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I compared the specs of some cameras.

The Canon EOS 5DS and the EOS-1D X MARK II each has a full-frame sensor. The second is more expensive. Their pixel counts are 8868 x 5792 and 5472 x 3648.

Nikon does the same. The the D810 and the D5 each has a full-frame sensor. The second is more expensive. Their pixel counts are 7360 x 4912 and 5568 x 3712.

Why does a more expensive camera have fewer pixels?

By searching, I found some possible explanations, but they are not convincing.

  • Having fewer pixels means faster processing. I know that more pixels mean slower processing. But I guess processing the picture is very parallel. So with the added price, the manufactures can afford putting in more circuitry. Even if the processing power can't keep up with the increased pixel count, the camera can be programmed to reduce the pixel count for processing by summing 4 pixels into 1 so less processing power is needed. But it still has more pixels when not shooting in burst mode.

  • Having fewer pixels makes a higher ISO setting possible. With the same sensor size and more pixels on a sensor, the amount of light reaching each pixel will decrease. So the higher pixel count hinders high ISO setting. But a sensor with more pixels won't need as high ISO setting. Say sensor A and sensor B have the same size and resolution. (Let's use the word "resolution" here to mean the resolution of the amount of light that reaches each pixel for example 14 bits on the D810.) Sensor B has 4 times as many pixels. With the help of a microlens in front of each pixel, light lost due to not hitting any pixel is negligible. A takes a picture with the appropriate ISO setting. B takes the same picture with a quarter of that ISO setting. Then the 2 most significant bits of B will be 0. So B's actual resolution will be 2 bits less. But if we only need as many pixels as A has, we can sum 4 pixels of B into 1. (I'm a little stuck as I don't know whether it's only for cost that the highest ISO setting of the D810 is so much lower than that of the D5 scaled inversely proportional to the pixel count.)

  • Many pixels are for marketing. People tend to only choose a camera based on its pixel count. It may apply when comparing between smartphones and serious cameras. But the said cameras are all for serious people. So it's useless to give the EOS 5DS and D810 more pixels to attract silly people. More pixels allow a photographer to take a clear photograph when there is no time to zoom. If a photographer who uses the D810 may need 36 megapixels, a photographer who uses the D5 may also need them.

  • Having fewer pixels means less shot noise. The signal-to-noise ratio of shot noise is proportional to the square root of the expected amount of light to arrive each pixel which is inversely proportional to the size of each pixel. But the output from the pixels can be combined to get a picture with fewer pixels and less shot noise.

I've read Why would a more expensive camera have lower megapixels?, but I still don't know.

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Why does the Canon 1D X MK 2 only have 20.2MP seems to be asking something alike, but I still don't understand after having read its answers. I don't understand these points in Michael Clark's answer:

  1. All pixels are not equal Each of the said cameras have a full-frame sensor. Increasing the pixel count may make each pixel capture less light, but more pixels give the ability to combine nearby pixels. See the second and forth points above.

  2. Data rates See my first point.

  3. Power consumption A photographer can always set the camera to take pictures with fewer pixels. That is if the increased power consumption is significant. And there are shutter and focus whose power consumption doesn't change as pixels are added.

  4. End use of the images produced If a user of the D810 may need that pixel count, a user of the D5 may also need that pixel count.

  5. Pixels aren't the only distinguishing features of top end cameras But it doesn't explain why a more expensive camera has fewer pixels than another high-end camera.

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    Possible duplicate of Why does the Canon 1D X MK 2 only have 20.2MP – Philip Kendall Aug 30 '17 at 11:28
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    A tentative response to your edit at least for some points. 1) More pixel on the same area means less active area. The pixels does not really touch each others, every pixel has it's own small lens and color filter. So the smaller the pixels are, the larger the space between them become. 3) Reducing the resolution is most likely done at a later stage, the full sensor will be read in any case, using the same power. – pim Aug 30 '17 at 14:45
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    Turn your question around: Why do you expect price to be proportional to pixel count? The answer to that question will highlight your flawed assumptions. – Caleb Aug 30 '17 at 15:14
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    note also that combining pixel data will add noise/errors of its own, so it's not as if combining 4 pixels will get you the same quality result as 1 pixel 4x larger – Gnudiff Aug 30 '17 at 18:02
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    @eiqfb The main thing you are missing, both here and in reading the other question/answers, is that sometimes having larger pixels is more important and trumps having more pixels. – Michael C Aug 30 '17 at 22:23
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Apart from everything in Why does the Canon 1D X MK 2 only have 20.2MP

For your two examples specifically:

The EOS 5DS shoots 7 frames/sec, the EOS-1D X MARK II 14. That alone could be a good reason why the latter needs to use a lower resolution. Not everything can be parallelized, as you assume, eg. the data transfer from sensor to memory and from memory to storage. Larger frames means more data and larger files, and SD cards have a finite writing speed.

