Why "crop factor" is so pervasive

[Michael Gerald-Yamasaki]

A larger sensor is always better than a smaller sensor for general photography because it collects more light.

Arash,

Couldn't one use the same argument for pixels? A larger pixel is always better than a smaller pixel for general photography because it collects more light?

Hard to make apples to apples comparison. A large sensor captures more light but also from increasingly worse parts of the lens (the edges worsening the more you include)... while with large pixels you have fewer edge effects (of the CFA) than a collection of smaller pixels (per area), I think. Unless the CFA performance improves with a smaller size (which would surprise me).

John, I'm a Rosling fan... you've seen Tufte's books?

Cheers,

-Michael-

[arash_hazeghi]

Arash,

Couldn't one use the same argument for pixels? A larger pixel is always better than a smaller pixel for general photography because it collects more light?

Hard to make apples to apples comparison. A large sensor captures more light but also from increasingly worse parts of the lens (the edges worsening the more you include)... while with large pixels you have fewer edge effects (of the CFA) than a collection of smaller pixels (per area), I think. Unless the CFA performance improves with a smaller size (which would surprise me).

John, I'm a Rosling fan... you've seen Tufte's books?

Cheers,

-Michael-

yes Mike larger pixels have higher SNR but if the pixels are scaled perfectly according to Moore's law (this addresses the issues you mention) ,like Nikon D800 you can combine the smaller ones to recover the SNR of the larger pixel. So you can get identical high ISO performance when you need it and at the same time get excellent resolution in low ISO. If you look at the NEF files from D800 and D4, the D800 is noisier at pixel level but when you down-sample to 16 mpixel at least the visual noise is comparable up to very high ISOs with more detail in D800 files. And needless to say D800 is much better than D700 despite having smaller pixels. However, this is not always true e.g. Canon 7D which suffers from poor FPN, bad CFA spectral response etc. which was failure of pixel scaling at its time. So it really depends.

BTW, BTW Nikon D800 just achieved the highest DxO sensor mark ever given to any digital camera (including medium format). the score is 95 while D4 scored 89. This shows the power of Moore's law scaling :D
http://www.dxomark.com/index.php/Pub...or-performance

[Roger Clark]

A larger pixel is always better than a smaller pixel for general photography because it collects more light?

I mentioned this myth in my post above: "but up and coming is the very misunderstood big pixels are less noisy idea."

A larger pixel enables the collection of more light, not that they collect more light. Consider this analogy: You have two buckets, one that holds 2 gallons of water and one that holds 1 gallon of water. You put the 2-gallon bucket under the faucet and turn on the water for 1 second. Now you put the 1 gallon bucket under the faucet and turn on the water at the same intensity for one second. Assume the amount of water was not enough to overfill either bucket. Which bucket has more water? (If you answer I hate story problems you fail the class.) If your answer is both buckets have the same amount of water, you are correct. Now what controls how much water is in the bucket? It is not the size of the bucket; it is the force and duration of the water controlled by the fawcet.

In digital photography, the bucket is the pixel, the faucet is the lens and the time the faucet is on is the exposure time. There is one thing missing in the analogy, and that is focal length which spreads out the light so if the faucet has a spray nozzle on the end the spray would expand a further distance from the faucet. Now for the larger bucket, if it has a larger diameter, it would collect more water because it sees a larger area. But if the smaller bucket were moved closer to the sprayer, so it collected the same angular area, it would also collect the same amount of water. People talk about the same sensor field of view, but there is also the same pixel field of view. When the pixel field of view is the same, regardless of pixel size, the two pixels collect the same amount of light in the same amount of time and produce the same signal-to-noise ratio.

So in the case of digital cameras, the amount of light collected is controlled by the lens, its focal length and the exposure time. The larger pixels only ENABLE the collection of more light when the exposure time is long enough. With digital cameras, that only happens at the lowest ISO. At higher ISO, the buckets (pixels) never get filled.

So to manage noise in digital camera images, one must manage the lens aperture, the focal length, and the exposure time. The focal length manages the pixel field of view. So it is not the pixel that controls the observed noise in an image.

