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What a larger sensor is NOT about...
- Thread starter Olivier GALLEN
- Start date
Trials Photographer
Leading Member
Yes and no. Here's the deal. The signal on the photosite is correllated and adds linearly. Noise is uncorrelated and adds as a power.
Double the size of the sensor in two dimensions 2.5um X 2.5um to 5um X 5um and you quadruple the area. With a correlated signal you also quadruple the available charge to be read.
With noise the formula for adding power is the square root of the sum of the squares (rms) in other words every time you multiply a unit of size you take the noise in a unit, square it, multiply it by the number of units and then take the square root.
The table below shows what happens with increasing photosite size. It assumes an equal charge for signal and noise so it is easier to fathom. It's easy to see the growth of signal is much faster then the growth of noise.
There are other factors to consider such as the noise of the readout circuitry and A/D conversion but this is the prime reason larger sensors are better at low light. It isn't marketing hype it's physics.
Linear dimension
of square pixel
area signal noise
1 1 0.0000001 0.0000001
2 4 0.0000004 0.0000002
4 16 0.0000016 0.0000004
8 64 0.0000064 0.0000008
16 256 0.0000256 0.0000016
32 1024 0.0001024 0.0000032
64 4096 0.0004096 0.0000064
Dan
Double the size of the sensor in two dimensions 2.5um X 2.5um to 5um X 5um and you quadruple the area. With a correlated signal you also quadruple the available charge to be read.
With noise the formula for adding power is the square root of the sum of the squares (rms) in other words every time you multiply a unit of size you take the noise in a unit, square it, multiply it by the number of units and then take the square root.
The table below shows what happens with increasing photosite size. It assumes an equal charge for signal and noise so it is easier to fathom. It's easy to see the growth of signal is much faster then the growth of noise.
There are other factors to consider such as the noise of the readout circuitry and A/D conversion but this is the prime reason larger sensors are better at low light. It isn't marketing hype it's physics.
Linear dimension
of square pixel
area signal noise
1 1 0.0000001 0.0000001
2 4 0.0000004 0.0000002
4 16 0.0000016 0.0000004
8 64 0.0000064 0.0000008
16 256 0.0000256 0.0000016
32 1024 0.0001024 0.0000032
64 4096 0.0004096 0.0000064
Dan
Trials Photographer
Leading Member
OK so that went horribly wrong. How about this instead.
Dan
Dan
Trials Photographer
Leading Member
Ah but now you bump into the dynamic range at the top limiting factor since a small pixel can only store so much charge before it saturates. There is no substitue for a big bucket.
Dan
Dan
I think your analysis is pretty good, except that I disagree with your analysis on aperture. You seem to be coming from a perspective of higher aperture being better. If your goal is to isolate the subject from the background, the lower the aperture value the better, and from that perspective, the R1 is far better situated. If you ask me, the most important feature of large sensors is the ability to have a system where you can isolate the subject in this way. You simply cannot do it with a camera with a small sensor. And that's simply because you're using a lens with a physical aperture that is much smaller.
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lingsb
Forum Enthusiast
I think that's what was claimed by the original poster (Big sensor == better dynamic range). It won't help you in 'high ISO' situations though...OK so that went horribly wrong. How about this instead.
Roland Karlsson
Forum Pro
Oh - yes it is
Oh - yes it is
That also
In a sense correct. It is the diameter of the aperture that determines the amount of ligth that you can gather. But - an 120 mm lens (at an APS sensor) have a larger diameter than a 21 mm lens (at a smallish sensor) - and therefore it lets in more light - and therefore you get a better signal to noise ratio - i.e. less noiy pictures.
So - indirectly will the larger sensor be less noisy.
Roland
Alan Martin256506
Forum Enthusiast
Actually, I agree completely that larger maximum apertures (i.e., lower equivalent F-numbers) are better. Shallow depth of field is often desirable, and when it isn't, there's always the option of stopping down. I've certainly seen plenty of examples in my own photos where shallow DOF is better - or would have been, had I used a faster lens.
