ISO 50

[TrumpetPower!]

The sensor does not care about iso, iso step is added after the readout.

Actually, that's not true -- and, presumably, at the heart of your misunderstanding.

Analog electronic (not digital) amplification / gain is applied to all ISO settings above the base ISO. It's like turning up the volume knob on your stereo. The readout is performed after that gain has been applied. And, just as your amplifier will start to produce more and more ugly-sounding distortion the louder and louder you crank that knob, your camera will produce more and more noise the higher you boost the ISO.

ISO 50 and highlight tone priority are two special cases. The exact same analog signal amplification (i.e., none) is applied with all three exposures: ISO 50, ISO 100, and ISO 200 w/ HTP. If you use the same shutter speed and aperture, you'll get the exact same raw file in all three cases. What changes is first the camera's metering system and second the digital (not analog) post-processing. That post-processing is quite simple, really...in the case of ISO 50, all the digital values from the initial sensor readout are exactly halved; with HTP, they're exactly doubled. (And, of course, with ISO 100, they're left as-is.) That's all simple integer math, too -- nothing fancy.

Hope that clarifies things somewhat....

Cheers,

b&

 

[Loswr]

the only purpose the sensor has is to collecting photons
agree?

Disagree - it does more. The photon wells collect photons, there is a lot of additional circuitry on a sensor besides just the photon wells.

TrumpetPower is absolutely correct in his statement that, "ISO 50 and highlight tone priority are two special cases. The exact same analog signal amplification (i.e., none) is applied with all three exposures: ISO 50, ISO 100, and ISO 200 w/ HTP. If you use the same shutter speed and aperture, you'll get the exact same raw file in all three cases." (well, almost correct - the base ISO isn't exactly 100 for all sensors, so even at ISO 100, some gain may be applied - but it is the same gain in all three cases). He's also correct about ISO 50 being a linear 50% reduction, but incorrect about HTP being a linear doubling (HTP processing is application of a tone curve to boost the shadows and midtones but not the highlights).

HTP. it is a halving of infaling light= halving of the amount of charged electrons= go from 100 to 200 iso and make a head room, the analog signal is different = 100iso and 200 iso
Agree?

Disagree. When you set ISO 200 with HTP, the camera is actually exposing at ISO 100, which is why ISO 100 cannot be set with HTP on, but (incorrectly) reporting ISO 200 in the metadata and setting the HTP flag. If you open an HTP RAW file in something like Rawnalyze, you'll see it's a stop underexposed - it's not halving the incoming light, it's reducing the analog gain applied to the same amount of light, by one stop.

 

[Loswr]

well then take a look what happening with HTP , 100 iso exposed as a 200iso exposure= under exposed and later adjusted .
not the same signal from the sensor ( and do nor mix apples with bananas this time)

The point is that ISO 50, ISO 100, and HTP ISO 200 are the same analog gain at the sensor, and if the aperture and shutter speed are held constant, the RAW data coming out of the ADC are the same for all three ISO settings.

regarding 50iso and 100 iso= not the same parameters regarding time

You're saying if I set my aperture and shutter in M mode, and then change from ISO 100 to ISO 50, my exposure time or aperture will change? I'd like to see some evidence for that...

 

[Loswr]

Well Neuro, read again what Im answering earlier, Im going to the gym for a hour , we can discuss it later

I don't need to read the same incorrect statements again, thanks. Enjoy your workout!

 

[TrumpetPower!]

He's [...] incorrect about HTP being a linear doubling (HTP processing is application of a tone curve to boost the shadows and midtones but not the highlights).

That makes sense -- thanks for the correction. At a rough guess, starting with the original linear data I'd suggest that the HTP curve is still a linear doubling from full dark until the last stop or two of brightness, and then a smooth curve from there back to unity at the saturation point. In Photoshop, create a new curve with one point, 92 as the input and 184 as the output, for an idea of what it might look like. Then apply white balance and color correction matrix and s-curve for contrast and the rest as usual. At least, that's how I'd do it....