Roger

[Hazel Grant]

I mentioned this myth in my post above: "but up and coming is the very misunderstood big pixels are less noisy idea."

A larger pixel enables the collection of more light, not that they collect more light. Consider this analogy: You have two buckets, one that holds 2 gallons of water and one that holds 1 gallon of water. You put the 2-gallon bucket under the faucet and turn on the water for 1 second. Now you put the 1 gallon bucket under the faucet and turn on the water at the same intensity for one second. Assume the amount of water was not enough to overfill either bucket. Which bucket has more water? (If you answer I hate story problems you fail the class.) If your answer is both buckets have the same amount of water, you are correct. Now what controls how much water is in the bucket? It is not the size of the bucket; it is the force and duration of the water controlled by the fawcet.

In digital photography, the bucket is the pixel, the faucet is the lens and the time the faucet is on is the exposure time. There is one thing missing in the analogy, and that is focal length which spreads out the light so if the faucet has a spray nozzle on the end the spray would expand a further distance from the faucet. Now for the larger bucket, if it has a larger diameter, it would collect more water because it sees a larger area. But if the smaller bucket were moved closer to the sprayer, so it collected the same angular area, it would also collect the same amount of water. People talk about the same sensor field of view, but there is also the same pixel field of view. When the pixel field of view is the same, regardless of pixel size, the two pixels collect the same amount of light in the same amount of time and produce the same signal-to-noise ratio.

So in the case of digital cameras, the amount of light collected is controlled by the lens, its focal length and the exposure time. The larger pixels only ENABLE the collection of more light when the exposure time is long enough. With digital cameras, that only happens at the lowest ISO. At higher ISO, the buckets (pixels) never get filled.

So to manage noise in digital camera images, one must manage the lens aperture, the focal length, and the exposure time. The focal length manages the pixel field of view. So it is not the pixel that controls the observed noise in an image.

Roger

Thanks. That "visual" helps me understand so much more.

[John Chardine]

I mentioned this myth in my post above: "but up and coming is the very misunderstood big pixels are less noisy idea."

A larger pixel enables the collection of more light, not that they collect more light. Consider this analogy: You have two buckets, one that holds 2 gallons of water and one that holds 1 gallon of water. You put the 2-gallon bucket under the faucet and turn on the water for 1 second. Now you put the 1 gallon bucket under the faucet and turn on the water at the same intensity for one second. Assume the amount of water was not enough to overfill either bucket. Which bucket has more water? (If you answer I hate story problems you fail the class.) If your answer is both buckets have the same amount of water, you are correct. Now what controls how much water is in the bucket? It is not the size of the bucket; it is the force and duration of the water controlled by the fawcet.

In digital photography, the bucket is the pixel, the faucet is the lens and the time the faucet is on is the exposure time. There is one thing missing in the analogy, and that is focal length which spreads out the light so if the faucet has a spray nozzle on the end the spray would expand a further distance from the faucet. Now for the larger bucket, if it has a larger diameter, it would collect more water because it sees a larger area. But if the smaller bucket were moved closer to the sprayer, so it collected the same angular area, it would also collect the same amount of water. People talk about the same sensor field of view, but there is also the same pixel field of view. When the pixel field of view is the same, regardless of pixel size, the two pixels collect the same amount of light in the same amount of time and produce the same signal-to-noise ratio.

So in the case of digital cameras, the amount of light collected is controlled by the lens, its focal length and the exposure time. The larger pixels only ENABLE the collection of more light when the exposure time is long enough. With digital cameras, that only happens at the lowest ISO. At higher ISO, the buckets (pixels) never get filled.

So to manage noise in digital camera images, one must manage the lens aperture, the focal length, and the exposure time. The focal length manages the pixel field of view. So it is not the pixel that controls the observed noise in an image.

