IQ in your mind.

Ray
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ondebanks

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Re: A 1/1.7 CDD sensor vs. a full frame CMOS sensor
« Reply #21 on: July 22, 2012, 07:39:12 am »

It is hard to wrap my brain around the fact that my small camera has a 7.6mm x 5.7mm CDD sensor and my larger camera has a full frame 36mm x 24mm CMOS sensor that is 20 times bigger yet not nearly 20 times better at producing quality images. I believe that the only difference in digital images is the number of pixels and the ability of the camera to deliver the most suitable color for each pixel (post-production aside). Is this true?

Ed(?) - your belief is pretty far off the truth, but you are at least asking the right questions and looking for the truth so I won't knock that.

Your question is a perfect candidate for a reductio ad absurdum way of answering. (This is a logical technique that you can employ yourself of course). If size doesn't matter, then let's take that 7.6mm x 5.7mm sensor and shrink it by a factor of 1000, so now it's a 7.6 micron x 5.7 micron sensor. And let's take that 36mm x 24mm sensor and enlarge it by a factor of 1000, so now it's a 36 metre x 24 metre sensor. Both still have the same number of pixels as before. Do you still think that they will give comparable or even same-ballpark image quality?

Well, am saying that, artistic rendering aside, image quality is a function of the selection of pixel color. The only difference between the output of a $300 sixteen megapixel digital camera and a sixteen megapixel flagship DSLR is the instrument's selection of color for each pixel.

That's twice you use the expression "selection of color" - "selection" is an unfortunate term because it implies that the camera has some choice, some informed decision to make about each pixel. It doesn't. It doesn't select or choose, it simply measures. (It also doesn't work with or measure "color" - it measures greyscale intensity at each pixel - the color comes afterwards, by interpolation across adjacent pixels which captured the light through different built-in coloured filters).

What distinguishes your two different sixteen megapixel sensors is how accurately they can measure the underlying rate of light flux falling on each pixel. You can check the references given above for a description of the Poisson nature of photon arrival times. To keep it simple, all I'll say is that the sensors only have a limited sample of that light flux to work with. The more light each pixel captured, the more likely that its measurement is closer to the true underlying rate of light flux. Given equal sensor technologies, this means that a larger 16MP sensor gives better IQ than a smaller 16MP sensor, due to its larger pixels capturing more light in a given exposure time. This scales nicely even when you apply the reductio ad absurdum test. At the microscopic end of the absurdum, you eventually hit the limits of discrete Poisson stats and quantum electronics.

Keep asking yourself such questions, keep researching and learning. In this age of depressingly rampant black-boxism ("it just works, I have no idea why"), you'll still find lots of people willing to help.

Ray
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marcmccalmont

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Re: A 1/1.7 CDD sensor vs. a full frame CMOS sensor
« Reply #22 on: July 22, 2012, 07:46:22 am »

Quote from Cambridge in colour

"even if two sensors have the same apparent noise when viewed at 100%, the sensor with the higher pixel count will produce a cleaner looking final print. This is because the noise gets enlarged less for the higher pixel count sensor (for a given print size), therefore this noise has a higher frequency and thus appears finer grained."

"No matter what the pixel size, larger sensors unavoidably have more light-gathering area. Theoretically, a larger sensor with smaller pixels will still have lower apparent noise (for a given print size) than a smaller sensor with larger pixels (and a resulting much lower total pixel count). This is because noise in the higher resolution camera gets enlarged less, even if it may look noisier at 100% on your computer screen. Alternatively, one could conceivably average adjacent pixels in the higher pixel count sensor (thereby reducing random noise) while still achieving the resolution of the lower pixel count sensor. This is why images downsized for the web and small prints look so noise-free."

http://www.cambridgeincolour.com/tutorials/digital-camera-sensor-size.htm

Marc
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Marc McCalmont

scooby70

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Re: A 1/1.7 CDD sensor vs. a full frame CMOS sensor
« Reply #23 on: July 30, 2012, 08:39:26 am »

Of course you see no drop in pixel IQ when you crop an image - they're the same pixels. But it's not the same picture. To get the same picture (same composition and angular coverage) from that cropped subset of pixels, you'd have to mount a shorter focal-length lens. Now you have a clear and obvious drop in image IQ - lower magnification, lower angular resolution, lower photon count per object within the photo (something which covered say 9 pixels in the original now only covers 4 pixels, if you employed a 1.5x crop factor).

I hope you believe it now? I think you just had to disentangle pixel IQ from overall image/picture IQ in your mind.

Ray

No. Because your argument is nothing to do with sensor seize affecting IQ in itself and by itself. Yes, a bigger chip will enable you to print a bigger picture but that isn't image quality...

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scooby70

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Re: A 1/1.7 CDD sensor vs. a full frame CMOS sensor
« Reply #24 on: July 30, 2012, 08:47:41 am »

Quote from Cambridge in colour....


Imagine a sensor... Take a picture and view it. Now imagine that we can electrically isolate rows of pixels making the resultant image smaller. Some cameras can actually do this.