Maybe my post wasn't clear about this, but I meant to say that the R1 lens is faster than the F828 lens, and thus the R1 lens is better specification-wise except in telephoto reach. This is the opposite to what some people were saying about the published specs, since they were just comparing F-numbers (f/2.8-4.8 vs. f/2.0-2.8) without taking sensor size into account.
Phil Askey said exactly this (that the R1 has better DOF control) here: http://forums.dpreview.com/forums/read.asp?forum=1000&message=14950457 ; But he didn't provide a way to make such comparisons in general, which is what I was trying to do.
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Alan Martin
Trials Photographer
Leading Member
Of course it does. What is referred to as "high ISO" is cranking up the gain of the preamp that feeds the A/D converter. Changing the ISO has no effect on the sensor whatsoever. When the gain is cranked up the noise is gained up by the same amount as the signal. That is why the signal to noise ratio is critical. The charge coming off the sensor is only dependant on light hitting the sensor. Fiddling with the knobs only modifies the circuitry after.
Dynamic range is just that, a range bounded by noise on the bottom and saturation of the photosite at the top. A bigger photosite has less noise relative to signal and that's the key. Not less absolute noise. With a bigger signal less gain is needed so it approximates a lower ISO. That's why noise is more problematic in the darker areas of a photo. The sensor noise is fixed so the lower signal means the signal to noise ratio in those parts of the photo is lower. The noise is there in the brighter areas but it is masked by the higher signal level.
There are a lot of smart people out there trying to balance sensor size with defect density with wafer cost and the cost of support circuitry and lens design.
I find these forums kind of amusing sometimes. The engineers who work on this stuff spend months/years trying to squeeze the tiniest performance gains out of a circuit. Rest assured what you get now is the best that the technology has to offer.
Now if you are willing to pay $??.??? for a high resolution camera based on a back thinned, LN2 cooled CCD you will get a whole new level of low noise. Kind of a B* to carry though. ;^)
Dan
Dynamic range is just that, a range bounded by noise on the bottom and saturation of the photosite at the top. A bigger photosite has less noise relative to signal and that's the key. Not less absolute noise. With a bigger signal less gain is needed so it approximates a lower ISO. That's why noise is more problematic in the darker areas of a photo. The sensor noise is fixed so the lower signal means the signal to noise ratio in those parts of the photo is lower. The noise is there in the brighter areas but it is masked by the higher signal level.
There are a lot of smart people out there trying to balance sensor size with defect density with wafer cost and the cost of support circuitry and lens design.
I find these forums kind of amusing sometimes. The engineers who work on this stuff spend months/years trying to squeeze the tiniest performance gains out of a circuit. Rest assured what you get now is the best that the technology has to offer.
Now if you are willing to pay $??.??? for a high resolution camera based on a back thinned, LN2 cooled CCD you will get a whole new level of low noise. Kind of a B* to carry though. ;^)
Dan
Shaun Williamson
Senior Member
One of the problems with this discussion is that all sorts of confusions
creep in. So we have people wanting to redefine aperture and ISO etc.
In terms of their redefinition they then want to claim that the Sony 828
is a superior camera to the R1. This is just silly since everyone knows that
the R1 can take better pictures with lower noise than the 828.
In photography aperture is a ratio the ratio between the diameter of the
lens opening to the lens focal length. ISO is defined relative to exposure
value i.e. shutter speed x aperture. These ratios are useful and no useful
purpose could be served by redefining them. If my exposure meter says
that this scene requires F2.8 at 1/60th at ISO 100 then no matter which
camera I am using I can set the correct exposure. That is the real use of
these ratios and that would be destroyed by redefining them. With regard
to exposure F2.8 means the same on any camera and so does ISO 100 and
so they should.
It has also been claimed that the larger absolute diameter of the lens
opening at an aperture of F2.8 on a large camera allows it to direct more
light onto the sensor. This is true of course but that light has to be
directed over a larger sensor area than it does in a compact digicam.
So just saying that the larger lens collects more light does not in itself
explain anything about the better noise performance of an APS v a
compact sensor. What is true is that the brightness of the image on the
APS sensor is probably just the same as the brightness of the image on
the small sensor. After all the small lens may not collect as much light as
the large one but the light it collects is concentrated on a smaller area.
