Small sensors gather less light? Wrong!

[Nordstjernen]

The brightness (f-number) of a lens tells about the light level over the whole image circle, no matter if the image circle is is covering a narrow or wide area.

No, this is incorrent.

The f number (or, to be pedantic, the t number) tells the intensity of the light, i.e. the brightness of the image per area unit, e.g. square centimeter.

Fact: an f/2.8 lens on a P&S will create an image that is equally bright as an image on a FF sensor. However, because of its size, the larger sensor collects more total light. This is exactly the reason of the FF advantage: the sensor, when used with larger optics, quite simply has more photons to work with.

Kind regards,

• Henrik

This is what I tried to express - the brightness of the image per area unit is given by the f-number. Therefore a large image circle delivers more light than a small image circle, simply because of the wider area.

Thanks a lot for your help!

 

[tko]

Now you are starting to understand equivalency theory.

For the same F-stop, a smaller sensor results in a smaller lens. Because the lens is smaller, it gathers less light. You can get more light by going w/a smaller F-stop, by some weird coincidence that smaller F-stop also gives the same DOF. That's equivalency!

So, if you keep the lens the same size, and make the sensor smaller, you will gather the exact same amount of light, and have the same DOF, but have a different FL and F-stop It's the lens, duh, not the sensor.

However, everyone is so hung up on F-stop and focal length they miss this very simple fact. Do you want to compare systems on the rather useless F-stop and FL numbers, or by actually performance? It's photography's years of dependency on F-stop and FL, which was fine w/one sensor size, that obscures a lot of basic, simple stated truths.

Now, to play devil's advocate, what would be the point of a small sensor with a full size lens? Not much, right? Or a FF sensor w/a tiny, slow lens? Each system needs to work together to be optimum, that leads to certain characteristics (not hard written truths, just generalities.)

 

[Matiasgh]

First: light can not be "Gathered", only concentrated
Sec: Sensors do not Gather light, only measure it

Third: Sensors are a concentration of photo-sensitive pixels, so its not the "sensor" that measures, its the pixel.

Fourth: Light works in volume, which is dependant on area and there-for we can deduce that "size" will matter, but only if we use the terms and theory correctly according to physics.

So...

To understand how a sensor gathers light you have to simplify your thoughts and remember that a sensor is a layer of pixels. The pixels are the ones that "gather" (I prefer to say "measure") the light, a sensor is nothing less than the term we use to describe the cluster of pixels that we understand as sensor (which is wrong in terms, for the "pixel" is actually the sensor, but I will use terms as we know and understand them)

Now, you must also understand that each sensor has on top of it, micro lenses that concentrate the light into the pixel. They are not continuously right next to each other... there is a gap... and micro lenses "remove" this gap. So this is an important factor in the argument, but I will not stretch into this... I find it quite irrelevant.

In the case of the D40, having a APS with 6MP, brings cleaner ISO 1600 than any other I've seen because the micro lenses are larger and there-for concentrate more light into each of the pixels, this allows a better reading and processing. But does not mean that it received more light, its simply more concentrated in less pixels... but the same at last.

What is important is to leave it clear is that the lens limits the amount of light that flows into it (you are right there), but at the end, the amount of light that the sensor receives, CONSIDERING THE SURFACE AREA, will be the same. That is how ISO works. ISO 100 in a 1/2.3 sensor, or a full frame Sony 900 measures the same amount of light. The quality of this measuring device is what brings different results.

More pixels, bring more measurements, more data and so, more information on a bigger image. The amount of light is NOT dependant to the size of the sensor.

Photography is all about physics... and there are rules to it.

 

[goetz48]

• nt -

 

[Anastigmat]

Actually, it is not the size of the sensor, but a lot more factors, such as the size of the photosite (the light sensitive portion of the pixel) and the opacity of the color filters that also factor in how light sensitive a sensor is. The microlens also plays a role. If someone comes up with a more light sensitive material than silicon, then that would have an effect. For now though, the larger sensors allow a manufacturer to increase the size of the pixel and therefore the light sensitivity of the sensor.

 

[DUSTY LENS]

I Think the OP and a few others are confused about the fact that a shorter focal length lens will concentrate any portion of the image to a smaller area on the sensor than a longer focal length will . ( The longer focal length will always spread the same portion of the image over a larger area , and the OP thinks this is causing a loss of light concentration at that point ) .

This is interesting , but still is not valid in the argument of large Sensor versus small sensor .

The fact which destroys his argument is the fact that a longer focal length lens with an aperture set to f 2 , will deliver the same light intensity of light as a shorter focal length lens with it's aperture set at f 2 , ( the same aperture ) .

It would only be the case which he believes it is , if the aperture is not diminished by the same ratio as the focal length is diminished . ( That is if the aperture were kept at the same physical size while the focal length only is shortened ) .

In this case the shortened lens would in fact have a larger aperture gathering more light and this greater amount of light would in fact be concentrated on a smaller area , therefore producing a greater light intensity at any point on the sensor .

