Why bother changing base ISO?

Started Jul 18, 2007 | Discussions
cycle61 Senior Member • Posts: 1,550
Re: ISO boost test shots re-processed

Here's 100% crops from the same area of the image as before. I can't discern any meaningful difference between the two now. Darn. Even on the noisiest area of the image (the inside of my trunk lid and the white jacket in the trunk) both ends of the scale look nearly identical to me.

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-Nick Davis
Please feel free to critique anything I post. I'm here to learn.

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cycle61 Senior Member • Posts: 1,550
Re: ISO boost test shots re-processed

Looks like a good raw converter has a lot to do with final image quality. Here's the underexposed ISO 100 shot pushed three stops, courtesy of RAWMagick vs ACR

The RAWMagick version has less noise in the sky, better detail in the shadows, a little less contrasty and maybe more chroma noise (look at the wall) but far more accurate and pleasing overall.
--
-Nick Davis
Please feel free to critique anything I post. I'm here to learn.

My galleries, such as they may be...
http://www.pbase.com/cycle61

Julia Borg Veteran Member • Posts: 7,280
Re: ISO boost test shots re-processed

Looks like light was changing a little between the shots, or your
camera has uncalibrated amplifier.

By the way, the shots were all taken within 30 seconds total elapsed
time on an essentially cloudless day. Is the amplifier thing
something I should look into?

the amplifier thing: not really worth the trouble. un-calibrated means that the ISO settings in the camera are not very accurate, like IS0 400 looks more like ISO 320. that is the difference small enough not to worry about.

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Julia

RichardPillay New Member • Posts: 1
Re: Why bother changing base ISO?

Jim Kaye wrote:

important thing is that I don't see any difference in the noise level
depending on whether I bump ISO in the camera or change the
"exposure" setting in post. Julia Borg and others have talked about
this in other posts -- changing ISO being equivalent to making
"exposure" adjustments in post -- but I've started shooting this way

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There may not be any difference in noise level, but there will be a diffeence in the image itself. Try it yourself with a more extreme example - say 5 stops underexposed.

First try a correctly exposed shot at ISO 100. A certain amount of light falls on the sensor, gets amplified and converted to digital. All well and good.

Now the light lessens by 5 stops.:

When you expose correctly by increasing ISO level, you are letting 5 stops less light reach the sensor, and the your change of ISO from 100 to 3200 has told the camera to amplify the light more than in your first shot so that the same amount of "electronic signal" from the sensor reaches the A/D convertor and gets converted to digital.

If instead you were to leave the ISO level at 100, again 5 stops less falls on the sensor (as compared to the first shot). However, this time, the lower signal gets amplified only as much as the first shot. What gets converted to digital is a very much lower signal, with most of your shadow detail lost, and simply not there to be boosted in PP.

The reason you are not really noticing a loss of shadow detail in the shots you've done is that the camera is working with 12 bits of A/D, while JPegs only use 8. You can get away with around 3 stops of underexposure and fix it in PP. More is a problem.

Best Regards,
Richard.

RAL Veteran Member • Posts: 6,377
Sounds wild to me

I haven't read all the posts, but keep seeing this here and checking a few. Sounds like the op is suggesting not worrying about exposure, setting the camera for some particular exposure that would probably be underexposed and then fixing the exposure in post process. If I am reading right, I can't comprehend this. Seems to me that it has to be much better to expose properly in the camera. Why else would even cheapie cameras have aperture and shutter variables provided? Also, it is not a difficult thing to get exposure right in the camera. With modern gear this is almost a given to be at least really close and seems to be much easier than having to post process to get the right exposure. I occasionally forget to change something and make a drastic mistake and have to fix it later, but for sure wouldn't want to have to do this to all my pics and the results certainly were not as good as they are when I get it right to start with. Just my 2 cents worth of opinion only.

rspino Contributing Member • Posts: 729
Re: Sounds wild to me

RAL wrote:

I haven't read all the posts, but keep seeing this here and checking
a few. Sounds like the op is suggesting not worrying about exposure,
setting the camera for some particular exposure that would probably
be underexposed and then fixing the exposure in post process. If I am
reading right, I can't comprehend this. Seems to me that it has to be
much better to expose properly in the camera. Why else would even
cheapie cameras have aperture and shutter variables provided? Also,
it is not a difficult thing to get exposure right in the camera. With
modern gear this is almost a given to be at least really close and
seems to be much easier than having to post process to get the right
exposure. I occasionally forget to change something and make a
drastic mistake and have to fix it later, but for sure wouldn't want
to have to do this to all my pics and the results certainly were not
as good as they are when I get it right to start with. Just my 2
cents worth of opinion only.

i do not think this is what the OP suggests.

rather, the point is that pushing ISO will NOT change the exposure, but only apply some sort of amplification to the signal (i have not quite understood wether this is analogue, digital, or both).

suppose your best estimated exposure be f/4, 1/250, ISO 800.

it appears that you'd better shoot at f/4, 1/250, ISO200 and then apply a +2 stops compensation in post.

that is because the data recorded by the sensor will be the same, but WITH SOME CONVERTERS you can get less highlight clipping and better colors.

