July 30, 2016, 07:06:25 PM

Author Topic: Just found ISO100 won't clip normally  (Read 3424 times)

rfdesigner

  • Canon 7D MK II
  • *****
  • Posts: 517
Just found ISO100 won't clip normally
« on: January 31, 2015, 03:11:52 PM »
I've been playing games with my ancient 30D..   I use it for various things including astrophotography.. and if you shoot the moon then you want a low ISO, fast shutter and ETTR as close as you can possibly get to the right.

I use a program called IRIS for astro image pre-processing and it allows me to inspect the un-debayered image, and to run a simple command that tells you things like the maximum pixel value..  handy if you're looking for clipped pixels.

So anyway I was thinking I had something utterly wrong when I was pushing exposures way beyond what they should be and at ISO100 I couldn't get more than 3500 out of 4095 ADC counts.   Then the penny dropped as I realised the sensor/buffer was clipping, but the ADC wasn't.

So go to ISO125 and I can just clip the image (get 4095), but also get about 25% less readout noise.

looks like I'll stop using ISO100.

I wonder if any other cameras out there are doing the same thing?
6D, 30D, 383L+, 28f1.8, 50STM, 100f2.0, 5.5"f8 scope, 12"f5 scope.
Personal Gripe:   "I could care less about.." = there are things less important too me, this may be my most important thing.
"I couldn't care less about.." = this is the bottom of my list.. it is so unimportant.

canon rumors FORUM

Just found ISO100 won't clip normally
« on: January 31, 2015, 03:11:52 PM »

dkaiser

  • Power Shot G7X
  • **
  • Posts: 12
    • 500px Profile
Re: Just found ISO100 won't clip normally
« Reply #1 on: January 31, 2015, 05:17:04 PM »
This test with a 60D tells that ISO 160 is better than ISO 100 which is better than ISO 125.

http://petapixel.com/2011/05/02/use-iso-numbers-that-are-multiples-of-160-when-shooting-dslr-video/

Of course this is another camera and it is related to video but I think that this supports the thesis that lowest ISO isn't always the best. (Which is most definitely true when also comparing with ISO 50)

There is also this video that shows the same as in the article but the person who uploaded this says that it is mostly with video and jpeg and not with RAW-files.

https://www.youtube.com/watch?v=LDsmVzp7p-w&feature=youtu.be

Marsu42

  • Canon EF 600mm f/4L IS II
  • **********
  • Posts: 6399
  • Canon Pride.
    • Der Tierfotograf
Re: Just found ISO100 won't clip normally
« Reply #2 on: February 01, 2015, 03:14:02 AM »
This test with a 60D tells that ISO 160 is better than ISO 100 which is better than ISO 125.

This is because iso160 is iso200 shifted, and because of Canon's read noise problem this has the weird effect of resulting in a bit more dynamic range. After many trial&error, I've personally chosen not to hassle with the tiny difference as lower analog iso values have better color fidelity.

So go to ISO125 and I can just clip the image (get 4095), but also get about 25% less readout noise.

That's weired because generally iso125 has iso100 as its base value and this pushing 1/3ev pushing usually makes things worse. Obviously on your old 30d, the image processing chain is older/different than on current camera bodies, Canon did a lot of work here recently.


mrsfotografie

  • 1D X Mark II
  • *******
  • Posts: 1613
  • M.R.S. Fotografie www.mrsfotografie.nl
    • MRS fotografie
Re: Just found ISO100 won't clip normally
« Reply #3 on: February 01, 2015, 04:34:04 AM »
I learned about the effect of the in-camera pushing/pulling base iso's years ago when I still had a 40D.  This was particularly interesting in the high iso range because it taught me I might as well use iso 1250 instead of iso 1000 when I needed sufficient shutter speed. And iso160 became my base iso except when I required longer shutter speeds. With my current camera bodies I don't worry about this anymore.
« Last Edit: February 01, 2015, 04:38:18 AM by mrsfotografie »
5D3, 5D2, Sony α6000, G16 | EF: SY14/2.8, V20/3.5, 28/2.8 IS, Ʃ35/1.4, 50/1.8, Ʃ50/1.4 EX, 100/2.8L IS Macro, 16-35/4L IS, 24-105/4L IS, 70-200/2.8L IS II, 1.4x II, 2.0x III, 70-300L IS, T28-300 Di VC PZD, Ʃ150-600 DG OS HSM S | E: SY12/2, 35/1.8 OSS, 16-70 ZA OSS, 55-210 OSS, SY300/6.3 ED UMC CS

rfdesigner

  • Canon 7D MK II
  • *****
  • Posts: 517
Re: Just found ISO100 won't clip normally
« Reply #4 on: February 01, 2015, 09:54:41 AM »
Well I just measured the read noise and well depth. (gain's a little hard to determine due to debayering, measuring raw (undebayered) pixels I get different gains for R vs G & B).