Lens aperture is a ratio of the front element diameter divided into the lens focal length . This is why the f # is a measure of light gathering power , and therefor is useful in determining exposure .

The sensor area has no effect on light intensity at any photo site when the size of the photo sites are equal in both sensors .

Light is very well understood and so are lenses . We do not need to be re-educated to the new math on any of this . We also do not need to understand the new string theory to understand any of this .

No matter how many additional pixels are present on large sensors compared to small sensors those additional pixels will not add useful information which was missing in the pixels present on the small sensor .

Multiple re-samplings of the same image file , however many times will never improve the IQ when there was no error in the first sampling .

That is like saying that if I count to ten , 1000 times , sometimes the ten will be more accurate than it was the first time .

Dusty Lens

 

[jerome_munich]

Someone could quite easily design a p&s camera witha 1/1.7" sensor that performed just as well as a A900 in terms of light gathering.

Leaving alone the problems of dead zones on the small sensors (or using a back illuminated 1/1.7" sensor...), the A900 sensor has 20 times the area of the 1/1.7" sensor, so it will gather roughly 20 times the light.

f/2.0 is the practical limit of P&S zoom lenses at the wide end (f/1.8 for the Samsung EX1). f/4.0 is half the light, f/5.6 a fourth, f/8.0 8 times less, f/16.0 16 times less. Your real-world P&S is as good as a A900 with a f/16 lens.

Supposing you could convince a manufacturer to produce a P&S with a f/1.0 lens, you still are only as good as an A900 with a f/5.6 lens. f/1.4 lenses are readily available for that camera.

f/0.7 is the fastest ever done. I think that Snell laws limit the maximal lens aperture (in air) around f/0.6

 

[Chippy99]

Now you are starting to understand equivalency theory.

For the same F-stop, a smaller sensor results in a smaller lens. Because the lens is smaller, it gathers less light. You can get more light by going w/a smaller F-stop, by some weird coincidence that smaller F-stop also gives the same DOF. That's equivalency!

So, if you keep the lens the same size, and make the sensor smaller, you will gather the exact same amount of light, and have the same DOF, but have a different FL and F-stop It's the lens, duh, not the sensor.

However, everyone is so hung up on F-stop and focal length they miss this very simple fact. Do you want to compare systems on the rather useless F-stop and FL numbers, or by actually performance? It's photography's years of dependency on F-stop and FL, which was fine w/one sensor size, that obscures a lot of basic, simple stated truths.

Now, to play devil's advocate, what would be the point of a small sensor with a full size lens? Not much, right? Or a FF sensor w/a tiny, slow lens? Each system needs to work together to be optimum, that leads to certain characteristics (not hard written truths, just generalities.)

I like this post ^

Sums things up very well for me.

However, what my thread does point out though, is that we are too hung up on sensor size per se, and especially so in the p&s sector where the lens geometry can be flexed more so than in a tightly specified system such as a DSLR fitting.

There's really little or no intrinsic advantage to a 1/1.7" sensor compared to a 1/2.3" one - it's down to the glass. Yes it's probably true that the photocells make up a larger percentage of the sensor surface on a larger sensor, but this effect is probably small, especially with the use of micro lenses and back-lit sensors.

 

[Chippy99]

Actually, it is not the size of the sensor, but a lot more factors, such as the size of the photosite (the light sensitive portion of the pixel) and the opacity of the color filters that also factor in how light sensitive a sensor is. ... For now though, the larger sensors allow a manufacturer to increase the size of the pixel and therefore the light sensitivity of the sensor.

What you say sounds obvious, but when I think about it, is it actually correct? Maybe, but I am not sure.

What I mean is, I don't doubt that larger pixels are more sensitive, but is that because they gather more light (compared to a smaller pixel under the same intensity of light). If a smaller pixel is fed the same number of photons, does it produce a weaker signal? I'm not so sure.

 

[Chippy99]

If what you're saying is true, a 12mpixel compact would have the same sensor performance as a 12Mpixel Full frame camera, which is simply absurd.

I don't see how that follows.

The 12mpixel compact sensor is receiving less light so obviously it doesn't perform as well.

The question is, how would it perform if it received the same amount of light.

It would still perform poorer due to individual photocells performing poorer due to physical constraints.

How so?

 

[goetz48]

closing the aperture by 1 step means reduction of light by 1/2. So f=2.8 means halfthe light of f=2.0.

 

[Chippy99]

With the greatest respect, I really don't think you have grasped what this thread is all about.

If you read and digest some of it, you will realise that what I am was suggesting is essentially correct. That the sensor size per se is not the major determining factor in the systems' noise performance. It is the glass that goes with the sensor.

Beyond this basic premise, we get into more detailed dialogue about inter-cell wiring and photosite area as a percentage of sensor size and pixel sensitivity and micro lenses and all sorts of stuff. But there are distractions (valid distractions, granted) from the central premise.