Cheers,

b&

 

[Loswr]

can you boys read?
http://theory.uchicago.edu/~ejm/pix/20d/tests/noise/noise-p3.html

it is hard to discuss HTP etc if you do not know what it means

Can you not be insulting, boy?

 

[TrumpetPower!]

can you boys read?

Yes, we can. Whether you can remains to be demonstrated. From your very link:

By the way, underexposing at lower ISO is precisely what Canon cameras do in the raw data when Highlight Tone Priority (HTP) is enabled; and what Nikon cameras do when Active D-Lighting (ADL) is enabled. Instead of using the ISO gain set by the user, the camera uses a lower ISO (but exposes with the indicated aperture and shutter speed), effectively underexposing the image; this provides more highlight headroom. In post-processing, the image data can be brought back up while preserving the highlights with a modified tone curve in higher exposure zones. The place where image quality suffers is in shadows at lower ISO, precisely as the above quantitative model predicts.

That's exactly what neuroanatomist and I have been writing all along. Instead of using the ISO gain set by the user (200), the camera uses a lower ISO (100) (but exposes with the indicated aperture and shutter speed). In other words, ISO 200 w/ HTP is exactly the same as ISO 100, but with a different tone curve.

b&

 

[Loswr]

No problem admitting when I am wrong. But I'm not.

You stated:

HTP. it is a halving of infaling light

That's flat out wrong. The light is not being halved, rather, as we and the page you linked correctly state, the gain is being halved, relative to the selected ISO value.

Further, you suggested that setting ISO 50 changes time value and/or aperture...and that's wrong, as well.

 

[Loswr]

HTP. it is a halving of infaling light

I think you must study the subject and understand how iso gain works together with full charge and then what it means by 200 iso= halving the number of electrons.

Which are we halving? Light or electrons? Are you going to admit being wrong about the light being halved, or just accuse others of not being able to admit when they're wrong?

Interestingly, I notice that you're consistently ignoring my question about ISO 50 changing aperture/shutter values...

last time I was answering you when you also where totally wrong I was turned off because of my language.it will not happen this time.

Questioning our ability to read, as I stated, is highly insulting, especially the fact that you took the time to edit your post to bold that remark. It seems you have a history of insulting behavior here, which resulted in past consequences and yet continues.

 

[TrumpetPower!]

Once more unto the breach....

The following "Sunny f/16" exposures all result in the exact same amount of analog gain applied to the readout of the sensor, and therefore the exact same raw file:

1/400s @ f/8 @ ISO 50
1/400s @ f/8 @ ISO 100
1/400s @ f/8 @ ISO 200 + HTP

Assuming the camera meter says you're properly exposed for the ISO 100 shot, it'll say you're one stop underexposed for the ISO 50 shot and one stop overexposed for the ISO 200 shot. But, if you ignore the meter and use the same shutter and aperture for all three shots, only changing ISO, you will get the exact same raw file.

...again, with the caveat that the metadata will indicate the ISO you had the camera set to, which will result in one stop of digital (i.e in ACR or DPP or LR or wherever) underexposure for the ISO 50 shot and one stop of digital overexposure for the ISO 200 + HTP shot. That means that, though the raw files are identical for all three, the numerical values recorded in them are divided by 2 for ISO 50 and multiplied by 2 for ISO 200 + HTP before any other processing is done. (It's a linear operation for ISO 50 and a non-linear graduated operation for ISO 200 + HTP.

In contrast, if you were to make the exact same exposure but with ISO 200 without HTP, analog electrical amplification would be applied to the signal before the analog-to-digital converter reads the data, sufficient to double the signal strength. The actual data recorded to the raw file would be different, though it would bear a superficial resemblance to the HTP shot after the initial digital compensation had been applied. Indeed, the regular, non-HTP shot would have more useful dynamic range. Set the ISO to 400 (still without HTP) and even more analog amplification is applied before the ADC digitizes the signal.