Binning pixels would reduce the resolution by a factor of 4, which is much more than the difference between the two cameras. Also, it adds processing overhead again...

In the end, it's not about pixel count. For certain applications, like sports photography, speed is, after a certain level, more important than resolution. A fine-art landscape photographer will probably value resolution more than speed, to the extent of paying ridiculous amounts for a medium format camera system that captures maybe 2 frames/sec.

  • Think you probably mean to refer to some version of the 5D in the beginning of your third paragaraph rather than referring to the 1D X mk II twice. – Caleb Aug 30 '17 at 15:10
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    @Caleb yes, thanks. i hate copy&paste errors when i see them in other people's texts :) – ths Aug 30 '17 at 15:13
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    @dgatwood Everything can't be easily parallelized. Conceptually, yes. On a bench, in the lab, or with desktop / server class computers, yes. But space, power, and manufacturing constraints limit easy parallelization. It comes at a huge cost to additional circuitry, which comes at a cost to power, etc. Yes, you could ostensibly have a CPU per JPEG macroblock, but then you have to solve the intercommunication pathways problem... the complexity begets higher complexity, ludicrously fast. – scottbb Aug 30 '17 at 19:59
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    A faster pipeline could probably be designed. The question is, at what cost, and would the resulting device still fit into a DSLR form factor? – ths Aug 30 '17 at 20:11
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    They've been using off-the-shelf ARM cores for years. The reason they stay one generation behind in terms of die size is the reliability factor. When the camera is designed the die process used is generally state of the art. By the time the camera has gone through intensive testing, both in-house, and then by their core user base, the technology has advanced. What the Sony fanboys don't seem to understand is how much that half-baked design released into the wild as 'cutting edge' costs the typical user of a 1D X II or D5. There's a reason many, many PJs who tried to switch to Sony didn't. – Michael C Aug 30 '17 at 22:53
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Reliability. Speed. Megapixels.

In a handheld, battery powered form factor you can only have two out of the three that are at the absolute state of the art. Cameras such as the 1D X Mark II and the Nikon D5 are for users who choose the first two.

Do you also expect the Formula 1 purpose built race cars made by Ferrari and Daimler-Benz to have leather interiors, heated seat cushions, and gold-plated instrument gauge bezels because they are the most expensive cars in the world? Or even tires that will last for more than a couple of hundred kilometers? I mean for goodness sakes, it only seats one medium sized or smaller person. What's up with that? For that price they ought to be able to carry an entire busload of people!


An addition to the question:

Why does the Canon 1D X MK 2 only have 20.2MP seems to be asking something alike, but I still don't understand after having read its answers. I don't understand these points in Michael Clark's answer:

  1. All pixels are not equal Each of the said cameras have a full-frame sensor. Increasing the pixel count may make each pixel capture less light, but more pixels give the ability to combine nearby pixels. See the second and forth points above.

  2. Data rates See my first point.

  3. Power consumption A photographer can always set the camera to take pictures with fewer pixels. That is if the increased power consumption is significant. And there are shutter and focus whose power consumption doesn't change as pixels are added.

  4. End use of the images produced If a user of the D810 may need that pixel count, a user of the D5 may also need that pixel count.

  5. Pixels aren't the only distinguishing features of top end cameras But it doesn't explain why a more expensive camera has fewer pixels than another high-end camera.

The main thing you are missing, both here and in reading the other question/answers, is that sometimes having larger pixels is more important and trumps having more pixels.

  1. You're missing the difference in dynamic range related to full well capacity which varies by pixel size when both sensors use the same generation of technology. Smaller pixel wells reach FWC with lower photon counts and thus have less DR than larger pixels. Binning multiple pixels together after the fact does nothing to change this.

  2. Due to the nature of Bayer masked sensors, binning four adjacent pixels into one would make all Bayer masked sensors monochromatic. It takes more processing power to combine pixels than to output at native resolution, because the most intense processing is the same demosaicing that must be accomplished before being output at native resolution OR before pixels can be binned! The processing required to bin the pixels is added to the total workload required to output at native resolution.

  3. See #2. Additional processing requires additional power.

  4. Another thing you are missing is that very few photographers who use a Canon 1D X Mark II or a Nikon D5 have only a Canon 1D X Mark II or a Nikon D5. Different tools for different jobs. Pros at that level often have multiple bodies with whatever features and performance they need for a particular job. They often use multiple bodies at the same time, especially when doing reportage and/or sports, although it is usually in order to have multiple lenses ready to go at an instant.