Roger

I've been ruminating a bit on this one Roger! What caught my eye on initial reading was the analogy of a faucet and a bucket, but particularly the faucet. The water coming out of your faucet is light = photons, and indeed if light poured onto a pixel bucket like a faucet, and the stream coming out of the faucet were a lot narrower than the mouth of the bucket, then I agree that buckets of different size would collect the same amount of light. Even buckets that had different sized openings at the top would collect the same amount of light. However, intuitively (remember, I'm biologist!) I don't view light this way- to me it's more like a heavy, drenching, minutely fine mist (of photons) falling on the pixels at the speed of light. If light is more like this than tiny streams from faucets, then the width of the pixel opening would hugely affect how much light the pixel would collect with the amount of light collected being proportional to the pixel area (pixel pitch squared). An 64 square micron pixel should collect twice as much light as a 32 square micron pixel, in the same amount of time. Therefore I would conclude that big pixels collect more light than small pixels per unit time, therefore have a higher signal to noise ratio, and therefore appear less noisy.

What is wrong with this logic? Can my intuitive view of light be so wrong???

[Roger Clark]

I've been ruminating a bit on this one Roger! What caught my eye on initial reading was the analogy of a faucet and a bucket, but particularly the faucet. The water coming out of your faucet is light = photons, and indeed if light poured onto a pixel bucket like a faucet, and the stream coming out of the faucet were a lot narrower than the mouth of the bucket, then I agree that buckets of different size would collect the same amount of light. Even buckets that had different sized openings at the top would collect the same amount of light. However, intuitively (remember, I'm biologist!) I don't view light this way- to me it's more like a heavy, drenching, minutely fine mist (of photons) falling on the pixels at the speed of light. If light is more like this than tiny streams from faucets, then the width of the pixel opening would hugely affect how much light the pixel would collect with the amount of light collected being proportional to the pixel area (pixel pitch squared). An 64 square micron pixel should collect twice as much light as a 32 square micron pixel, in the same amount of time. Therefore I would conclude that big pixels collect more light than small pixels per unit time, therefore have a higher signal to noise ratio, and therefore appear less noisy.

What is wrong with this logic? Can my intuitive view of light be so wrong???

Hi John,

While part of the explanation is correct, you changed a critical parameter in the middle. When you changed the area of the pixel, you reduced the resolution (e.g. pixels on subject). While the mist idea is a first approximation, light in a camera system is not like mist falling uniformly all over. It is more a focused beam, so more like a spray nozzle spraying water and that is why I used the spray nozzle idea in my explanation.

So what you say is correct, that the larger area pixel collects more light, and one can certainly look at the problem that way (and most photographers do just that). But it is a situation like crop sensors giving the impression of greater telephoto reach. The crop is not the reason for more telephoto reach, it is pixel pitch. Similarly, it is not pixel size that determines the amount of light gathered, it is the lens diameter and the angular area of the pixel that enables the pixel to collect the light. So if you equalize the angular area of the pixel, the amount of light collect is the same for a given exposure, regardless of pixel size. In your example of the 32 and 64 square micron pixels, put a 2x TC on the camera with the 64 sq micron pixels using the same lens (lets say a 300 f/2.8 lens) then the 64 sq micron pixel system puts the same pixels on subject as the 32 sq micron pixel system and then both cameras get the same amount of light in a given exposure time. The larger pixels didn't magically get more light.

In summary, the control of how much light we get in the camera is: 1) lens diameter, 2) angular area seen by the pixel, and 3) exposure time. (I'm assuming all similar focal length lenses would be pretty close in transmission--which they are.) So like crop factor, pixel area is actually not part of the equation for the resolution on subject and the amount of light a pixel receives. The angular area of a pixel is proportional to the ratio of the pixel size and lens focal length, not pixel size alone. I've written this up in more detail at:
http://www.clarkvision.com/articles/...m.performance/
but I plan to add a lot more to the page with examples of how this works.

But once one realizes this paradigm, we see that adding a TC is no different than decreasing pixel size by a corresponding factor (when working at higher ISO so the pixels don't overflow). This then leads to no high ISO/low light advantage to large pixels.

Did you every decide on the 300 f/2.8 versus 500 f/4? Given the choice today, I would choose a 300 f/2.8 and body with 5 micron pixels. Gee, that's a D800.

Roger