The Myth of the Exposure Triangle

Capturing
38

And I doubt there is a practical one. The right ISO 800 image seems to be a bit more saturated.
Assuming you increased the exposure of the left ISO 64 image in a program, how does its algorithm work? If it only increases a luma channel then it would explain the difference.
Would it be the same result if you use another program or another method (brightness, exposure, gamma or whatsoever)?
If you’re precise, both images are not comparable.

But in reality you would just move a ‘saturation’ slider some more pixels in the left picture and won’t have a remarkable difference.

39

@pphoto

I just set black and white point in the histogram, which was perhaps not completely identical for both images. Thus the appearance of a slight difference in saturation. That was all for both images. So I did not change the pixel values – in contrast to the procedures you mention (brightness, exposure, gamma …)!

Concerning the issue with ISO I want to quote from the book by Tooms (2016): “Colour reproduction in electronic imaging systems”: On page 407 he writes: “The image sensor will have an opto-electro-conversion factor (OECF), which is normally linear and will output its signal to variable gain amplifiers which are adjusted for minimum gain, consistent with providing a standard level signal after the desired adjustments to the iris and shutter speed exposure settings. The gain adjustment is usually calibrated in terms of the ISO rating, which in turn is related to the ASA film sensitivity or speed rating familiar to traditional photographers.”
This confirms that the signal is just multiplied by a gain factor, which in turn does not change the SNR.

I also found a webpage, confirming what I just wrote. Scroll down to the part about “Common ISO Myths and Misconceptions” and read the paragraph “Is ISO “Sensor Sensitivity”?” The fundamental difference to the ISO/ASA of a photographic film is, that a film with high ISO does indeed have a higher sensitivity. If you raise the ISO setting of your camera, it does not increase the sensitivity of the detector.

However, one never knows what the firmware in the camera does in detail, if one raises the ISO setting. Thus it may well be, that due to some clever algorithm the image with high ISO setting does look somewhat better than what you get with a lower ISO setting and some image processing with the software of your choice.

Hermann-Josef

40

I suspect the SNR comparison depends on the camera model (which includes the physical sensor, any analogue gain, the ADC, any digital gain, and whatever post-processing is done. Especially any noise-reduction, obviously.

I don’t know exactly how my camera works. When I twiddle the ISO dial, what changes? Is there a change in analogue gain, or what?

This is somewhat frustrating, but I never fully understood how film worked, even just B&W, let alone colour. I think my D800 uses analogue gain for different ISO, up to the “extra” ISO called H1, H2 etc which are digital. But I could be wrong.

Personally, I’m not much bothered about what the theory says should happen. Instead, I look at what actually does happen. For example, an out-of-focus gray card, under office lighting (LED), constant exposure 1/800s f/2.8, at ISO 6400 and ISO 100:

set AREA=3500 2500 300 300
set CROP=-crop 300x300+3500+2500

%DCRAW% -v -A %AREA% -6 -o 1 -T -O 4398.tiff AGA_4398.NEF
%DCRAW% -v -A %AREA% -6 -o 1 -T -O 4402.tiff AGA_4402.NEF

%IMG7%magick 4398.tiff -strip %CROP% -auto-gamma +write isonse_4398.png -colorspace HCL -format "%%[fx:mean.g]\n%%[fx:standard_deviation.g]\n" info:
%IMG7%magick 4402.tiff -strip %CROP% -auto-gamma +write isonse_4402.png -colorspace HCL -format "%%[fx:mean.g]\n%%[fx:standard_deviation.g]\n" info:

The numbers are:

ISO 6400:
0.156747
0.0983761

ISO 100 (given post-proc gain):
0.170328
0.132537

The numbers show that post-proc gain has higher saturation (mean chroma), and a wider variation (standard deviation of chroma).

In real photography, I use dcraw’s noise reduction for high ISO, which improves those images.

So, if post-proc gain is noisier, do I avoid it? No. On the contrary, I generally set the camera to auto-expose with exposure compensation up to -2 stops. I deliberately underexpose to avoid clipping highlights, then correct with post-proc gain. For me, some extra noise is a price worth paying to avoid clipping.