  5. Sure it does: Those other differences are deemed more desirable by the target buyer for the purpose for which that camera will be used. So are the larger, fewer pixels on the sensors of those cameras.

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Let's address your issues with existing answers :

All pixels are not equal Each of the said cameras have a full-frame sensor. Increasing the pixel count may make each pixel capture less light, but more pixels give the ability to combine nearby pixels. See the second and forth points above.

Combining adjacent pixels presents two issues :

Firstly the pixels are filtered using something like a Bayer array. So the adjacent pixel of the same color filter class is a little further away than directly adjacent. This makes errors more likely in combining them.

Combining pixels means adding noise. The mathematics is a bit hard to explain, but it's worse than using a big pixel.

Data rates See my first point.

Which is wrong.

Having fewer pixels means faster processing. I know that more pixels mean slower processing. But I guess processing the picture is very parallel.

No.

Normal affordable pro cameras use sensors that have each line of image read in turn.

This data is stored internally (buffered) in a limited memory (and memory costs money and uses power !) and processed by specially designed chips that convert them to the required form.

And all that resulting data has to be written to a card. Cards, even the fastest, are slow as molasses compared to the rates data can be produced by a computer. This, incidentally, is why CPUs have complex multi-tiered memory caching. But that strategy doesn't work for image processing like a camera has to do.

Processing in parallel also generates enormous heat.

Take for example a modern GPU. Many image processing applications on PCs use these to do complex image processing tasks quickly as they can exploit parallel processing.

And the GPUs use huge amounts of power and generate huge amounts of heat.

You can't do that in a normal camera. You'd need a mains supply and the cooling system would add a lot of mass and weight (and it's own power demand).

So engineering trade-offs require than you can't do everything in parallel.

There's no free lunch.

Power consumption A photographer can always set the camera to take pictures with fewer pixels. That is if the increased power consumption is significant. And there are shutter and focus whose power consumption doesn't change as pixels are added.

The senor can't change the number of pixels it has.

When you choose to get a JPEG from a camera that has a particular size, that size is a scaled image constructed from the full native sensor size data.

So we still have to read, process and store all that data, regardless of the size of image you ask for.

As we have to read all that data and process it that's most of the power budget for taking a shot. So we don't save much power when shooting at a lower resolution.

End use of the images produced If a user of the D810 may need that pixel count, a user of the D5 may also need that pixel count.

No.

If I am e.g. a fine landscape shooter I may want as many (good) pixels as I can get.

If I am a fine portrait shooter, I almost certainly do not need all those pixels because no-one needs (or wants) a photo that shows details like skin pores and nose hair in excruciating detail.

And if I'm a sports shooter for a magazine or paper, I don't need more than a few megapixels typically. What I really need is blinding speed, high rates of fire and focus and reliability. If sacrificing some pixels is the prices, it's a trivial one as I didn't need all those pixels in the first place. To put this in perspective, note that you can print an full color high quality 300 dpi A4 image from about 8 megapixels ! That's a magazine full page.

Pixels aren't the only distinguishing features of top end cameras But it doesn't explain why a more expensive camera has fewer pixels than another high-end camera.

For a professional photographer the other things are more important.

If I can gain a whole stop by using a lower resolution that could, for some photographers, be way, way more valuable than resolution.

If I can gain an extra 1 frame per second by dropping to a lower resolution camera, that could be the difference between a sports pro getting the shot that pays this month's mortgage and not getting it.

Pro photography is about getting the shot, not pixels.

  • The days of paying the mortgage with one sports shot, no matter how good, are long past. Things that used to pay $500/image are now lucky to bring $5/image. Nowadays you not only have to get the shot, but you have to push it to the wire services before the guy kneeling next to you does - which is likely before the next play starts on the field/court in front of you. – Michael C Aug 30 '17 at 22:33
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The 1D and the D5 are built for specific purposes. They have the best auto focus system on the market. They are the most rugged being better weather sealed, better shock resistance and double the expected shutter count. It has the processing power to do the fast auto focusing and 14 frames per second RAW with a huge buffer. For people buying these cameras the extra resolution is just not necessary and would slow the camera down. If these features are important to you then no other camera will do. Most will prefer the higher image quality and are willing to sacrifice the speed. Also the durability of the D810 is usually sufficient for most. The 1D and D5 generally appeals to the professional sports and news photographers, wildlife and National Geographic type photographers. If you don’t need the ultimate speed, durability and auto focus then choose the 5DS or D810. Its all about priorities.

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