There is never a case where the starting point with ISO 50 or ISO 200 + HTP is any different from the exact same shot (same aperture and shutter) at ISO 100. The difference is entirely in the value displayed for the camera's meter and the way the raw file is processed. Any time you shoot with ISO 50 or ISO 200 + HTP, you can do the exact same thing by using the same aperture and shutter as you would at the expanded ISO settings but using ISO 100 instead, and then doing your own digital push or pull in post-processing.

So, why would you want to use either?

If you're shooting JPEGs, ISO 50 is useful when you wish you had a one-stop neutral density filter but you don't.

If you're shooting JPEGs and you care more about the highlights than the shadows -- such as when photographing a bride in a white dress -- HTP will cause the JPEG to render those highlights with more visible detail.

If you're shooting raw and doing ETTR (expose-to-the-right), you're doing the exact same thing as ISO 50. So, you might as well set the camera to ISO 50 and thereby get a preview image on the back of the camera that's closer to your desired final rendering.

If you're shooting raw and you're doing ETTL (expose-to-the-left) in order to capture as much bright detail as possible, you again might as well turn on HTP again to get a more accurate preview.

Lastly, if you're doing a very methodical manual HDR shoot, you might want to consider using ISO 50 for the shadow exposure and HTP for the highlight exposure -- being careful to actually properly adjust the shutter. That is, you might shoot, all at f/8, 1/200s @ ISO 50, 1/400s @ ISO 100, and 1/800s @ ISO 200 + HTP. All three images will be processed internally as if all were shot at ISO 100, but the JPEG previews for all three will be rendered to look very similar. However, the shadows will be cleanest with the ISO 50 shot and there will be more highlight detail with the ISO 200 shot. You could then process them identically in Photoshop and mask in the highlights and shadows from the respective files and get a seamless, natural-looking image with cleaner shadows and more highlight detail. You could, of course, do the exact same thing by shooting them all at ISO 100 and manually applying the digital exposure compensation before layering and masking them in Photoshop.

Cheers,

b&

 

[TrumpetPower!]

I give up....

b&

 

[Loswr]

What is it you Neuro and TrumpetPower not understand ?

Well, I'd like to say I don't understand your rudeness and steadfast refusal to admit when you're wrong, but I'm afraid I understand them all too well.

You say read your earlier comments? Ok.

I read that you stated HTP halves the amount of light. Do you believe that?

The data recorded by the sensor (and, presumably, written to the raw file) is identical for ISO 50 and ISO 100; all that's changed is the meter is told to overexpose by a stop and the raw processing engine is told to underexpose by a stop.

no they are not
At 100iso with the same metering the sensor charge is under 100 % and at the read out =before overload and clipping
with 50iso your double the time or open up the lens 1 stop and therefore blow one stop of highlight.

I read that you stated that ISO 50 changes shutter speed or aperture by one stop. Do you believe that?

Neuro
put your camera in front of a white wall, see what you get for values ​​at 100 iso then compare with 50iso
you will find that the exemplel 1/60sec f-5.6 will be with 50iso 1/60 sec f-4, 0 = 1 stop richer exposed compared to 100iso and you lose 1 stop of high light

I'm talking about the RAW file, not the metering. Unless I change the aperture or shutter, or let the camera do that (in an auto exposure mode like Av or Tv), there will be no difference in the sensor-derived image data between ISO 50 and ISO 100. Both are ISO 100 exposures, tthe ISO 50 data will merely be pulled down a stop by the RAW engine.

As for HTP, it's possible we're saying the same thing in different ways about the what (HTP ISO 200 is actually being underexposed at ISO 100 then brought back up). But where you seem to be wrong is the when - your contention is apparently that the data are altered on the sensor. Perhaps I misunderstand you, and you are referring to the actual ISO 100 exposure when ISO 200 is set in camera as that alteration. But what I'm saying, and TrumpetPower is saying, is that the HTP ISO 200 exposure RAW file is the same as an ISO 100 (non HTP) RAW file, and your statement that, "you halve the number of electrons" indicates you believe the RAW file is different. If you mean relative to an actual ISO 200 (non HTP) exposure, fine - but that's not what we've been talking about.