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41

@snibgo

This is what is happening, according to what I understand and read. However, this does not change the SNR, which is the main issue.

What do you mean by that?

I think one should be careful in the discussion to separate theory and praxis. My emphasize was on theory in order to understand what is going on. The practical part are the personal preferences.

If the variance in the higher ISO image is lower, then this could well mean, that the firmware is applying some algorithm to reduce noise (e.g. wavelet filtering). I doubt that the manufacturers will tell us exactly what they do.

Hermann-Josef

42

@Jossie: by “post-proc gain” I mean afre’s “gain via post-processing”, ie setting a low ISO such as 100, then multiplying the image in the computer, after the work in the camera.

The other case is “gain via ISO”, where we set a higher ISO eg 6400 in the camera, so the camera does the multiplying by some secret process that may include some smoothing or other noise-reduction.

Yes theory is useful. And so is testing.

43

Adding additional analogue gain will add noise (and possibly non-linearity) to the signal. Digital gain will increase quantisation noise in the signal. A certain amount of noise on the input to the ADC is a good thing as it will dither away the effects of quantisation.

Which one is best will depend on the details of the signal processing hardware…

I believe most cameras end up doing a combination of both.

44

Assuming you keep same exposure for two photos of same scene , one taken at ISO 6400 and one taken at ISO 100.
Knowing that the analog amplification is done as far upstream as possible, the ISO 6400 image is theoretically less noisy than the digital amplified ISO 100 image.
In case the noise introduced by amplifier and downstream electronics (mainly ADC) is sufficiently small, it could be negligible versus the upstream noise. Then , the two images above (ISO6400 and ISO100X64) will present the same noise level.
I think In-camera digital gain (extended ISO) is of no value if you process raw file.

45

Is there any domain, where the digitization noise is comparable or larger than the shot noise?

@gaaned92

See comparison above.

If the camera is well designed, the shot noise should always be the limiting factor.

Hermann-Josef

46

But the sensitivity of the system. It might be true that

But it is always wise to amplify early to not amplify later noise sources. If there are no highlights clipped, it is wise to amplify the sensor’s signal to best utilize the AD converter’s dynamic range, that means, ETTR. And reasonable ISO settings would do that.

If it is true that some cameras do not change this first amplification’s gain based on ISO setting, there may be a possibility to implemet ISO invariance in the sense that there would be no difference if I set ISO in-camera or boost exposure later. However, there are still some noise sources along the way, analog and digital ones, in camera and in the raw procesor, until the signal gets to the exposure setting in the latter. However it is implemented in-camera, I clearly see advantage in setting the ISO correct. The only way I see how this could be changed is some auto-gain in the camera. Maybe that’s how these ISO invariant cameras are implemented? One could easily check this with an underexposed raw file.

47

I was explaining @snibgo observation.

I suppose that for professionnal astronomy sensors, to fight thermal electronic noise, you can cool sensor and the electronic devices with some coolant (liquid helium for instance) and use special electonic , ADC and sensors designs…
In this case, it is possible that the only noise you have to take into account is the photon shot noise.
Your system is thus iso-less

It’s not at all the case for commercial camera (see also Emil Martinec paper). And in certain situations, camera electronics can be very hot thus generating a lot of thermal noise.

Thats surely mandatory for astronomy systems, but not for commercial cameras where the upstream (front end ) noise includes other noise sources.(see Emil martinec paper)

In this case your camera is ISO-less. That’s not always the case for commercial cameras at low ISO but becomes rather true beyond some ISO depending on the camera.

About ISO invariance see

and
http://www.photonstophotos.net

2 Likes
48

This is actually funny. It is clearly visible that the image boosted from ISO 400 has much worse magenta-type noise as expected. Examine carefully the shadow below the painting at 7:03.

