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neek
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PostSat Jul 18, 2020 9:12 am 
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Can't believe I misunderstood this for so long (due to being lied to by almost everyone), and figured others might be in the same boat.

ISO has nothing to do with sensitivity (we're talking digital here, not film).  That's a fixed property of the sensor.  Also, a higher setting with everything else held constant does not introduce noise.  ISO is simply a post-exposure amplification factor.  Noise is mostly a function of how many photons hit the sensor.  Not that the goal is ever simply to minimize noise, but if it were, you'd just want to maximize the amount of light you let in, via aperture and shutter time, without clipping.  Almost.  One small detail: post sensor electronics actually introduce a small amount of noise, and you want the ISO high enough to minimize this.  The optimal setting varies per camera but is generally around 1600.

To prove this to myself, I took a series of shots, varying only the ISO, then adjusted curves to match.  By a subjective assessment of noise, can you tell the difference between ISO 200 and 3200 without looking at the filename?  Both are f/9, 1/2 sec, 1:1 crops when viewed at full 1600x1600 size.  The subject is the crust of some sourdough bread.


If you're taking this to mean higher ISO is better, you've misunderstood.  The point is more subtle than that.  For sure, unless you really know what you're doing, keep your ISO as low as possible, as you most likely have been doing.  Otherwise you're just asking your camera to starve the sensor of photons.  All this is really only relevant in astrophotography and other cases where you're trying to make the most of extremely low light situations.

Please let me know if my understanding needs any refinement.

Reference: https://clarkvision.com/articles/nightscapes/
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Cyclopath
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PostMon Jul 20, 2020 12:58 pm 
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I agree sensitivity is determined by the CMOS/CCD itself, and that ISO is handled by software at some level.

You're "basically" asking to double the photon count from each sensor pixel when you double the ISO.  In theory you can go the same thing in post processing (curves, layers) but if you're starting from jpeg files you only have 8 bits whereas the camera has 12.
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Justus S.
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PostWed Jul 22, 2020 2:16 am 
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Interesting…Took me a bit to see what was being said, and I’m still not sure I understand this. I agree with what your are saying. This is my novice take.

Yes, sensitivity appears to be fixed based on the physical properties of the sensor etc… However, there is still lots of hardware/processing after the sensor that can contribute to image quality.

ISO I think is basically a definition of light exposure over a specific time based on a target brightness goal. For ISO settings this is my take on what might be going on, and I think they play with both hardware and processing to achieve the end goal.

On the hardware front to get the best performance in very low light you need to increase ISO or gain to a certain level. Above a certain ISO it doesn't really buy you anything. This analog gain allows the system to resolve smaller levels of signal and overcome some system/read noise. However, this low light benefit comes at the expense of two things in the out of camera image.
1: Overall dynamic range will be reduced in hardware. This means you get a flatter image and for daytime imaging often means you either saturate or through out darks. For nighttime you have fairly low dynamic range so there is benefit.
2: A raised visual appearance of the noise floor appears due to increased analog gain and signal processing gain(also perhaps dynamic range compression artifacts). Post capture signal processing is thus required to get this “digital noise” out and if your captured dynamic range is low things just don’t look good and will always appear noisy or look flat. Note:If using JPEG you also reduce your post processing abilities. Side note: to really reduce noise you need frame averaging/stacking.

IMO…The best daytime image will still always be to capture the highest dynamic range “lots of light” hence you use low ISO settings and change your aperture/shutter settings to maximize light.
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Cyclopath
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PostWed Jul 22, 2020 9:17 am 
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Because the "ISO isn't really sensitivity, that's fixed" idea isn't super intuitive, here's a quick digression on how digital cameras work:

If your camera has X megapixels, that means it has a physical chip inside with X million photon wells.  During an exposure they all capture incoming light, and turn that into a visual image.

Everybody knows bigger pixels (physically larger wells) are better because they capture more light.  Those wells have walls between them to prevent leakage, so they have lenses over them.  The shape and quality of those lenses also helps determine how much light gets into each pixel.

You can't change the sensitivity without making the pixels bigger, changing those lenses, etc.  Sensitivity is a hardware thing, how efficiently can it capture available light?
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InFlight
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PostWed Jul 22, 2020 3:27 pm 
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If one considers a single pixel (red, blue or green); it will accumulate an electric charge (photo-electric effect) based on the shutter speed and aperture.

Larger pixels can accumulate more charge, but the trend to higher pixel counts results in smaller (and somewhat nosier) pixels.

In creating the photo, each pixels charge is read.  The ISO adjustment is literally the amount of amplification that is applied to the pixel charge before it is converted in the Analog to Digital Convertor.

Based on the Pixel size and sensor semi-conductor type; the ISO number to amplification curve is a sensor specific curve.  The camera's EV adjustment simply alters this amplification curve.

The photograph light levels can obviously be easily altered in post processing. It's hard to fix low light noise; so it's it best to limit the allowed ISO range and deal with shutter speed that falls out of your preferred depth of field (aperture).  (Low Light - Aperture Priority with ISO Limited to 800)

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OutOfOffice
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PostThu Jul 23, 2020 2:50 pm 
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So, ISO is not changing the actual hardware's sensitivity, but is amplifying the effect after the image is taken?

In other words, its an "artificial" change to the sensitivity (if that makes sense?). Its not changing the amount of the sensor is absorbing, but its digitally altering the effect afterwards.

This is my 2 min read through understanding. Need to read the source article. Interesting post. Thanks
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Bedivere
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PostMon Jul 27, 2020 11:58 pm 
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OutOfOffice wrote:
So, ISO is not changing the actual hardware's sensitivity, but is amplifying the effect after the image is taken?

In other words, its an "artificial" change to the sensitivity (if that makes sense?). Its not changing the amount of the sensor is absorbing, but its digitally altering the effect afterwards.

This is my 2 min read through understanding. Need to read the source article. Interesting post. Thanks

This is my takeaway from this as well.  Actual sensitivity is fixed, ISO is just the amount of gain or boost applied to what the sensor actually captured.

I can say one thing - ISO performance is sooooo much better in newer cameras.  My first DSLR was a Nikon D200.  10.2 mp sensor.  That camera took beautiful pictures in good light but when it started getting dark the images got noisy quick.  Anything above ISO 800 was essentially useless.  Next camera was a D7000. Same size sensor, 16 mp, so smaller photo-sites/pixels/wells/whateveryouwannacallem but either the sensor itself was much more sensitive or the algorithms used to boost the signal from the sensor were a LOT better (or both) because that camera's low light performance blew the D200s out of the water.  Usable pictures up to ISO 3200 when lighting was good, but still pretty noisy beyond that and even above 1600 things were noisy if you were pushing it.  Present camera is a D7200 and it's another step better.  still the same size sensor but 24mp, so smaller pixels still than either of the previous two, yet it's quite capable of capturing relatively clean images at ISO 6400 if lighting is good enough for perfect or slight over-exposure.  Now the D500 is out and I've been salivating over that.  Reports are that it will match my camera's performance at ISO 12800.  Lower resolution, 20mp, so not much lower though.  Or, I could step up to full frame with a D750 or maybe migrate to the new Z mirrorless architecture and get even better performance.

The processing that's available now for low-light images is pretty mind blowing, too. Phone cameras are able to produce nice pictures with good saturation and even gradients in low light and they have tiny sensors and very basic lens structures.

I guess it's a good time to be alive if you like digital cameras that can produce good results in less than optimum light.

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