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04-16-2014, 01:11 PM   #1
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Help me understand the 645Z sensor's resolving power

I was comparing the sensor dimensions and resolutions in the newly announced 645Z to the Sony A7R. Like most number games, the results are inconclusive but it made me think a bit. I'd like to share my thoughts and ask for your opinions.

The sensor in the 645Z is 43.8mm by 32.8mm with an output image resolution of 8265 by 6192 pixels. I calculated one pixel to be 5.3 micrometers square. Yes, I understand that the sensor uses a bayer array of individual photonic cells and that the individual cells are actually smaller.

The sensor in the Sony A7R is 35.9mm by 24mm with an output image resolution of 7360 by 4912 pixels. That pixel size is 4.9 micrometers square. A small photonic cell will have weaker low light performance since there is less surface area for the photons to strike. This tells me that the 645Z should have better low light performance with less noise at higher ISO settings. Or, perhaps I can capture greater details in the shadows since the darker areas of the image will have less noise to battle against.

Now I'm going to mix my units a bit and this is where I might get in trouble. With a horizontal resolution of 8265 pixels across 43.8mm (or 1.72 inches) the 645Z sensor achieves 4793 dots (or pixels) per inch. The Sony A7R with its 7360 pixels across 35.9mm (or 1.41 inches) achieves 5207 dots (or pixels) per inch. This tells me that the Sony A7R could resolve finer details although the overall size of the resolved details in the final image would be smaller.

This is where I get hung up. Numerically it seems that each camera has a strength over the other. But, these kinds of comparisons fall apart when shifted to another extreme. The Q has a 6.17mm x 4.55mm sensor with a resolution of 4000 pixels by 3000 pixels. With a horizontal resolution of 4000 pixels across 6.17mm (or 0.24 inches) the sensor achieves 16467 dots (or pixels) per inch. It sounds impressive until you consider the 1.54 micrometer pixel size which means higher noise.

I completely understand that the proof is in the pictures, not the numbers. The 645Z sample pics I saw on Flickr were absolutely amazing! There were images of trees and I could resolve almost every twig and leaf on them. I have no complaints about that kind of resolving power! But, could the A7R resolve even more?

04-16-2014, 01:29 PM   #2
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I've had the same thoughts about resolving power and pixel density when thinking about using the Q (or Q10/Q7) for macro. Since the sensor packs so many pixels in such a small area, you ought to be able to resolve incredibly small details, no? If both a Q and my K-01 are at 1:1 macro ratio, the Q is putting all its pixels into a much smaller (image) area than my K-01 (at the same 1:1 macro ratio). I'm not sure how/why this reasoning breaks down, so I'm in the same boat as you...
04-16-2014, 01:50 PM - 1 Like   #3
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If you are PRINTING an image then a high number of dpi is fine. But this has nothing to do with the amount of pixels on your chip...
04-16-2014, 02:14 PM   #4
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Well the sensor resolution in terms of pixels per inch is bigger, this also means that, added to the increased noise, the optical flaws of the lens will be more amplified.
I have seen large printouts of the 645 D in an exhibition, 645Z is even better, that's an achievement !

04-16-2014, 02:27 PM   #5
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The most important other factor is that larger lenses going to a larger 50MP sensor will give you better resolution that smaller lenses going to a smaller 50MP sensor (which will probably exist by the end of the year).
04-16-2014, 02:44 PM   #6
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I think what's happening in all the math is you're comparing pixels per inch of sensor, but what matters when printing (or viewing on the internet for that matter) is pixels per inch of viewing surface. Regardless of sensor size, a 24" x 36" print is still 24" x 36", and a 50mp file will give you higher dpi (and higher resolution) than 36mp, which is higher than 24 mp, etc etc. Or maybe I'm totally daft. Someone smarter than me will have to figure that out.
04-16-2014, 03:06 PM   #7
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QuoteOriginally posted by 6BQ5 Quote
I was comparing the sensor dimensions and resolutions in the newly announced 645Z to the Sony A7R. Like most number games, the results are inconclusive but it made me think a bit. I'd like to share my thoughts and ask for your opinions.