(if anyone can point me in the direction of how to post tables I'd be greatful..   things like <table> etc don't work on this site)

ISO   Gain   Readout noise   max pixel value   max (e)   readout nosie (e)   DR   Tonal Range
3200   0.259375   9.2   4095   1062   2.38625   8.80   5.03
1600   0.51875   4.6   4095   2124   2.38625   9.80   5.53
1200   0.6916666667   3.6   3284   2271   2.49   9.83   5.57
1000   0.83   4.2   4095   3399   3.486   9.93   5.87
800   1.0375   3.2   4095   4249   3.32   10.32   6.03
625   1.328   2.5   3283   4360   3.32   10.36   6.05
500   1.66   3.3   4095   6798   5.478   10.28   6.37
400   2.075   2.5   4095   8497   5.1875   10.68   6.53
320   2.59375   2   3283   8515   5.1875   10.68   6.53
250   3.32   3   4095   13595   9.96   10.41   6.87
200   4.15   2.3   4095   16994   9.545   10.80   7.03
160   5.1875   1.8   3283   17031   9.3375   10.83   7.03
125   6.64   2.9   4095   27191   19.256   10.46   7.37
100   8.3   2.2   3521   29224   18.26   10.64   7.42

Looks like the main ISOs are not worth deviating from, and that my assumtion that more ISO would reduce read noise only happens in steps (as pointed out by other posters..  many thanks)

« Last Edit: February 01, 2015, 10:02:44 AM by rfdesigner »
6D, 30D, 383L+, 28f1.8, 50STM, 100f2.0, 5.5"f8 scope, 12"f5 scope.
Personal Gripe:   "I could care less about.." = there are things less important too me, this may be my most important thing.
"I couldn't care less about.." = this is the bottom of my list.. it is so unimportant.

mrsfotografie

  • 1D X Mark II
  • *******
  • Posts: 1613
  • M.R.S. Fotografie www.mrsfotografie.nl
    • MRS fotografie
Re: Just found ISO100 won't clip normally
« Reply #5 on: February 01, 2015, 11:09:22 AM »
(if anyone can point me in the direction of how to post tables I'd be greatful..   things like <table> etc don't work on this site)

Take a screenshot and upload an image file? ;)
5D3, 5D2, Sony α6000, G16 | EF: SY14/2.8, V20/3.5, 28/2.8 IS, Ʃ35/1.4, 50/1.8, Ʃ50/1.4 EX, 100/2.8L IS Macro, 16-35/4L IS, 24-105/4L IS, 70-200/2.8L IS II, 1.4x II, 2.0x III, 70-300L IS, T28-300 Di VC PZD, Ʃ150-600 DG OS HSM S | E: SY12/2, 35/1.8 OSS, 16-70 ZA OSS, 55-210 OSS, SY300/6.3 ED UMC CS

Actionpix

  • Canon AE-1
  • ***
  • Posts: 52
Re: Just found ISO100 won't clip normally
« Reply #6 on: February 01, 2015, 11:30:17 AM »
Nice to see this. For years, with Canon, when I use higher ISO, I use steps of 160, 320, 640 etc and now I it gets little confirmed what I suspected. Like this. :-)

canon rumors FORUM

Re: Just found ISO100 won't clip normally
« Reply #6 on: February 01, 2015, 11:30:17 AM »

WIDEnet

  • PowerShot G1 X II
  • ***
  • Posts: 41
  • Where does this go? Oh, that's where.
Re: Just found ISO100 won't clip normally
« Reply #7 on: February 01, 2015, 11:41:55 AM »
If you really want to learn more about how ISO, DR, and SNR really work on Canon sensors, take a gander at this thread on the ML forum; it's probably the most authoritative source out there on the internals of the Canon RAW image readout and processing pipeline I know of.
A camera or two. Some lenses. An old flash. Lots and lots of photos.

rfdesigner

  • Canon 7D MK II
  • *****
  • Posts: 517
Re: Just found ISO100 won't clip normally
« Reply #8 on: February 01, 2015, 12:38:15 PM »
(if anyone can point me in the direction of how to post tables I'd be greatful..   things like <table> etc don't work on this site)

Take a screenshot and upload an image file? ;)

I found the solution..  I run NoScript, and occasionally it gets set back to zero..  at which point I lose all the little extras here..  like the table button.
6D, 30D, 383L+, 28f1.8, 50STM, 100f2.0, 5.5"f8 scope, 12"f5 scope.
Personal Gripe:   "I could care less about.." = there are things less important too me, this may be my most important thing.
"I couldn't care less about.." = this is the bottom of my list.. it is so unimportant.

canon rumors FORUM

Re: Just found ISO100 won't clip normally
« Reply #8 on: February 01, 2015, 12:38:15 PM »