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Roberto (equipment in my profile)
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Julia Borg Veteran Member • Posts: 7,280
Re: Sounds wild to me

RAL wrote:

I haven't read all the posts, but keep seeing this here and checking
a few. Sounds like the op is suggesting not worrying about exposure,
setting the camera for some particular exposure that would probably
be underexposed and then fixing the exposure in post process. If I am
reading right,

you are not. one of the differences between film and digital is that in digital ISO has nothing to do with the exposure.

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Julia

MK914 Senior Member • Posts: 1,202
Nick...

When I do similar tests, I see less noise in the shadows with the "properly" exposed, higher ASA shots. With the pushed shots there's more noise but sharper edges. I'm using a D200.

Is this what you're seeing?

RAL Veteran Member • Posts: 6,377
Re: Sounds wild to me

It sure seems to show up as problem in my pictures. Actually, I in theory understand what you are saying, but feel there is maybe more to it than that. That aside, it still seems to be an awful lot more trouble to have to make it right in post process than having it right out of the camera. It would also take a lot of convincing for me to believe that it was as good in the end also.

Julia Borg Veteran Member • Posts: 7,280
Re: Sounds wild to me

it still seems to be an awful lot more
trouble to have to make it right in post process than having it right
out of the camera.

one slider move seems to you like a lot of trouble?

It would also take a lot of convincing for me to
believe that it was as good in the end also.

nobody can convince you except for yourself. one way or another.

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Julia

Tom Christiansen Senior Member • Posts: 2,239
Re: ISO boost test shots re-processed

cycle61 wrote:

Here's 100% crops from the same area of the image as before. I can't
discern any meaningful difference between the two now. Darn. Even on
the noisiest area of the image (the inside of my trunk lid and the
white jacket in the trunk) both ends of the scale look nearly
identical to me.

That's what it looks like to me, too. I'd say that at least
when it comes down to ISOs 100 to 800 here, aperture
times exposure time equals photons, and you'll (be able
to) get the same picture with the same photons.

I wonder whether the D200's hitting unity gain at ISO 800
is the ceiling for this sort of thing. That is, if you compared
between an ISO 1600 shot and one at ISO 200 3-stops
underexposed, or a HI-1 shot (which apparently is not actually
ISO 3200) and ISO 400 3-stops underexposed. I think the
conventional wisdom runs that because you poked your
head through the unity gain ceiling, your dynamic range got
squashed a bit and you don't have very attractive highlights
(although a nonlinear shoulder roll-off would help if it existed).

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tom

Tom Christiansen Senior Member • Posts: 2,239
Succinct poster case for ACR inferiority

cycle61 wrote:

Looks like a good raw converter has a lot to do with final image
quality. Here's the underexposed ISO 100 shot pushed three stops,
courtesy of RAWMagick vs ACR

The RAWMagick version has less noise in the sky, better detail in the
shadows, a little less contrasty and maybe more chroma noise (look
at the wall) but far more accurate and pleasing overall.

Wow. That's extremely revealing; convincing, even.

ACR is doing a poor job here with respect to what RML is doing.
I imagine the RML image was generated using the slowest,
highest-quality rendering. But that's ok, because it gives you
chance to let your processor put in more time working on it so
you can get out a superior result. There's something satisfying
about the equation "more work equals better output"; that's how
it should be, of course, but as it so often doesn't make a difference,
it's refreshing to encounter a case where it does. Shows that
the coder(s) knew what they were doing, and cared enough to
put in the work up front to write strong code.

This is a good demo page for the next time someone advocates
ACR as a fine raw processor, eschewing NC, NX, or anthing else.
It's a pretty solid and stark comparison, albeit with RML not N[CX].

Thanks for posting these.

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tom

Tom Christiansen Senior Member • Posts: 2,239
Re: Nick...