The sensor in the 645Z is 43.8mm by 32.8mm with an output image resolution of 8265 by 6192 pixels. I calculated one pixel to be 5.3 micrometers square. Yes, I understand that the sensor uses a bayer array of individual photonic cells and that the individual cells are actually smaller.

The sensor in the Sony A7R is 35.9mm by 24mm with an output image resolution of 7360 by 4912 pixels. That pixel size is 4.9 micrometers square. A small photonic cell will have weaker low light performance since there is less surface area for the photons to strike. This tells me that the 645Z should have better low light performance with less noise at higher ISO settings. Or, perhaps I can capture greater details in the shadows since the darker areas of the image will have less noise to battle against.

Now I'm going to mix my units a bit and this is where I might get in trouble. With a horizontal resolution of 8265 pixels across 43.8mm (or 1.72 inches) the 645Z sensor achieves 4793 dots (or pixels) per inch. The Sony A7R with its 7360 pixels across 35.9mm (or 1.41 inches) achieves 5207 dots (or pixels) per inch. This tells me that the Sony A7R could resolve finer details although the overall size of the resolved details in the final image would be smaller.

This is where I get hung up. Numerically it seems that each camera has a strength over the other. But, these kinds of comparisons fall apart when shifted to another extreme. The Q has a 6.17mm x 4.55mm sensor with a resolution of 4000 pixels by 3000 pixels. With a horizontal resolution of 4000 pixels across 6.17mm (or 0.24 inches) the sensor achieves 16467 dots (or pixels) per inch. It sounds impressive until you consider the 1.54 micrometer pixel size which means higher noise.

I completely understand that the proof is in the pictures, not the numbers. The 645Z sample pics I saw on Flickr were absolutely amazing! There were images of trees and I could resolve almost every twig and leaf on them. I have no complaints about that kind of resolving power! But, could the A7R resolve even more?
I believe you are correct for the same size image projected by the lens, e.g. if you used a 50mm lens on the 645D and the A7r, the Sony would in theory be able to see more detail in the image in terms of ppi, but the total image on the 645 is much larger and the FOV quite different (roughly 40mm equivalent vs. 50mm). A better comparison would be a 50mm on the Sony vs. 62mm on the 645; in that case, it's the same FOV and same image, but larger and higher resolution on the 645 (more pixels for a given image and FOV.)
04-16-2014, 03:13 PM   #8
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I think the best analogy to use would be a jumbotron screen.

would you rather have one jumbotron that's 100 feet long and 50 feet tall? or the same jumbotron but made up of 500 smaller screen, each 10 feet by ten feet.
the smaller photosites require more photosites to record the same portion of the image, thereby introducing fuzzy logic interpolation errors.
since 5 micrometer details are completely indistinguishable to the human eye, there is a limit to how small a photosite can be to gain an advantage.

i'm not 100% certain im right, but I think on on the right path

04-16-2014, 03:29 PM   #9
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QuoteOriginally posted by Thomas Quote
I believe you are correct for the same size image projected by the lens, e.g. if you used a 50mm lens on the 645D and the A7r, the Sony would in theory be able to see more detail in the image in terms of ppi, but the total image on the 645 is much larger and the FOV quite different (roughly 40mm equivalent vs. 50mm). A better comparison would be a 50mm on the Sony vs. 62mm on the 645; in that case, it's the same FOV and same image, but larger and higher resolution on the 645 (more pixels for a given image and FOV.)
Yeah, that.
04-16-2014, 03:45 PM   #10
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QuoteOriginally posted by 6BQ5 Quote
I was comparing the sensor dimensions and resolutions in the newly announced 645Z to the Sony A7R. Like most number games, the results are inconclusive but it made me think a bit. I'd like to share my thoughts and ask for your opinions.

The sensor in the 645Z is 43.8mm by 32.8mm with an output image resolution of 8265 by 6192 pixels. I calculated one pixel to be 5.3 micrometers square. Yes, I understand that the sensor uses a bayer array of individual photonic cells and that the individual cells are actually smaller.