I am not suggesting I have solved the mystery of dark matter. I am sure all camera system designers understand the principles far better than I. I merely point out that the perceived wisdom of "large sensor = good, small sensor = bad" is not a simple as people might imagine.

That is all.

 

[harold1968]

small sensors capture less light as:
1. they are smaller

small sensors produce more noise as:
1. their photosites are smaller

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

Its physics. you can't fight it.

sensor electronics gets better over time and small sensors have less noise (or better software algorithms). but large sensors also get better

rgds

 

[Frenske]

True if all things are equal e.g. using the same lens!

small sensors capture less light as:
1. they are smaller

small sensors produce more noise as:
1. their photosites are smaller

 

[TrojMacReady]

small sensors produce more noise as:
1. their photosites are smaller

This isn't true.
Example:

http://forums.dpreview.com/forums/read.asp?forum=1018&message=28607494&q=sheehy+fz50+canon&qf=m

Some quotations from Eric Fossum (inventor of CMOS sensors) and other scientists on the matter:

http://forums.dpreview.com/forums/read.asp?forum=1018&message=31741682&q=eric+fossum+noise&qf=m

DXO on the issue:

http://www.dxomark.com/index.php/eng/Insights/More-pixels-offsets-noise ! (add the exclamation mark at the end manually).

 

[newstemp]

All other things being equal, and for the same lens area, you are asking for an approximate halving of lens focal length for a halving of sensor width (or quartering of sensor area), and lenses are already heavily optimized to minimize non-ideal effects. Since P&S lenses do not have to perform at DSLR levels it can be expected that the focal length/lens area ratio does not have to be as large, however there will be limits to how far this can be taken.

For the same class & performance level of camera smaller sensors require smaller lenses and are then supplied with less light.

Other issues related to shortening focal length include microlens performance. This is dependent upon spread of angles of light entry, and microlenses are also strongly optimized. For the same microlens technology shorter focal lengths produce wider microlens entry angles which means more lost and spilled (blurred) light.

Reduce the level of performance of the camera (sharpness, chromatic aberration etc.) and you can reduce the sensor area (saving some fraction of the sensor's contribution to the camera bill of materials) while maintaining light entry to the lens (though not necessarily to the sensor), and sell the camera for a lower price. This equation however will of course be very precisely tuned by each manufacturer.

 

[Chippy99]

small sensors capture less light as:
1. they are smaller

small sensors produce more noise as:
1. their photosites are smaller

Both the above comments are flawed. I won't get drawn into repeating why so, over and over again.

 

[Chippy99]

All other things being equal, and for the same lens area, you are asking for an approximate halving of lens focal length for a halving of sensor width (or quartering of sensor area), and lenses are already heavily optimized to minimize non-ideal effects. Since P&S lenses do not have to perform at DSLR levels it can be expected that the focal length/lens area ratio does not have to be as large, however there will be limits to how far this can be taken.

For the same class & performance level of camera smaller sensors require smaller lenses and are then supplied with less light.

Other issues related to shortening focal length include microlens performance. This is dependent upon spread of angles of light entry, and microlenses are also strongly optimized. For the same microlens technology shorter focal lengths produce wider microlens entry angles which means more lost and spilled (blurred) light.

Reduce the level of performance of the camera (sharpness, chromatic aberration etc.) and you can reduce the sensor area (saving some fraction of the sensor's contribution to the camera bill of materials) while maintaining light entry to the lens (though not necessarily to the sensor), and sell the camera for a lower price. This equation however will of course be very precisely tuned by each manufacturer.

Sensible post. I agree with all of that.

 

[J R R S]

To reitterate your point - Why the hell dont camera manufactures stop giving us stingy amounts of glass on good point and shoots? -- i.e. give us a bit more glass and f0.95 zoom lenses (24mm - 100mm) I would quite happly lug around the extra glass on the front of my TZ7

 

[Stepan Kudryashov]

I would like to put my 2 c.
I generally agree with author of this topic.
I Just want to summarize.

1. Quantity of light depends only on physical size of opening at front of front glass (physical aperture)

2. F stop is kind of misleading for most people. F-stop is geometrical measure and defines intensity of light in current application but not a quantity of light.

3. For the same physical size of aperture you can project light to any size (size of sensor) and this projection will get the same amount of light, just with different intensity.

Also I found missing point in this conversation. Initial post did not say, but implicated that sensor size doesn’t matter in final quality of digital camera output. In abstract world yes, sensor size doesn’t matter. In physical reality it is crucial. The issue is not with optical geometry of lenses, optical degradation, spacing in sensor, the matter of physic of sensor itself. Sensor consists from few million open transistors. Each transistor can absorb only limited number of photons, after it become saturated and doesn’t measure number of photons anymore (it translated usually in clipped highlights). This defines maximum acceptable intensity of light for photo sell (and for sensor itself). This means that small sensor couldn’t receive and translate the same amount of light as a big sensor by definition. All of this means that big sensor is better.

--
Stepan
http://www.flickr.com/photos/stepank/