By the reasoning of many the IQ of the remaining image will drop. I still remain to be convinced of this and in fact I don't believe it.

Yes the smaller image will degrade if magnified more to match the size of the larger image but taking a picture with a smaller sensor should be the same as cropping an image taken with a larger system using the same technology. The IQ in the areas of the image that both systems capture should be the same at the same image size as all you are doing is throwing away the edges of the larger picture or not capturing them with the smaller system.
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marcmccalmont

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Re: A 1/1.7 CDD sensor vs. a full frame CMOS sensor
« Reply #25 on: July 30, 2012, 11:58:23 am »

Scooby70 you are changing the argument, please reread the original post! Of course each pixel is equivalent that is why many go for apsc sized cameras when the pixels are the same, the apsc sized cameras take advantage of the sweet spot of the lens. But the original statement was comparing cameras, for instance if I had a 16mp db and took a picture and say compared the same picture to a 16mp point and shoot camera is there an IQ difference? the OP thought the only difference was how they interpret color. This is not true the larger sensels and the larger sensor will in general have higher IQ.
Marc
« Last Edit: July 30, 2012, 01:20:44 pm by marcmccalmont »
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Marc McCalmont

ErikKaffehr

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Re: A 1/1.7 CDD sensor vs. a full frame CMOS sensor
« Reply #26 on: July 30, 2012, 02:55:51 pm »

Hi,

I'm looking into this a bit, right now. I happen to have three cameras with similar resolution but different sensor sizes.

Sony Alpha 900, full frame, 24MP
Sony Alpha 77SLT, APS-C 24 MP
Sony RX100, small sensor, 20MP

I plan to do some kind of write up on the issue. Anyway, what I see is that the difference between the two large sensor cameras may be small. The Alpha 77 SLT  has live view while the Alpha 900 has not, and that may be the most important factor between the two.

The small sensor camera can hold it's own at large apertures and short focal lengths. It is quite impressive, but it cannot really match the larger sensors. The RX100 is really good at handheld low light shooting at short focal lengths as the lens is incredibly good at full aperture!

Best regards
Erik

Scooby70 you are changing the argument, please reread the original post! Of course each pixel is equivalent that is why many go for apsc sized cameras when the pixels are the same, the apsc sized cameras take advantage of the sweet spot of the lens. But the original statement was comparing cameras, for instance if I had a 16mp db and took a picture and say compared the same picture to a 16mp point and shoot camera is there an IQ difference? the OP thought the only difference was how they interpret color. This is not true the larger sensels and the larger sensor will in general have higher IQ.
Marc
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Erik Kaffehr
 

NancyP

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Re: sensor physics - book recommendation
« Reply #27 on: July 31, 2012, 12:03:18 pm »

Charles Johnson, Science for the Curious Photographer. It does have some math, but it is written in such a way that you don't need math skills to understand most of it.
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ErikKaffehr

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Re: sensor physics - book recommendation
« Reply #28 on: July 31, 2012, 12:08:25 pm »

Yes, that is a very good book. Thank you!

Erik


Charles Johnson, Science for the Curious Photographer. It does have some math, but it is written in such a way that you don't need math skills to understand most of it.

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Erik Kaffehr
 

EricV

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Re: A 1/1.7 CDD sensor vs. a full frame CMOS sensor
« Reply #29 on: July 31, 2012, 02:51:31 pm »

No matter what size the sensor is the number of photons which are recorded at each individual pixel (assuming for a moment for easy of argument that pixels are all the same size no matter what size the sensor) will be the same regardless of sensor size. All the larger sensor does is receive more photons in the area which is beyond the physical area of the smaller sensor.

Imagine a brick wall facing the sun, each brick illuminated equally. Now draw two squares on it, one within the other and each enclosing a number of bricks. The larger square will capture more light but the quality and strength of the light falling on each brick is exactly the same. Bricks / light outside of any square we draw can not alter the amount or quality of light inside of the square. You could prove this by nailing a light meter to each brick. Adding or removing bricks will not affect the light meter readings on other bricks. How could it.

I presume your desired end product is a photograph of the inner section of the brick wall?  If you take images as you have described above, both cameras will produce identical results, as you note.  But your setup deliberately throws away pixels from the larger format camera.  Let's change the rules so that both cameras photograph the same subject, namely the inner section of the brick wall, without cropping.  This could be accomplished for example by switching to a longer focal length lens on the larger format camera.  Now the larger camera gets to use its entire sensor, and the resulting image will almost certainly be superior.  How could it not be better to use more pixels?

In this revised setup, if both lenses are set to the same aperture (say f/8 in both cases), then the light intensity per unit area on the sensor will be the same, so each pixel will still collect the same number of photons.  The longer lens collects more light through a larger physical opening, but that light is distributed to a correspondingly larger area on the sensor.  What is different in this setup is the resolution -- a single brick is imaged across more pixels on the larger sensor.  This is what potentially gives the larger sensor its advantage.  This resolution advantage also provides a signal/noise advantage to the overall image, assuming that noise per pixel is the same on both sensors.
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