49

Yes, astronomical CCD cameras are cooled by liquid nitrogen (He is not necessary) to about 150K and the electronic parts are in most cases selected for minimum noise. However, the photon flux levels are orders of magnitude less than in photography. Exposure times (relevant for dark current and associated noise) are also on the order of up to an hour, instead of 1/100 of a second. If you look at Fig. 13 in Martinec’s work you see that at average illumination the shot noise in digital SLRs is, as I had written, the dominant part.

The discussion is going now in a somewhat different direction from the original topic. But still the bottom line here is that ISO does not change the sensitivity of the sensor but only the gain, i.e. the amplification factor of the voltage, delivered by the sensor.

Hermann-Josef

3 Likes
50

Unfortunately, my electronics knowledge has atrophied somewhat since I left university 30+ years ago and started writing software, so I’m really out of date on the specifics.

51

Others correct me if wrong, “digitization noise” is probably more specifically termed “quantization error”, where a specific analog value has to be shunted to the nearest digital quantity. N’est ce pas?

If so, then the domain of interest would be where the destination bit depth becomes “comparable” to the error magnitude characteristic of the shot noise… ??

Thinking out loud…

52

I think this is correct, it is a “rounding error”. So this may only be relevant for 8bit/pixel data and for those in the darker parts, I would conclude.

Hermann-Josef

1 Like
53

That’s my surmise also, but I’m not sure how I’d quantify it.

54

Quantization error, aka rounding error, is discussed at http://theory.uchicago.edu/~ejm/pix/20d/tests/noise/index.html#quanterror :

It is also relevant when deliberately underexposing, then multiplying in the computer (“post-proc gain”). In my post above, the ISO 100 photo is underexposed by 6 stops. The camera records 14 bits/channel/pixel, so the top 6 bits are zero, so the camera is using only the bottom 8 bits. The raw files are effectively 8-bit. Of course, they are linear.

This causes some noise. Worse, we can expect severe banding in the shadows. Is this banding worse in post-proc gain, or in ISO gain? I haven’t tested this.

55

The only shadow banding posterization I’ve experienced was due to big-integer → little-integer colorspace conversion to my display profile. Vexed me for a bit, as the output JPEGs always looked fine. Went away when I refactored my display color conversion pipeline to go straight from the internal float to the 8-bit integer for display.

Noise has been my challenge, just as you describe. My new camera has a highlight-weighted matrix metering mode, which apparently places the brightest parts at middle gray. I like that images exposed with that mode don’t have saturated channels, but if there’s a high dynamic range in the scene, those dark parts pulled up with tone curves can be noisy.

Next experiment is to shoot that metering mode wtih a +1EV exposure compensation. Some say I’ll blow out some images, but I’m not sure; in my last locomotive shoot I took pairs of exposures, one with regular matrix, quickly followed by one with HW-matrix. The EV differences of the pairs ranged from 2 to 5, but in all the HW images the bright parts were at least a full stop below saturation.

Edit: banding posterization, following @paulmiller… :smile:

56

Actually, the noise helps to hide the posterisation in the same way that dithering on a ink jet print makes smooth looking tones from only 1-3 possible ink droplet sizes.

If the ISO gain in the camera is applied as an analogue gain stage before the DAC, there isn’t any extra posterisation at all - just more noise.

Edit 1: Aargh… ADC not DAC!

Edit 2: I mean posterisation, not banding. The dither effect only works if the noise is uncorrelated with the location on the sensor. If you have pattern noise (which shows up a streaks or stripes in the shadows) or hot pixels, digital scaling will make it more visible. Analog gain will probably make it more visible as well, depending on where in the system the pattern noise is being generated.

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57

Sorry if I’ve caused confusion. Yes, representing an input range of values (eg photon counts) by a small range of numbers (typically integers, or some other quantization) reduces the number of distinct values, and we call this “posterization” (or perhaps “posterisation”).

However, the effect is typically not noticable except where the input range represents a gradient. A gradient input is transformed by posterization into visibly distinct bands. In linear encoding, the effect of posterization is strongest in the shadows. Shadow areas often have a strong gradient component, so we get “banded shadows”.

Yes, adding noise before the posterizing (or even after) will reduce or eliminate banding, thus concealing the posterization.