To see how much more light is reaching each photodiode during the
exposure in the large sensor camera (assuming both are 6mp cameras)
we need to compare both the diameter of the lens opening in each camera
and also the relative sizes of the sensor photosites.
If it then turns out that more photons are hitting each photosite in the
large sensor camera we could say that the better noise performance of
the large sensor camera is due to the greater light collecting ability of
its lens but if you do the calculations then I think you will find that each
1 square mm of each of these two sensors receives the same number of
photons at the same aperture and shutter speed.
So why does a 6mp APS sensor perform so much better than a 6mp
compact sensor. The answer is that the photosites (photodiodes) on the
APS sensor are much larger than the ones on the compact sensor. Each photdiode is hit by more photons during exposure than the small
photodiode on the compact camera. This means that they generate a
larger current that requires less amplification and is much higher than the
noise level of a photodiode. So it seems to me that it is larger photosite
size that makes one sensor better than the other.
Constuct a small sensor with large photosites and you will get the same
noise (but not much resolution) on your compact digicam. Put small
photosites on a large sensor and you will get the same noise as you get
in a compact digicam.
So it is misleading to say that the light collected by the lens causes the
difference between large and small sensors.
The more important question is how can we get a small sensor to
perform as well as a large sensor. One answer to this would seem to be
to make lenses with a larger maximum aperture. In theory a compact
small sensor with a F1 lense would give lower noise than a F2.8 lens
on an APS camera. The F1 lens would also give the same DOF on our
compact as a F4 gives on an APS camera (approximately). However
constructing such wide aperture sharp lenses would not be easy and it
is not clear to me that the small photodiodes on a small sensor could
cope with such higher levels of light.
creep in. So we have people wanting to redefine aperture and ISO etc.
In terms of their redefinition they then want to claim that the Sony 828
is a superior camera to the R1. This is just silly since everyone knows that
the R1 can take better pictures with lower noise than the 828.
In photography aperture is a ratio the ratio between the diameter of the
lens opening to the lens focal length. ISO is defined relative to exposure
value i.e. shutter speed x aperture. These ratios are useful and no useful
purpose could be served by redefining them. If my exposure meter says
that this scene requires F2.8 at 1/60th at ISO 100 then no matter which
camera I am using I can set the correct exposure. That is the real use of
these ratios and that would be destroyed by redefining them. With regard
to exposure F2.8 means the same on any camera and so does ISO 100 and
so they should.
It has also been claimed that the larger absolute diameter of the lens
opening at an aperture of F2.8 on a large camera allows it to direct more
light onto the sensor. This is true of course but that light has to be
directed over a larger sensor area than it does in a compact digicam.
So just saying that the larger lens collects more light does not in itself
explain anything about the better noise performance of an APS v a
compact sensor. What is true is that the brightness of the image on the
APS sensor is probably just the same as the brightness of the image on
the small sensor. After all the small lens may not collect as much light as
the large one but the light it collects is concentrated on a smaller area.
To see how much more light is reaching each photodiode during the
exposure in the large sensor camera (assuming both are 6mp cameras)
we need to compare both the diameter of the lens opening in each camera
and also the relative sizes of the sensor photosites.
If it then turns out that more photons are hitting each photosite in the
large sensor camera we could say that the better noise performance of
the large sensor camera is due to the greater light collecting ability of
its lens but if you do the calculations then I think you will find that each
1 square mm of each of these two sensors receives the same number of
photons at the same aperture and shutter speed.
So why does a 6mp APS sensor perform so much better than a 6mp
compact sensor. The answer is that the photosites (photodiodes) on the
APS sensor are much larger than the ones on the compact sensor. Each photdiode is hit by more photons during exposure than the small
photodiode on the compact camera. This means that they generate a
larger current that requires less amplification and is much higher than the
noise level of a photodiode. So it seems to me that it is larger photosite
size that makes one sensor better than the other.
Constuct a small sensor with large photosites and you will get the same
noise (but not much resolution) on your compact digicam. Put small
photosites on a large sensor and you will get the same noise as you get
in a compact digicam.
So it is misleading to say that the light collected by the lens causes the
difference between large and small sensors.