If you mean ISO 100 RAW is different than HTP ISO 200 RAW, that's wrong, and if you think that's what the page you linked and keep on re-quoting is saying, you're misinterpreting it. It states: "In post-processing, the image data can be brought back up while preserving the highlights with a modified tone curve...." Post-processing, not the RAW data file, as your statement about halving the number of electrons indicates. There is no change in the number of electrons between ISO 100 and HTP ISO 200, both are ISO 100 exposures, same number of electrons, just handled differently in post. If you don't understand that, perhaps you should re-read those links of yours.

 

[Marsu42]

The sensor does not care about iso, iso step is added after the readout.

Actually, that's not true -- and, presumably, at the heart of your misunderstanding.

Thanks for explaining Trumpetpower & Dr. Neuro (which I usually trust to be correct), htp & iso50 aren't self-explanatory and hardly documented anywhere - and in other articles there's still the theory that htp does some magic because it's done inside the image pipeline - probably because no one knows the exact tone curve Canon is applying.

 

[Don Haines]

"Halves the amount of light"? Please correct me if I am wrong..... But there is nothing you can do with sensor settings or camera modes that will change the amount of light.... You can play with gain and linearity and mapping color depths, but the amount of light remains unchanged. You can change the amount of light with shutter speed, aperture, or slapping on a neutral density filter, but not with the sensor.

 

[Loswr]

Thanks for explaining Trumpetpower & Dr. Neuro (which I usually trust to be correct), htp & iso50 aren't self-explanatory and hardly documented anywhere - and in other articles there's still the theory that htp does some magic because it's done inside the image pipeline - probably because no one knows the exact tone curve Canon is applying.

Yep. Most RAW converters see the HTP flag in the metadata, and apply their version of Canon's tone curve. But some RAW converters ignore the metadata flag (e.g. Rawnalyze) and just show you the 1-stop underexposed image as it's actually recorded in the RAW image data.

 

[Meh]

Oh boy

 

[Meh]

"Halves the amount of light"? Please correct me if I am wrong..... But there is nothing you can do with sensor settings or camera modes that will change the amount of light.... You can play with gain and linearity and mapping color depths, but the amount of light remains unchanged. You can change the amount of light with shutter speed, aperture, or slapping on a neutral density filter, but not with the sensor.

that is not what Im saying, every iso step =is a halving of the number electroner, the sensor has no knowledge of iso at all, it collects photons and the number of photons / electrons is determined by time and the light inlet.

That's exactly what you said... "HTP. it is a halving of infaling light" are your exact words. Even this last statement from you is not even internally consistent.

 

[TrumpetPower!]

that is not what Im saying, every iso step =is a halving of the number electroner, the sensor has no knowledge of iso at all, it collects photons and the number of photons / electrons is determined by time and the light inlet.

That is an absolutely incorrect statement.

With the now-beaten-to-death exceptions of ISO 50 and HTP, ISO is entirely a function of the sensor. As I've repeatedly attempted to explain, with increasing ISO the sensor applies increasing amounts of analog amplification, and all this happens on the sensor, well before the analog signal is digitized.

Yes, the number of photons that impinge upon the sensor is dependent upon the aperture and shutter (and, of course, the luminance of the scene as projected by the lens). But the number of electrons that reach the analog to digital converter (ADC) depends on how much analog amplification the sensor applies to the readout -- and the amount of amplification is directly set by the user (or the autoexposure system) with the ISO control.

With ISO 50, 100, and 200+HTP, the number of electrons per photon is the same. With ISO 200 (without HTP), thanks to analog amplification, twice as many electrons per photon make it to the ADC. The number of electrons per photon is doubled again with each additional stop of ISO.