MK914 wrote:

When I do similar tests, I see less noise in the shadows with the
"properly" exposed, higher ASA shots. With the pushed shots there's
more noise but sharper edges. I'm using a D200.

Is this what you're seeing?

That's not what I see in the two comparison postings I just responded
to, the ones kindly provided by Nick Davis. Do you see those pairs
differently than I'm seeing them?

On your own tests, what raw processor were you using, and how much
did you push the exposure? Also, from where to where; that is, did
you cross the D200's unity-gain threshhold of ISO 800, or were you
always below that?

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tom

kevm14 Senior Member • Posts: 2,253
Re: There's no red data in the sky

Tom Christiansen wrote:

But that was a daylight shot, so working from a daylight WB
perspective, the red channel wouldn't have had to be pushed to obtain
the correct WB.

Sure it would.

http://www.pochtar.com/NikonWhiteBalanceCoeffs.htm

If daylight is 5200K, then your red channel gets multiplied by 1.8x
and your blue channel by 1.4x on a D200. That on top of the 3
stops of push from the ISO.

So you have a big 1.8 push on the red channel already (daylight
is mostly green, then blue; incandescent is mostly red, then green),
which you further distort with 3 stops of ISO pushing, and you do
this in an area where there is virtually no red signal at all. That
doesn't seem a good thing.

Wow, I guess I have zero understanding about how WB really works. I thought most digital camera sensors were "daylight balanced" requiring little to no red/blue correction for daylight shots. Interesting.

OP Jim Kaye Senior Member • Posts: 2,794
Re: Sounds wild to me

one of the differences between film and digital is that
in digital ISO has nothing to do with the exposure.

Julia, why is that a difference with film? With film, ISO also has nothing to do with exposure (shutter speed/aperture combination), either -- doesn't it have to do with how "sensitive" the components of the emulsion are to undergoing certain chemical reactions when they are exposed to a given quantity of photons?

Maybe what you mean is that we don't change from a sensor with one (lower) native sensitivity to one with a different (higher) native sensitiivity when we adjust "ISO" in a digiital camera. As I understand it, we just change how the signals coming off the sensor are processed (either in the camera or on our computer).
--
Jim Kaye

'I believe that the electronic image will be the next major advance. Such systems will have their own inherent and inescapable structural characteristics, and the artist and functional practitioner will again strive to comprehend and control them.' -- Ansel Adams, 1981

 Jim Kaye's gear list:Jim Kaye's gear list
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Julia Borg Veteran Member • Posts: 7,280
Re: Sounds wild to me

we don't change from a sensor with one
(lower) native sensitivity to one with a different (higher) native
sensitiivity when we adjust "ISO" in a digiital camera.

yes indeed.

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Julia

Julia Borg Veteran Member • Posts: 7,280
Re: Succinct poster case for ACR inferiority

I imagine the RML image was generated using the slowest,
highest-quality rendering.

not the slowest as I turned NR off. NR in RML is done during demosaicing. highest quality, yes. rendering time was 117 secs per frame on MacBook Pro Duo 2Gb RAM

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Julia

Tom Christiansen Senior Member • Posts: 2,239
Re: Succinct poster case for ACR inferiority

Julia Borg wrote:

I imagine the RML image was generated using the slowest,
highest-quality rendering.

not the slowest as I turned NR off. NR in RML is done during
demosaicing. highest quality, yes. rendering time was 117 secs per
frame on MacBook Pro Duo 2Gb RAM

That's not too bad, then. I was somewhat fearing one of those
go-out-for-a-5-course-dinner sorts of processing times.

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tom

Julia Borg Veteran Member • Posts: 7,280
Re: Succinct poster case for ACR inferiority

I was somewhat fearing one of those
go-out-for-a-5-course-dinner

no way, promise you. I like my jeans.

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Julia

cycle61 Senior Member • Posts: 1,550
Re: Succinct poster case for ACR inferiority

Julia Borg wrote:

I imagine the RML image was generated using the slowest,
highest-quality rendering.

not the slowest as I turned NR off. NR in RML is done during
demosaicing. highest quality, yes. rendering time was 117 secs per
frame on MacBook Pro Duo 2Gb RAM

Wow. ACR is definitely taking some huge shortcuts then, given that on an ancient 1.8Ghz Pentium with 512mb ram, ACR processed the images at around 10-15 seconds per file. That was coming from DNG, however. Maybe this has something to do with it?
--
-Nick Davis
Please feel free to critique anything I post. I'm here to learn.

My galleries, such as they may be...
http://www.pbase.com/cycle61

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