Push vs. ISO

[Edwin Herdman]

It should also be mentioned that some empirical testing (i.e. DxOMark's data, recompiled at sensorgen.info) demonstrates that benefits from increasing ISO start to decline, on any given sensor, past a certain point; this decline is accompanied by decreases in saturation capacity and dynamic range.

Therefore, I have said in the past (but honestly haven't found anything in real shooting to back up this claim, since I'm often trying to keep in line with "reasonable" exposures that look good on the camera's screen review, while avoiding ISO settings above 400 in most all cases) that there ought to be a point where increasing ISO on the camera makes less sense than pushing in post.  But it might be the case that such a result will not be found as the camera manufacturers won't offer an ISO setting that is worse than pushing in post - at worst, this should only effect the most extreme ISO settings, and then it'd still need to be proven.

However, it ought to be considered that complex noise reduction processing is available for computer users that isn't possible in-camera, so quite often you should be able to get away with being a bit conservative or liberal with your ISO use, depending on the situation.

I'm no ETTR expert but if I recall correctly the main idea is to reduce the noise found in shadow areas, by exploiting unused headroom past the highlights.  If that headroom was unused, shadow areas would be squeezed out.

[wockawocka]

It's well known that an incorrect digital exposure will create noise as a result.

If you are in a situation where you take a picture at the incorrect settings when you correct this in post production you will introduce more noise.

I'm not too sure how well this stands up if the ISO used was in the expanded range though as generally things get nasty up there anyway.

[GeorgeMaciver]

Wow, there is some photography expertise around here, for sure. I enjoy reading these posts, but don't often feel qualified enough to jump in. Thought I'd mention that I do love the forums though. Respect. Regarding ISO and cameras, I'm not sure if it's just me, but the 50D does seem to perform better around 400 to 500 ISO in most situations, though I've no idea why this would be, if indeed it is the case.

[awinphoto]

Meh,  I did a quick search and this is what Canon says about the current digic 4 processor:

The performance of a digital camera is largely determined by three key components: the lens, the image sensor (CMOS or CCD), and the image processor. The image sensor, which takes the light entering the camera through the lens and converts it into electrical signals, can be compared to the film used in a traditional film camera, while the image processor "develops" the image. If any one of these three components is of substandard quality, it will negatively affect the images that the camera produces. Canon, which has been making cameras for over 70 years, develops its own lens units and CMOS sensors in-house, and has for some years now also been designing its own image processors: the DIGIC digital image processors.

The evolution of the DIGIC image processor has been the result of an ongoing search for higher and higher levels of image quality and faster speeds. DIGIC 4, which Canon began using in its camera products in the fall of 2008, represents the sixth generation of DIGIC processors. Compared with first-generation DIGIC processors, device performance has improved 50-fold in the space of just 10 years.
Today's DIGIC image processor does more than mere image processing: It controls a wide range of functions and circuits, including automatic exposure control, exposure mode control, image file compression/playback control, LCD display control, and more. All of these functions are contained in a single-chip system large-scale integrated circuit (LSI), and the same image-processing platform can be used in any Canon digital camera, including the Digital ELPH/IXUS and PowerShot series of compact digital cameras, and the EOS digital SLR (single-lens reflex) camera series.

One could say that DIGIC is the "brain" of Canon digital cameras. DIGIC 4, the latest version of the DIGIC digital image processor, provides dramatic improvements in terms of image quality and processing speed. So how have these advances in processor design been achieved? We talked to four members of the DIGIC 4 development team to find out more.

** On the digic5 press release I vaguely remember a key note at reduced noise... Canon claims in the above statement that the digic processes the image, hence controlling the noise...  So it make perfect sense that the digic, if you take the statement above at face value, that the digic controls the noise better than the intel computer in this regards. 

[neuroanatomist]

"RAW data" implies a digitized value (i.e. post ADC) so "Analog gain applied to RAW data" seems contradictory unless you are using "RAW data" to mean the direct voltage values as read out from each photosite.   There is definitely an amplifier for each photodiode but I don't know if this amplifier has adjustable gain depending on the ISO setting but your statement that there is some analog gain based on an ISO setting up to about 800 indicates that it is applied prior to the ADC.  Anything after the ADC is obviously digital and includes all the readout noise in the digitally encoded luminance values for each pixel.

TI guess I should have said 'raw signal coming off the sensor' vs. RAW data.  As I understand it, there's an analog gain applied to the signal from the sensor before the ADC, up to a certain ISO setting.  Above that threshold, additional digital gain is applied after the ADC.

[branden]

Does the analog gain vs digital gain correspond directly to the regular ISO numbers (100-6400) and then the "expanded" ISO (L, H1, H2)?

[neuroanatomist]

Does the analog gain vs digital gain correspond directly to the regular ISO numbers (100-6400) and then the "expanded" ISO (L, H1, H2)?

I don't think so.  I think analog gain stops much lower than ISO 6400.  But that's a guess, as I haven't seen data for the 5DII on this.

[Flake]

Just to add another factor and confuse everything a little bit more!

The other unmentioned issue is that of shutter speed, and all of this will be a little less accurate if shutter speed isn't considered.  a 1/500 sec exposure and a Push, or high Iso is going to have very different results to a 5 Sec exposure.  In most cases a 1 stop on the shutter speed will not have as large an effect as a push or Iso stop, but then that assumes that you're photographing a still subject with a tripod, if it's moving and you need fast speeds it's not of as much importance, but still needs consideration in the round.

