Photography Chat

jrista

2:01 AM

so, it seems, at least according to Wikipedia

that gain per stop is 6dB

and that seems to work out correctly regardless of what FWC is

Michael Nielsen

so its power gain not aplitude gain

jrista

I've run the numbers for FWC's of 100,000, 60,000, 40,000, and 25,000

they all come out the same

it may be power gain

I don't know the amount of current running through any sensor though

w = v*a

if we knew how many amps for a given sensor, we could compute power gain

and since that is 10 log, it probably is 3dB per stop

the problem with Canon sensors is that damnable bias offset

at lower ISO settings, the bias offset allows read noise to intrude into signal

because of how DXO computes S/N

the ratio between maximum signal and the average of read noise

Canon sensor DR flattens out at low ISO

usually, the first two or three ISO settings have exactly the same dynamic range

the next stop tends to have slightly less, but usually not a full stop less

from that point on dynamic range drops off as a linear function

Canon's problem is how they deal with read noise...which these days, is quite ineffectively.

Their CDS design is ancient, and clearly not up to the task.

Canon has been on a 500nm process for over a decade, where as most other sensor manufacturers started moving to the 180nm process several years ago.

Canon seems to have a 180nm process of their own, they used it in both the 50mp and 120mp APS-H sensors. They also have lightpipe technology on a 180nm Cu process.

They don't seem to have ramped up any kind of FF or APS-C fabrication capacity on that process yet, for whatever reason.

Michael Nielsen

yes, 6db per stop

jrista

The design of the 120mp APS-H also seems to indicate some kind of CP-ADC technology

and the press releases stated "on die image processing", which sounds veyr much like Sony's digital noise reduction in Exmor's CP-ADC

I just wish Canon would get on the ball and use their advanced process alrady.

Michael Nielsen

I dont think I ever saw a camera with less than -3db

jrista

2:11 AM

that would have to be an analog approach to ISO 50 or something

rather than a change to exposure and digital compensation

Michael Nielsen

which makes a true 50 iso pretty hard to make

jrista

yeah, true ISO 50 doesn't exist in consumer DSLR's as far as I know

some MFD sensors are based at ISO 80

rather than ISO 100

Michael Nielsen

yes I think I had one

jrista

making the next full stop ISO 160, then 320, 640, etc.

Michael Nielsen

a dslr wannabe fixed lens mirrorless

fujifilm

jrista

2:13 AM

lol

Michael Nielsen

electronic pixelated viewfinder :)

flickering too

jrista

yeah...not an EVF fan here

even the newer ones

they still suck

Red even makes a high end high resolution EVF for like $3800

it is pretty nice

Michael Nielsen

I had it 6 months then I bought the 400D

jrista

but still nothing close to an OVF

Michael Nielsen

I think I found teh source of my wrong info about 3db per stop

jrista

2:15 AM

?

Michael Nielsen

it was a datasheet for an adc that had gain in 3db steps

14bit adc

jrista

ah

Michael Nielsen

so I assumed that would be a direct mapping to iso

jrista

so, that would mean gain is divided up into 1/2 stop adjustments

Michael Nielsen

so it seesm the adcs have half stop increments

jrista

2:16 AM

Canon ADC's can do some gain

but as far as I know, it is only applied at high ISO

Michael Nielsen

wich explains why the 1/3 stops are "tricks"

jrista

there are actually amplifiers for each pixel that apply most gain

and 1/3rd stops are only "tricks" with Canon cameras

Nikon cameras seem to have real third stops

without the exposure shifting and compensation (and corresponding 1/3rd stop loss to DR) that Canon has

that is, until high ISO

Michael Nielsen

hmm teh read noise is chaotic on nikon graphs:

sensorgen.info/NikonD5000.html

jrista

in Canon, historically, ISO 1600 was the last setting that used per-pixel gain

Michael Nielsen

canons graphs are more like ski jump hills as expected

jrista

2:18 AM

above ISO 1600, ADC gain was used to achieve each additional stop, with a 1/3rd stop push or pull post-ADC

Michael Nielsen

nikon is up and down

jrista

in the 1D X and 5D III, I think the last native stop is ISO 12800

with ADC gain above that

Well, sensorgen.info is based on DXO data

which is not all that accurate

Michael Nielsen

a professor at the uni working with vision claim that they all use a mix of analogue and digital gain already from low isos

jrista

even Canon read noise fluctuates a bit

@MichaelNielsen read this: photography-on-the.net/forum/showthread.php?t=1081982

Bargain ISO uses per-pixel gain

Numpty ISO uses ADC gain

Michael Nielsen

Ive read that before

jrista

2:21 AM

Idiot ISO uses digital "gain"

which is rally just a digital exposure push or pull

Michael Nielsen

thats why I mentian "trick" 1.3 stops

jrista

yeah, it's idiotic :)

I wish Canon didn't do that....it is incredibly cheap

Michael Nielsen

and its wierd since machine vision cameras have gain in e.g. 0.0359 db increments

jrista

I would assume that is because they are working at a much finer grain than "stops"

even 1/3rd of a stop is a pretty significant difference in exposure

I'd expect a machine that can "see" to have a much finer grained ability to tune sensor "sensitivity"

Michael Nielsen

but it is still based on a cmos and an adc, like hte dslr, so tehres no reason for tham to make them such that they cant control 1/3 stops on the sensor gain

actually my camera that has that 0.0359 value is ccd. could it be a ccd vs cmos thing

jrista

2:34 AM

?

