ISO Woes

[jaapv]

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Yes, and the point is that cranking up the ISO doesn’t alter the exposure (amount of light falling on the sensor). To do that you must change the only the two exposure parameters: shutter time and aperture. 

[jkcampbell2]

Very interesting discussion. This has peaked my curiosity so I did run a quick test. Below are crops of two M10 files. One shot at ISO 3200, the other at the same exposure setting but at ISO 200 and then corrected in Lightroom. First off, amazing shadow recovery without blowing out the highlights or mid-tones.  I do see a bit more noise in the corrected image vs the one shot at 3200 but you have to look close. I'll assume you'll see even less difference if the exposure was say 1600 or 800 and even more difference if you go up to a 5 or more stop difference. I'm not sure I'd run this way all the time but its nice to know that if needed you have that much flexibility. Cheers, jc

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M10 ISO Test by John Campbell, on Flickr

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[evikne]

2 minutes ago, jkcampbell2 said:

Very interesting discussion. This has peaked my curiosity so I did run a quick test. Below are crops of two M10 files. One shot at ISO 3200, the other at the same exposure setting but at ISO 200 and then corrected in Lightroom. First off, amazing shadow recovery without blowing out the highlights or mid-tones.  I do see a bit more noise in the corrected image vs the one shot at 3200 but you have to look close. I'll assume you'll see even less difference if the exposure was say 1600 or 800 and even more difference if you go up to a 5 or more stop difference. I'm not sure I'd run this way all the time but its nice to know that if needed you have that much flexibility. Cheers, jc

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Simply register for free here – We are always happy to welcome new members!

M10 ISO Test by John Campbell, on Flickr

Exactly!

[adan]

1) it looks like we scared off the original poster - we still don't know what his "ISO woes" were.

2) A digital sensor is a pretty simple analog device. It captures photons of light and outputs an analogous number of electrons, which are read out as a charge (CCD) or a voltage (CMOS). Those are measured to produce the brightness values assigned to each pixel of the image. It doesn't matter what ISO is set - the sensor puts out a constant value of electrons based on the inflow of photons received per pixel. 1 photon = 1 electron (more or less) regardless of whether the ISO is set to 200 or 6400.

3) There is (generally speaking) no way to force the sensor to put out a different value - it can't be "amped up" with more or less power, for example. But the identical signal it puts out at ISO 200 or ISO 6400 can be "amped up" farther down the digital imaging chain.

4) When you change the ISO on a digital camera, you are doing two things:

4A) you are telling the camera processing electronics (not the sensor) to amplify the signal after exposure. Not unlike push-processing film - the weak signal gets multiplied after the fact to produce more brightness than the exposure would normally produce at "box speed" (or "base ISO"). And 4B ) you are telling the metering system to compensate for that amplification (which in turn tells you reduce the exposure via shutter and aperture).

As jkcampbell2's example shows, there is not much difference between the camera amplifying the signal (by setting the ISO dial to 3200) or the photographer amplifying the signal in Photoshop/ACR/LR later on. It's all just computer math - a brightest pixel value of 15 gets multiplied 2^4x (16x) to 240, along with a certain loss of quality (grain with film, noise with a digital signal).

However.....

1 hour ago, jaapv said:

Yes, and the point is that cranking up the ISO doesn’t alter the exposure (amount of light falling on the sensor). To do that you must change the only the two exposure parameters: shutter time and aperture. 

...or use a neutral density or other filter. Or add light to the scene, with anything from car headlights to a flash to a full Hollywood lighting setup. Or simply chose a brighter or darker scene to begin with. All of those also determine the amount of light falling on the sensor or film. There are not "only two exposure parameters" - and never really have been.

Box Brownies and similar came with a fixed shutter speed ("Instantaneous" - somewhere around 1/30th sec). Many came with a fixed aperture (f/11 or so). About the only thing you could change was the ISO used - generally something in the ISO 125 range like Verichrome Pan, but eventually faster (and slower) films. Nevertheless, billions of family snapshots were made with such devices.