The sensor in the Sony A7R is 35.9mm by 24mm with an output image resolution of 7360 by 4912 pixels. That pixel size is 4.9 micrometers square. A small photonic cell will have weaker low light performance since there is less surface area for the photons to strike. This tells me that the 645Z should have better low light performance with less noise at higher ISO settings. Or, perhaps I can capture greater details in the shadows since the darker areas of the image will have less noise to battle against.

Now I'm going to mix my units a bit and this is where I might get in trouble. With a horizontal resolution of 8265 pixels across 43.8mm (or 1.72 inches) the 645Z sensor achieves 4793 dots (or pixels) per inch. The Sony A7R with its 7360 pixels across 35.9mm (or 1.41 inches) achieves 5207 dots (or pixels) per inch. This tells me that the Sony A7R could resolve finer details although the overall size of the resolved details in the final image would be smaller.

This is where I get hung up. Numerically it seems that each camera has a strength over the other. But, these kinds of comparisons fall apart when shifted to another extreme. The Q has a 6.17mm x 4.55mm sensor with a resolution of 4000 pixels by 3000 pixels. With a horizontal resolution of 4000 pixels across 6.17mm (or 0.24 inches) the sensor achieves 16467 dots (or pixels) per inch. It sounds impressive until you consider the 1.54 micrometer pixel size which means higher noise.

I completely understand that the proof is in the pictures, not the numbers. The 645Z sample pics I saw on Flickr were absolutely amazing! There were images of trees and I could resolve almost every twig and leaf on them. I have no complaints about that kind of resolving power! But, could the A7R resolve even more?
I've written an article showing when one can start to appreciate having a larger sensor, and what effects this has on image quality. There are ultimately three factors to consider: 1. pixel count 2. pixel density and 3. sensor age.

https://www.pentaxforums.com/forums/55-photography-articles/228535-how-why-se...s-compact.html

So, in this case the 645Z has 51 megapixels (>3x more than the A7s) on an area that's only 65% larger than a full-frame sensor. So this suggests that it will have more noise than the A7s out-of-camera, but after scaling the image down to 12 megapixels you should be able to get a cleaner file, since all that extra data (resolution) will greatly help differentiate noise from true color data.

Adam
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04-16-2014, 04:07 PM   #11
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QuoteOriginally posted by 6BQ5 Quote
could the A7R resolve even more?
you're forgetting one important factor - lenses. The Sony A7R with a good copy of the Calr Zeiss 50mm f/2 Makro planar would be roughly equal to the 645Z with an older film era 75mm f/2.8 - MF sensors don't strain lenses as much as smaller formats do. However if you put a newer Pentax Zeiss or schneider lens on the 645Z the equation tilts drastically in favor of the 645Z. A sensor is only as good as the lens you put in front of it.
04-16-2014, 05:17 PM - 1 Like   #12
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Leaving out the theoretcials, the first thing you have to realize is that you may be talking apples and oranges. What's important for low light and noise is not the size of the pixel site, but the actual size of the light sensitive sensor...

see this article for a brief explanation....
What is a backlit CMOS sensor? | News | TechRadar

So the first question becomes which camera has the largest photo receptor, that is different from which has the biggest photo sites as determined by how much room each occupies on the chip.

I've also seen a breakdown of a Sony technology that makes the photosensitive part of the sensor almost the whole size of the space allotted by placing the circuitry under the sensitive array.

That just means you need a lot more knowledge than I'd know where to find before you even start.

But just for a comparison, if you measure resolution by line widths per picture height (lw/ph)
A D610 FF camera with 24 MP will produce ~2,800 lw/ph from a raw image under ideal conditions based on a pixel def. of 6016x4016. So the D600 produces 2800/6016 or .465 distinct lines per pixel.

A D7100 also 24 Mp but APS-c will also produce 2800 pixels from raw.. from 6000x4000... Nikon D7100 Review - Exposure
A Pentax K-3 resolved about 2700 lw/ph.. Pentax K-3 Review: Now Shooting! - Exposure

So in terms of lw/ph, there is no advantage to a larger sensor. At least looking at current models, all tested with the same lens as far as I know.