(As a side note, "inbetween" ISO settings, those not powers of two times 100 or whatever the base ISO is, are again, with most camera systems, achieved by digital pushing or pulling from the nearest full-stop ISO. ISO 125 produces the same raw file as ISO 100 but with a metadata flag telling the raw processor to add 1/3 stop of digital push, and ISO 160 is really ISO 200 with 1/3 stop digital pull.)

And just, to further clarify what digital versus analog exposure adjustment means...if you were to write a computer program that translated the data in a raw file into a massive spreadsheet, divide every number in the spreadsheet by 2, and then translate from the spreadsheet back to the original raw file format, you'd do exactly the same thing that ISO 50 does. If you were to translate ISO 50, ISO 100, and ISO 200+HTP (assuming identical scenes, apertures, and shutter speeds) files each into separate spreadsheets, they'd all have the exact same numbers in them. But, if you exposed at ISO 200 (without HTP, but still keeping the shutter and aperture and everything else the same), the numbers in your spreadsheet would be twice as big...but they'd also have a higher standard deviation, indicating additional noise due to the distortion from the higher analog gain applied to the sensor readout. If you then, say, made the shutter a stop faster, the ISO 200 numbers would be back in line with the ISO 100 numbers, but you'd still have a higher standard deviation because of the additional noise from the increased analog amplification.

I really don't know how to express this any more clearly. If you still don't understand, then perhaps you should explain how you think the whole shebang actually functions, rather than just vaguely handwave about photons and electrons with unspecific and irrelevant references to Web pages that actually describe things correctly, and the opposite of what you describe.

Cheers,

b&

 

[Loswr]

that is not what Im saying...the sensor has no knowledge of iso at all, it collects photons and the number of photons / electrons is determined by time and the light inlet.

...and apparently, by the HTP setting as well, which reduces by half the number of photons reaching the sensor. Or so you stated earlier, in multiple posts and more than one way:

HTP, here you have exposed the motive 1 stop shorter, halving the number of photons and you get a High light head room and then another curve is applied with a lift in lower areas/levels and a softer curve at the top / high lights
Here 100iso are exposed as i where 200iso which means 1 stop shorter exposure , the sensor collect less photons who are converted in to a charge/signal.

HTP. it is a halving of infaling light

Do you stand by those assertions, Mikael? :

That's exactly what you said... "HTP. it is a halving of infaling light" are your exact words. Even this last statement from you is not even internally consistent.

Factual consistency does not seem to be one of Mikael's strong suits... But in other ways, he's quite consistent - his use of bold text, his derogatory questioning of others' understanding coupled with urging others to 'read' and 'try to understand', his repetition of the same statements in post after post, in these and similar areas, he seems consistent to the point of boredom.

 

[Meh]

With the now-beaten-to-death exceptions of ISO 50 and HTP, ISO is entirely a function of the sensor. As I've repeatedly attempted to explain, with increasing ISO the sensor applies increasing amounts of analog amplification, and all this happens on the sensor, well before the analog signal is digitized.

Yes and no, depends how you define the "sensor". It could be that this is a fine semantic point. There is a lot of circuitry on the same chip but does that make it part of the "sensor"? If the ADC were to be fully integrated onto the same chip is that part of the "sensor"? What about memory?

So, it is imperative then that we define what we mean by "sensor" and discuss based on a common definition. In my view, it is most practical to refer to the sensor as the "light/electron gathering" parts. Including much more than that gets right into the quagmire I described above.

The amount of light incident on the sensor is NEVER a function of ISO. It is only a function of aperture and shutter speed. ISO gain is applied after the exposure. The so-called analog gain is applied prior to readout but that is only up to some point (something like ISO 1600?) beyond which it is applied digitally after readout.

So, the statement that "ISO is entirely a function of the sensor" might be somewhat correct depending on how we define "sensor" but only up to about ISO 1600.