The fact that one can get a "bad exposure" by using the wrong ISO, or the wrong filter - just as easily as using the wrong shutter speed or aperture - shows that ISO is just as much a determinant of correct exposure as the others. Exposure is not just the amount of light falling on the sensor - it is the correct final range of tones in the finished picture.

I want to make a picture. I want to use a specific shutter speed (for motion control, not just exposure and amount of light). I want to use a specific aperture (for DoF control, not just exposure and amount of light). ISO becomes another variable I can use, just like changing the lighting, or adding an ND filter, to make the exposure correct.

I can walk around all day with a camera set to a fixed ISO (with film cameras, I have to). And vary the aperture and shutter speed to get a correct exposure. Or set the camera to "A" and let it vary the shutter speed (with a fixed aperture - the old photojournalists' mantra, "f/5.6 and be there").

But these days, I can also walk around all day with a camera set to a fixed aperture and shutter speed (say, f/4 and 1/2000th sec.). And let the camera automatically vary the ISO to get correct exposures everywhere I go. ISO has a more limited range (8 stops with the M10 Auto ISO, from 200 to 50000), while using auto-shutter gives me at least 12 stops (1/4000-1 sec) and 19 stops if I use shutter and aperture together. But that's just a question of degree - doesn't change the underlying principle.

[jaapv]

Come on, Andy, those are things that happen outside the camera.  You have to alter the camera exposure to compensate for them.

[LocalHero1953]

As an aside......

When Jim Kasson wrote his piece about using a lower ISO in low light (same aperture and shutter speed) and simply boosting the image in post, I tried it on my M240. And it simply wasn't true that I could get the same image quality compared to shooting at the 'correct' ISO. I usually saw more noise in the shadows in the low ISO images. It wasn't a big difference, and often it was easily sorted by shadow noise reduction in post. But there was a difference. My totally ignorant interpretation was that the camera ISO boost occurred at an early stage in sensor output processing. With it, the signal was boosted S/N before later processing introduced more noise; without it, boosting in LR also boosted that noise.  

So as far as I'm concerned, setting ISO right in camera is necessary. 

But what do I know? I'm just a geologist and I hit things with a hammer to find out what's inside.

Edited September 13, 2019 by LocalHero1953

[pedaes]

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28 minutes ago, jaapv said:

Come on, Andy, those are things that happen outside the camera.  You have to alter the camera exposure to compensate for them.

Sorry, jaapv, how is letting the camera  adjust the ISO to get correct exposure "something that happens outside the camera"?

Edited September 13, 2019 by pedaes

[jaapv]

1 hour ago, adan said:

...or use a neutral density or other filter. Or add light to the scene, with anything from car headlights to a flash to a full Hollywood lighting setup. Or simply chose a brighter or darker scene to begin with. All of those also determine the amount of light falling on the sensor or film. There are not "only two exposure parameters" - and never really have been.

 

[lct]

10 hours ago, pedaes said:

Sorry, jaapv, how is letting the camera  adjust the ISO to get correct exposure "something that happens outside the camera"?

Indeed and isos determine exposure the same way as asas even if they are adjusted manually by the photographer.

[marchyman]

23 hours ago, jkcampbell2 said:

 I do see a bit more noise in the corrected image vs the one shot at 3200 but you have to look close.

Not surprising.

 

21 hours ago, adan said:

3) There is (generally speaking) no way to force the sensor to put out a different value - it can't be "amped up" with more or less power, for example. But the identical signal it puts out at ISO 200 or ISO 6400 can be "amped up" farther down the digital imaging chain.

I take minor exception to the word "digital" in that statement.   ISO amplification, at least at the lower values, is done in the analog domain, before A-D conversion.  Some cameras use analog amplification up to some ISO number and then digital amplification for higher ISO values.   I suspect that is the reason behind the noise differences found by jkcampbell2. I don't know specifically what the M 240/262 does.

 

[adan]

True. But I expect many, especially on this forum, might take "analog imaging chain" to mean "film > developer > enlarger > print."

Sometimes, in the telegraphy of the internet, one can sacrifice a bit of precision in favor of comprehension.

(And of course, by "bit" I don't mean a 1 or 0, but a tiny amount - although a 1 or 0 is also a tiny amount).

Edited September 14, 2019 by adan