[awinphoto]

Just to add another factor and confuse everything a little bit more!

The other unmentioned issue is that of shutter speed, and all of this will be a little less accurate if shutter speed isn't considered.  a 1/500 sec exposure and a Push, or high Iso is going to have very different results to a 5 Sec exposure.  In most cases a 1 stop on the shutter speed will not have as large an effect as a push or Iso stop, but then that assumes that you're photographing a still subject with a tripod, if it's moving and you need fast speeds it's not of as much importance, but still needs consideration in the round.

Are you referring to reciprocity and or long exposure noise referring to shutter speed?  In the past with leaf shutters there used to be wide varaitions of speeds hence you had to have your leaf lenses with shutters built in tested regularly to insure your exposure was correct (wrong exposure could add noise especially if you had to correct in developing and or printing) but I'm just trying to get on the same page here....

[Meh]

** On the digic5 press release I vaguely remember a key note at reduced noise... Canon claims in the above statement that the digic processes the image, hence controlling the noise...  So it make perfect sense that the digic, if you take the statement above at face value, that the digic controls the noise better than the intel computer in this regards.

Awinphoto, yes absolutely the DIGIC processor "processes" or "develops" the image from the original read out values coming out the sensor.  It also creates the RAW file which is more than just the pixel values and is a proprietary format.  However, in terms of processing or developing an image, I believe that has more to do with producing the JPEG rather than creating the RAW file.  Good point about "if we believe" as certainly anything we read from Canon has marketing spin (not that they're lying but could be exaggerating).

The DIGIC processor only sees the output from the A/D converter... in other words it's already digitally encoded and therefore no more noise will be added (processing artifacts possibly, but again that's more about producing the JPEG).   DIGIC5 is getting quite powerful but in general an in camera processor is subject to constraints (speed, power) that the computer is not and because of this some noise reduction algorithms, etc. that are too slow for in camera use could be used in post processing allow additional tools but not less.

Other than the fact CR2 is proprietary and Canon might know things about the data in the file that a 3rd party application would not be able to access, I doubt the DIGIC processor can do anything that can't be done on a computer and even then Canon would build that functionality in to DPP.

[Picsfor]

Blimey, thought the term "push" had long since disappeared into the ether of film.

I still do "push" my exposures because situations arise where i really needed a flash or a tripod and have neither.
Also, because i want a certain amount of noise/ grain in my shots, as i've said before.

But as i used to push in film days, it's something i've always done.
I think for me, noise/ grain is something i've learned to use to create an appearance.

The most i ever got to in film days was pushing a 3200asa 2 stops to 12800, but wow, some of those would never have been gained without it.

I'll leave you scientists to debate the fromulae and algorithms that prove your point. I just know that i use the camera, lens and some otheer bits of kit to create an image based on various settings applied!

[Meh]

I'll leave you scientists to debate the fromulae and algorithms that prove your point. I just know that i use the camera, lens and some otheer bits of kit to create an image based on various settings applied!

Fair enough     +1

[awinphoto]

** On the digic5 press release I vaguely remember a key note at reduced noise... Canon claims in the above statement that the digic processes the image, hence controlling the noise...  So it make perfect sense that the digic, if you take the statement above at face value, that the digic controls the noise better than the intel computer in this regards.

Awinphoto, yes absolutely the DIGIC processor "processes" or "develops" the image from the original read out values coming out the sensor.  It also creates the RAW file which is more than just the pixel values and is a proprietary format.  However, in terms of processing or developing an image, I believe that has more to do with producing the JPEG rather than creating the RAW file.  Good point about "if we believe" as certainly anything we read from Canon has marketing spin (not that they're lying but could be exaggerating).

The DIGIC processor only sees the output from the A/D converter... in other words it's already digitally encoded and therefore no more noise will be added (processing artifacts possibly, but again that's more about producing the JPEG).   DIGIC5 is getting quite powerful but in general an in camera processor is subject to constraints (speed, power) that the computer is not and because of this some noise reduction algorithms, etc. that are too slow for in camera use could be used in post processing allow additional tools but not less.

Other than the fact CR2 is proprietary and Canon might know things about the data in the file that a 3rd party application would not be able to access, I doubt the DIGIC processor can do anything that can't be done on a computer and even then Canon would build that functionality in to DPP.

For the most part, I get your question but to be honest, i'm not that geeky to pretend to have have that answer... I could pose a question to canon's cps reps next time we meet however i'm sure there's information there where they could tell me but then they would have to kill me.  =)  All i know is, in practice, in camera ISO boosting is cleaner than pushing in post production... I cant quite tell you why but it is what it is, for now.   

[Meh]

All i know is, in practice, in camera ISO boosting is cleaner than pushing in post production... I cant quite tell you why but it is what it is, for now.

+1

Agreed, if the empirical testing shows it's cleaner then it is (although sometimes what we see leads to incorrect conclusions).  Neuro's answer that up to a certain ISO setting the analog gain applied to the readout voltage before passing to the A/D converter make sense since that amplification is applied before noise is picked up from the ADC.

OP has been answered.   In camera ISO results in less noisy images than underexposing and correcting in post.  The caveat though is that might only be true up to a certain ISO setting.

[ejenner]

http://www.clarkvision.com/articles/digital.sensor.performance.summary/

This might be of interest if you want to start understanding sensors.  Definitely worth a read IMO.

Also it shows why a 36MP FF sensor is either going to need major new technology (like doubling the capture of photos) or trade off IQ.