The more important question is how can we get a small sensor to
perform as well as a large sensor. One answer to this would seem to be
to make lenses with a larger maximum aperture. In theory a compact
small sensor with a F1 lense would give lower noise than a F2.8 lens
on an APS camera. The F1 lens would also give the same DOF on our
compact as a F4 gives on an APS camera (approximately). However
constructing such wide aperture sharp lenses would not be easy and it
is not clear to me that the small photodiodes on a small sensor could
cope with such higher levels of light.
Wonjae Lee
Well-known member
Your 'effective ISO' concept is useless in practice, and misleading in photographic sense.
Wonjae Lee
Well-known member
I agree.
Small photodiodes on a small sensor can not cope with such higher levels of light due to small full-well capacity.
Thus, bright lens can not improve SNR at base ISO because of small full-well capacity. At base ISO, bright lens gives faster shutter speed but it can not improve SNR. SNR at base ISO is defined by sensor, not by lens.
Joe0Bloggs
Veteran Member
It depends on whether you are satisfied with the base ISO performance on the small sensor. If ISO 80 is not too noisy for you on a 1/2.5" sensor, you can in principle use it for all of your photography if you have a big enough lens.
Roland Karlsson
Forum Pro
Exactly
Correct.
Correct.
Correct
Here I start to disagree some
And those two reasons are exactly the reasons why I started to disagree. You cannot make the lenses you need to make a high sensitivity system with a small sensor. It is not physically possible to do so. There is a hard limit at F/0.5 and a construction limit (for fixed focal length) at F1.0 or so and a practical limit (for zooms) at F2.0 or so. Therefore - a camera with a larger sensor has a potential (if it has the same number of pixels) of being more sensitive to light.
Then I fully agree that muddying things up by new kind of ISO and F-numbers is just confusing.
Roland
lingsb
Forum Enthusiast
Care to explain why?
Play of words...
In fact we have what we are talking about - less noises. We as photographers are not interested in deep phisics and we can avoid some steps in the chain, leading to the same result.
Your second statement about no influence to camera and lens size is total nonsense - sorry. Bigger sensor needs longer focal distance for the same view angle and coming from that bigger lens diameter for the same aperture - that's the size of body and lens!
If You want to pay attention to wider DR - just say so...
--
http://zoom.tinkle.lt
In fact we have what we are talking about - less noises. We as photographers are not interested in deep phisics and we can avoid some steps in the chain, leading to the same result.
Your second statement about no influence to camera and lens size is total nonsense - sorry. Bigger sensor needs longer focal distance for the same view angle and coming from that bigger lens diameter for the same aperture - that's the size of body and lens!
If You want to pay attention to wider DR - just say so...
--
http://zoom.tinkle.lt
Wonjae Lee
Well-known member
Shaun Williamson has explained well.
http://forums.dpreview.com/forums/read.asp?forum=1000&message=14998064
Simply put, it doesn't reflect reality at all. ISO 25 on 4/3rds is not available currently, and will not be in the near future.
Who would use 'effective ISO' of your definition for determining exposure?
lingsb
Forum Enthusiast
He confuses aperture with aperture ratio
That was my point. That's one of the actual advantages of larger sensors: lower possible sensitivities.
No-one. The point of using 'effective ISO' is for comparing different camera systems, with different sensor sizes--not for calculating exposure. People compare focal lengths at 35mm equivalents. Why not ISO ratings and f-numbers?
Focal length is a physical quality of a lens system, yet it can be multiplied by a crop factor to compare the field of view with different sensor sizes. What is being suggested that the same is done for f-number and ISO. (To compare equivalent light-gathering, DoF, etc)
Lord Coe
Senior Member
Do they? I've seen prints from an 828, I've never seen anything from an R1.
--
Seb
Roland Karlsson
Forum Pro
Because those terms are already in use for something totally different. Why confuse people?
Focal length is focal length - often meassured in mm. It is sometimes used as a comparision to 35 mm FF. This is not all that useful IMHO. The 4:3 ratio (often used for digital cameras) is not comparable to the 2:3 ratio used for 35 mm FF. Depending on how you crop you get different results. I surely hope that this practice will just die out.
Roland
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