When you start discussing artifacts like moire and noise at low light, the smaller sensor seems to be at a disadvantage.

A Zeiss Makro-Planar T* 100mm f/2 ZF (ZE) (on Canon EOS) records 3776 lw/ph on a 21 Mp camera.

The Sigma 70 macro tested at ƒ4 also tested out at 3736 on the Canon body, and that is the lens used to test the D610, D7100 and K-3 tests posted above... there may be other attributes that make the lens desirable...

I don't know what lens as use to test the 645D but it also resolves down to 3700... but the conclusion would that a Zeiss Makro-Planar T* 100mm f/2 ZF (ZE) (on Canon EOS 21MP), is as good as an image taken on a 40 Mp 645D... pretty scary.
A D800 on the other hand also probably with a Sigma 70 produces 3400 lw/ph Nikon D800 Review - Exposure, this is also similar to an A7r.

Looking at all the numbers, they just don't add up in any real way, except that as pointed out above, a top quality lens may be the biggest contributing factor in maximum resolution...
My conclusion would be , test the lens you want on the camera you want and see what it gives you... there are just way to many variables to make sense of, trying to come up with some kind of mathematical formula to understand it all.

Last edited by normhead; 04-17-2014 at 05:52 AM.
04-17-2014, 12:18 PM   #13
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Thanks for everyone's replies! Lots of good points were brought up here. I understood that I was looking at the sensor's resolution in a vacuum without taking into account low light performance, noise, dynamic range, etc. What I completely missed is how the lens interacts with the sensor. Now that I think about it some more I wonder what limits the image more : sensor or lens? The sensor photocell site (or resulting pixel) has a certain dimension. So anything smaller than that dimension cannot be resolved. My lens could be so awesome that it shines through a million distinct lines onto one site/pixel and my sensor won't distinguish it. I would guess that even a perfectly constructed lens has a theoretical resolution limit. Even if I get rid of all the impurities my light still has to rub and bounce against the individual molecules making up the glass that make randomly or chaotically oriented. I know that this venturing into the ridiculous since it's an extreme but it's kind of interesting to think about. It brings up the age old question of how good is old glass on a new sensor.

A bigger sensor requires a bigger lens for the bigger image circle. Making something bigger is usually a lot easier than making something small. Some manufacturing requirements may be relaxed. That's probably another area where larger format photography wins a bit.
04-17-2014, 02:11 PM - 1 Like   #14
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No matter the conclusion, after spending some time with an IQ250, I seriously cannot wait until my 645z pre-order is fulfilled!
04-17-2014, 04:09 PM   #15
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QuoteOriginally posted by 6BQ5 Quote
Thanks for everyone's replies! Lots of good points were brought up here. I understood that I was looking at the sensor's resolution in a vacuum without taking into account low light performance, noise, dynamic range, etc. What I completely missed is how the lens interacts with the sensor. Now that I think about it some more I wonder what limits the image more : sensor or lens? The sensor photocell site (or resulting pixel) has a certain dimension. So anything smaller than that dimension cannot be resolved. My lens could be so awesome that it shines through a million distinct lines onto one site/pixel and my sensor won't distinguish it. I would guess that even a perfectly constructed lens has a theoretical resolution limit. Even if I get rid of all the impurities my light still has to rub and bounce against the individual molecules making up the glass that make randomly or chaotically oriented. I know that this venturing into the ridiculous since it's an extreme but it's kind of interesting to think about. It brings up the age old question of how good is old glass on a new sensor.
The thing is, "lens resolution" is essentially an analog signal (unless you can count individual photons), while image sensors are a form of digital sampling. You can pretty much keep sampling indefinitely and keep getting more information out, even beyond the so called "diffraction limit" (because airy disks are not uniform blobs, but complex patterns). So, you can never really be hard limited by the lens. The question is, how useful is the information you gain by continuing to sample (adding more pixels) vs. the compromises you make in implementing the smaller pixel technology?
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