I wouldn't think so

it is a FINER gradation of amplification

Michael Nielsen

and why would it be more expensive to make

it seems more expensive to so the humty dumpty smart switching

jrista

which means you should be able to achieve third stops (or close enough) by amplifying in multiples of the base gain

Michael Nielsen

well the "measured iso" is often off by up to 50 anyway

so 0.0359 should be fine enough

jrista

yeah

Michael Nielsen

approx.55-56 is 1/3 stop

and maybe 0.0359 isnt the physical sensor limitation but thats just the grains of the interger gain parameter set in the firmware

better get to bed at 3:42 am

Aaron

7:58 PM

so one of my co-workers wants a good low-light camera that is small, smaller than most DSLRs, even

suggestions?

dpollitt

8:18 PM

@Aaron well a few things play into that equation

Johan Larsson

hej (Swedish)

dpollitt

sensor size, maximum aperture, and high ISO performance(which is usually tied to sensor size).

and even better yet, we have a question on this site that explains all of that detail:

34

Q: How do I tell which point-and-shoot cameras take good low light photos?

I have a Canon Powershot A710 and I previously had a previous Powershot A-series model. I love them due to the high amount of control they offer. Sadly, they both suck in low light conditions, their flash is very unresponsive, and the camera takes several seconds preparing itself for the next sho...

I personally really like the Canon S series of compacts. The S90, s95, S100, S110

They are pretty good in low light

Johan Larsson

I have mailed Canon that they should make a model in the same price range as 5D with ~2 Mpixel

dpollitt

@Aaron If you want a small DSLR, you could get any entry level DSLR with a 50mm f/1.8 lens.

Johan Larsson

Focus on dynamic range and noise

dpollitt

8:29 PM

@JohanLarsson hahah, good luck!

Johan Larsson

8:53 PM

@dpollitt yeah, I know but I still find it strange that they stack all models on each other 18-25 Mpixel

dpollitt

@JohanLarsson why strange? isn't that what all of the manufacturers basically do?

Johan Larsson

@dpollitt but they already have that segment covered x5

I think there might be market for an uncompromising 2 Mpixel fullformat

I guess a large % of all pictures taken 2012 is meant for web/screen

and besides, it is very rare that a pic is sharp/noise free enough to have 18 effective Mpixels

Aaron

Hmmm... anyone used Magic Lantern firmware?

@dpollitt thanks for the URL, I was already reading through that one

jrista

10:23 PM

@JohanLarsson Noise free is relative. If you take a photo at 18mp and scale it down to 2mp size, it will have far superior noise characteristics than a native 2mp sensor. You have the benefit of nine-fold pixel averaging.

Evan Krall

@MichaelNielsen 3db per stop sounds correct. 10^(0.3) is approximately 2.

jrista

A lot of high resolution sensors these days also have increased quantum efficiency. If we take an 8mp camera from years ago and compare it to an 18mp camera of today, the 18mp camera will perform better on all levels thanks to improved manufacturing quality and higher quantum efficiency.

@EvanKrall The math seems to come out to 6dB though...

Evan Krall

which math?

6db can show up when you've got quadratic terms

jrista

Gain = 10 log (Vout/Vin)^2

Gain = 20 log (Vout/Vin)

either one

the 20 log rule just makes it simpler

I don't know about power gain

I am not even sure exactly how gain is done in a CIS

the only numbers I have handy is charge in electrons, so I used voltage gain

Evan Krall

I mean, power gain is 20 log(Vout/Vin) when you have an ohms-law load - when you double voltage, you quadruple power

/me reads wikipedia

jrista

10:31 PM

ok

so, in that case, it seems that each stop is about 6dB

at least, that's what my math came out to last night

Evan Krall

Decibel

{| class="infobox" style="padding:0;" border="0" cellpadding="0" cellspacing="0" width="1" ! align="right"|dB ! align="center" colspan="2"|power ratio ! align="center" colspan="2"|amplitude ratio |- | align="right"| 100 | align="right"| 10 000 000 000|| | align="right"| 100 000|| |- | align="right"| 90 | align="right"| 1 000 000 000|| | align="right"| 31 620|| |- | align="right"| 80 | align="right"| 100 000&nbs...

