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RAW and ISO and Processing -- Is there a difference?
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Apr 30, 2019 16:01:30   #
CatMarley Loc: North Carolina
 
Bobspez wrote:
Setting the iso and aperture and shutter speed in the camera to get as close to a good exposure as you can is best. If I increase exposure in post processing it seems to add noise. For underexposed pics I find the Photoshop auto tone can often correct the exposure with minimal noise added. If not, then brightness/contrast and shadows/highlights in Photoshop works well.


Some sensors are ISO independent, and PPing the raw underexposed files up to about 5 F stops does not increase noise. Other sensor/processors are more noise sensitive. Also some PP software is better than others.

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Apr 30, 2019 16:20:03   #
boberic Loc: Quiet Corner, Connecticut. Ex long Islander
 
I allways try to get the best exposure in camera. I try to use the lowest ISO that gets me to that goal. Do I always get it right, Of course not. But if I get close then fiddling swith the image in post is that much easier.

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Apr 30, 2019 16:54:26   #
CHG_CANON Loc: the Windy City
 
boberic wrote:
I allways try to get the best exposure in camera. I try to use the lowest ISO that gets me to that goal. Do I always get it right, Of course not. But if I get close then fiddling swith the image in post is that much easier.


Our OP asked a generic question. The technical discussion that followed in the responses is applicable for certain high-end models from certain manufactures. If you don't have an ISO invariant camera, those aspects of the discussion are not relevant to your usage of your specific camera.

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Apr 30, 2019 18:30:42   #
DrPhrogg Loc: NJ
 
jlg1000 wrote:
The ISO value of a sensor is a fabrication which was made in the 90's (I believe by Kodak) to facilitate the transition of photographers from film to digital and still sticks around. Computer vision system cameras do no declare ISO values, because they are of no use to calculate the correct illumination and exposure values to feed the RAW data to some algorithm..


I don't follow most of your post, not because of error but because it doesn't impact me. However. ISO was not Kodak and was not part of the digital revolution. It was first established by the International Organization for Standardization in 1974 to unify ASA and DIN.

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Apr 30, 2019 19:12:08   #
jlg1000 Loc: Uruguay / South America
 
DrPhrogg wrote:
I don't follow most of your post, not because of error but because it doesn't impact me. However. ISO was not Kodak and was not part of the digital revolution. It was first established by the International Organization for Standardization in 1974 to unify ASA and DIN.


You are right of course.

Yet the original ISO standard related to the effects of exposure on a chemical emulsion. When Kodak introduced the digital sensor, the standard was no longer applicable and had to be updated.

I really don't recall who had the initiative to create an ISO committee to write an update ISO (the 12232) so the new sensors would yield somewhat comparable results to the chemical films... I heard that it was Kodak who encourage the ISO organization to summon to existence that committee, but I'm not sure.

Maybe a member with a better memory can help with that historical matter.

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May 3, 2019 20:30:34   #
bonjac Loc: Santa Ynez, CA 93460
 
jlg1000 wrote:
I'll elaborate a bit on my previous post.

All camera sensors have one and only one sensitivity, which depends on the Quantum Efficiency, the Full Well Capacity of each pixel and the Conversion Noise.

The ISO value of a sensor is a fabrication which was made in the 90's (I believe by Kodak) to facilitate the transition of photographers from film to digital and still sticks around. Computer vision system cameras do no declare ISO values, because they are of no use to calculate the correct illumination and exposure values to feed the RAW data to some algorithm.

To attain some (made up) ISO value, the camera performs a two stage process:
1) Amplification of the analog signal produced by each individual photodiode in the sensor
2) Normalization, which means multiplication by some numerical constant

There is a practical limit to amplification which is dictated by the maximum signal amplitude that the ADC converter can manage. Beyond that, only normalization is possible.

*BUT* by normalizing, the camera is doing exactly the same procedure than increasing the exposure value in any post processing software (Lightroo, CaptureOne, etc.)

So we can divide the ISO value in two ranges:
1) LOW ISO: Less than ISO invariance threshold (amplification dominates)
2) HIGH ISO: More than ISO invariance threshold (normalization dominates)

The HIGH ISO range is only of interest when shooting JPEG, because all the Post Process is done in camera, and a final developed image is produced. There is no intention to perform PP at a later stage, so the product is final.

But when shooting RAW, using HIGH ISO is of no use whatsoever, because PP in camera does the same operation than PP in a computer.

If you are shooting RAW, of course your are *planning* to perform some Post Process, in which case it is better to never ever use HIGH ISO, because:
1) Trying to conform the exposure triangle, HIGH ISO will probably blow up the highlights
2) HIGH ISO will create noise which could be better handled by a full blown PC than a the small CPU in the camera.

In the PC you can manually enhance the shadows and the midtones without burning up the detail in the highlights. You can create masks (Capture ONE, ON1, PS, Affinity, etc.) and reveal detail of select features of the photograph without introducing noise in the rest. You can then dampen the noise only in the areas where it is visible, without softening the whole photo.

I'd recommend always to stop at the ISO invariance threshold and then PP. Then you-ll have all the headroom to perform PP as you wish... Just set the maximum ISO at that value.

This link has the ISO invariance threshold for most cameras:
http://www.photonstophotos.net/Charts/Sensor_Characteristics.htm

Just click on your model or look in the table at the bottom of the page. The "Unity ISO" will be the threshold value which divides the LOW and HIGH ISO ranges. Just don't increase ISO beyond that value if you are shooting RAW and plan to post process later.

Hope this clarifies the subject.
I'll elaborate a bit on my previous post. br br A... (show quote)


I'm the OP for this topic. I over much appreciate your time in responding. I've done some reading and looked at the DXO data. In that data I noted there are a UNITY ISO, READ NOISE, and ISO INVARIANT values. From what I have read, if the read noise is below 5 that is a good sign. When I read your post, you say that setting ISO at UNITY ISO is a point that distinguishes between amplification and normalization. So, presumably you go beyond that point, as you state, and would use PP from there. But, what is the ISO VARIANT? I can figure out what meaning is has or it's utility. Elsewhere, you state stopping at the ISO VARAIAnce. Are specifying a range for ISO settings between UNITY ISO and ISO VARIANT? When I look at the DXO data, it is a puzzle in the sense that there ae situations where the VARIANT value is less than the UNITY value. Can't wrap my head around that. I know I am asking to spend more time on this and there are probably a lot of things you would rather be doing. Nonetheless, any time you give this would really be appreciated.

Regards, Jack

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