(click the link, that summary didn't turn out well)

jrista

yeah, that one got botched good :P

hmm

although...I guess technically I'm using eV

rather than V

I wonder if that might change things...

Evan Krall

eV is logarithmic

V is linear

jrista

does it matter if you use electron volts rather than volts?

Evan Krall

oh

sorry not EV

jrista

10:33 PM

not EV as in stops, electron volts :P

Evan Krall

yeah

eV is a unit of energy

jrista

I mean, charge in electrons is effectively charge in units of electron volts

Evan Krall

at a particular voltage, yes.

jrista

I guess eV would actually be joules of energy, though....

Evan Krall

yeah eV and joules are directly convertable

if you have 1 electron at 1 volt, you've got 1 eV of energy

if you have 2 electrons at 1 volt, you've got 2 eV of energy

or 1 electron at 2 volts

jrista

10:35 PM

right

you wold really need to know the current as well

to be able to actually figure out how many dB per stop

because from the information generally available, we only seem to have electrons at max saturation for each stop

we would need to know voltage and current to be able to figure anything else out

so, I can't say

it may be 3dB

DXO indicates that the signal in dB is in the 40's at ISO 100 for modern sensors

Evan Krall

I mean it all depends on the base unit

is that dB of eV, V, photons...

jrista

S/N

Evan Krall

ah

jrista

and as far as I know, the unit is simply "e-" (electrons)

Evan Krall

that makes sense to me

jrista

10:42 PM

but still, what is that?

electrons...

I mean, pixels are electron counters

photon strikes "usually" convert to an electron

Evan Krall

so I think photodetectors basically work by emitting an electron when they get hit by a photon

jrista

some convert to heat or reflect

@EvanKrall correct

Evan Krall

so it should be proportional to number of photons hitting the sensor.

or rather, that site

jrista

That would only be the case with 100% Q.E. for the photodiode

but not every photon strike, for photons that make it through the filter stack, the microlens, the color filter, and down the wiring channel and actually reach the photodiode in the first place

will actually convert...most do, but some convert to heat or reflect

and there is also the conversion response level

I don't think it is always a 1:1 ratio of photons to electrons

I think it is a slight fractional ratio

so, it might take like 1.137 photons per electron or something around there

I think it depends on the exact nature of the silicon

and maybe applied voltage

Evan Krall

but in any case it should be proportional, right?

jrista

10:45 PM

roughly, yes

Evan Krall

so 40 dB S/N ratio means (or should mean) there are 10^4 times as many electrons due to the signal as there are due to noise

jrista

due to read noise

read noise explicitly

there is still photon noise in the signal itself

Evan Krall

yeah

not much you can do about that.

jrista

nope

from sensorgen:

1D X read noise: 38.2e-

1D X ISO 100 signal: 90367e-

Evan Krall

(and that's the basis of my question about the theoretical limits of ISO)

jrista

10:49 PM

hmm

wouldn't the limits have to do with Q.E.?

I mean, lets say we DO have a perfect sensor

Evan Krall

what's Q.E.?

jrista

0e- read noise

quantum efficiency, which is effectively the rate of successful photon to electron conversions

modern sensors have around 50% Q.E.

the 1D X is 47%, the 5D III is 49%

Evan Krall

ah

yeah

so let's say we had a Q.E of 100%

and 0e- read noise

jrista

the D800 is 56%

well, before we get that far...it seems Canon gains about 1 stop of ISO for every 8% gain in Q.E.

(Based on the progression of the 5D line)

If we project that forward through to 100% Q.E.

100-49 = 51

51/8 = 6.375

Assuming the improvements are linear

we could see about 6 additional stops of usable ISO at Q.E. 100%

Evan Krall

an 8% gain in Q.E. doesn't seem to me like it'd give you a stop on its own

jrista

10:53 PM

technicaly speaking, we would only need 97% Q.E., but that is also more realistic...we'll never actually achieve 100% anyway

well

Evan Krall

it's probably just correlation

and the gains are mostly read noise I'd suspect

jrista

the 5D C had 25% Q.E., the 5D II had 33%, the 5D III has 49%

8% between 5DC and II, 16% between 5D II and III

8% per stop so far

Johan Larsson

Interesting discussion, I'm not very up-to-date with the physics/electrnics

jrista

@EvanKrall Read noise doesn't really apply at high ISO

Since canon uses a bias offset

Evan Krall

read noise is the sensor itself, yes?

jrista

10:54 PM

they effectively clip read noise at higher ISO

Evan Krall

I'm not sure what you mean by that

jrista

@EvanKrall read noise is dark current, fixed pattern (intrinsic pixel hot spots fixed in layout for a given sensor), banding (both dynamic and fixed, crosshatch pattern caused by signal interference and some other fixed artifact (usually vertical) that is caused by sensor design), and PRNU (pixel response non uniformity...differences in the response rate of each pixel...very small contributor to read noise)

Then there is downstream amplifier, conversion, and quantization noise added by the ADC as well (in a Canon design...Sony Exmor works in a radically different way)

@EvanKrall Well, according to both DXO and Roger Clark

read noise in a canon sensor falls off to around 2e- at high ISO

Evan Krall

hm.

jrista

I have an article bookmarked at home that explaines the exact mechanics...I'll link it when I get back home

Evan Krall

yeah I should really learn about this

jrista

10:57 PM

either way, by around ISO 800, read noise is a minimal contributor

so you can effectively ignore read noise at high ISO

Evan Krall

hmm

I mean in terms of electrons it makes sense that it falls as you raise ISO

jrista

it is pretty much all photon shot noise, in a Poisson distribution

@EvanKrall It depends on sensor design. Exmor has flat read noise, at around 3e- for ALL ISO settings. An exmor sensor is effectively ISO less...you could take a shot at ISO 100 and amplify it in post and get pretty much the same result as using a higher ISO. You might exhibit slightly more noise in the shadows by boosting ISO 100 in post, but in general you wouldn't notice.

Exmor has the benefit of CP-ADC and digital noise mitigation, though, so it is tough to compare it with a Canon sensor (or even an older gen Nikon sensor, for that matter.)

anyway

if we actually COULD achieve 6 more stops of ISO with 100% Q.E. (ignoring the caveats in doing that for now)...that would be a native ISO 3276800

And that ISO 3 million should look similar to ISO 51200 on the 1D X right now

(Assuming 8%/stop Q.E. is actually enough of a photon conversion improvement to actually hold up through six more stops of ISO)

Evan Krall

gotta go be back in an hour

jrista

k

Johan Larsson

I think noise is proportional to photodetector size

And if that is the case it would be beneficial with large sensor and few pixels

There is a border around each detector right? That border would approach 100% if pixel count went to infinity and it would be as leaving the lens cap on

jrista

11:05 PM

Size plays a roll...but size is just a way of improving Q.E.

Larger pixel surface area, greater chance of converting a photon to an electron.

However, assuming the same pixel size, a pixel that converts more incident photons will generally have less noise at all ISO settings than one that converts fewer incident photons.

@JohanLarsson As for pixel structure, in a Front-Side Illuminated CIS design, the photodiode is at the back. In front of that is a channel, and around the sides of that channel (around the sides of the photodiode) is readout wiring.

Johan Larsson

Do you happen to have a number for how many % of the photons that gets detected

jrista

@JohanLarsson A Backside Illuminated CIS design flips the whole structure, directly exposing the photodiode, with all the readout wiring underneath, thus eliminating the problem entirely.

@JohanLarsson generally speaking, for cameras released over the last four years, anywhere from 25% to 56%

the 5D II was 25% Q.E. (quantum efficiency, the conversion rate as a percentage of photons)

the 5D III is 49%

the D800 is 56%

There are limits to how far we can push Q.E. in a bayer design

each pixel has a color filter

the dyes in the CFA eliminate a LOT of light per pixel

56% is actually a pretty amazing feat, and probably required some special high transmittance dye formula for the color filters

I think all manufacturers are weakening their CFA's these days to improve Q.E., but with that comes more "color blindness".

Color blindness is probably not a problem at 50-60% Q.E., as we seem more than capable of mathematically correcting that.

I am not even sure if it is possible to achieve 70-90% Q.E. with a bayer type sensor...the color filters have to filter out a certain amount of light at each pixel to achieve color...so we are going to hit a wall at some point, and it is probably not far off.

The only way to achieve near-100% Q.E. would be with a monochrome sensor

many $100,000 scientific grade supercooled sensors are monochrome, and those are achieving 90% or so Q.E. at temperatures of -80°C

sensorgen.info

layered sensor designs might be interesting, once they go mainstream

I know Canon has at least one, maybe two patents for a layered CIS design

Q.E. per photosite might be able to push higher than 60% then

however per color it would still be lower

so you would still have per-color channel noise differentials

Johan Larsson

@jrista You happen to know the number for 20D?

You know much more than I do, I'm wasting your time :)

jrista

11:24 PM

np

I don't know for the 20D

I would guess around 20% or so

maybe as low as 15%

it can't be too low, otherwise the camera simply would not work

but older cameras natively amplified their signal more at base ISO, because they had such low Q.E.

that is why older cameras used to have a minimum ISO of 200

rather than 100

Evan Krall

11:54 PM

I think I've heard of an idea where instead of filtering per pixel, you split the light with prisms

directing all of the red light for a pixel to one sensor, all the blue light to another, etc.

if you did it right you might be able to direct all the photons into photo sensors

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