ISO 50

[sanj]

Here is another picture that helps me understand this better.
I had shot this on 5d2 just when I was starting off with photography and I used to often wonder why certain portions of the clouds lacked detail. I hunted this photo down in an old HD and yes, it is shot at ISO 50.
Hmmm I too will now avoid ISO 50...

 

[sanj]

There's a reason ISO 50 needs to be unlocked to be used. It's not actually ISO 50. It's ISO 100 being reduced in camera to ISO 50. The same as shooting ISO 100 and then in post bringing the exposure down a stop. I would never shoot with it.

Exactly. There's no real benefit to ISO 50, except perhaps convenience if you're shooting in Av mode and want a stop slower shutter speed. But if a highlight would be blown at ISO 100 with a given aperture/shutter combo, it'll be just as blown at ISO 50.

Neuro, my tests indicate differently to this. Iso 50 highlights are clearly more blown. I can never contradict you, so please tell where I am going wrong here..

 

[TheSuede]

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&

Well, this was a long thread, and there's to little time...
I'll take this post from the start of the thread and start there. If I missed something in the in-between pages, shoot me!

I'm KIND of reading this as you're talking past each other, but anyway:
No, the ISO amplification is added after what is usually called the "read-out". The readout in itself is the analog coupling between the individual pixel charge storage where the charge is translated into a feed line voltage via the cell capacitance, a resistor and the reference level - one of the base definitions of a CMOS sensor.

The conversion from charge to voltage at pixel level is done at a fixed rate, that can only be adjusted by changing the pre-charge or the reference voltage feed, and you usually don't do that. Some machine vision sensors have facilities for this (absolute conversion rate calibration), but no sensors in the commercial area AFAIK. After this conversion has been done, the voltage is "transported" - shorted, connected to - a voltage amplifier that sits at the edge of the sensor, just before the signal leaves the sensor plate. The ISO amplifier.

In a Canon sensor, you usually have somewhere between 4-12 communication lines out from the sensor plate edge - each of those has it's own voltage amplifier where the ISO gain is applied. This is also what causes most of the vertical banding - the gain in those amplifiers aren't matched perfectly.

At base ISO, the "whites" are in the green channel usually quite close to the absolute clipping point of the individual pixel - the full well capacity. As you approach FWC, no more charges can be stored, no more voltage can be read out from the pixel. At this point, it doesn't matter if you try to lower the ISO amplifier gain, the data is still just a flatline, pinned at the maximum value of the pixel. Like trying to salvage a pure blown white in a jpg file - it's quite impossible.

So what would really be good to know if you're shooting raw is if the maximum pixel voltage that you can convert into an ADU value increases when shooting at one step below "normal" base ISO. And usually it does - by some 5%. The 5Dmk3 goes from about 67ke- to 70-71ke- The added range is mostly compression effects, distorted data - but that doesn't matter much in the highlights. On the other hand, 5% doesn't matter much anyway - +5% is the same as 0.07Ev or 1/14 Ev. Not much.
............

I have no idea how Canons HTP works in the transition from base ISO to higher ISO in newer cameras. If the camera is at ISO400, it's simple - just tell it to halve analog amplification (to ISO200) and compensate in post, pulling in the highlights as you do. In this case you don't lower the actual photometric exposure, you just adjust the amplification. The actual photoelectric conversion rate is constant.

But at base ISO this is meaningless, since there's no data to pull in. The pixels themselves are what they are, they can't send the amplifier any higher voltages, FWC sets the upper limit. Trying to lower amplification in that case is counterproductive, you're just hurting the shadow tone resolution.

compare it to an analog sound chain.
You have a microphone that can convert sound into a voltage, maximum output is 2v. This is the pixel.
You have an amplifier that can amplify the signal if needed (ISO gain, this is NOT in the pixel)
You have a loudspeaker that needs exactly 2v to play at the sound pressure that you want, no more and no less (your image brightness)

You adjust sound pressure at the output - image brightness - by adjusting the amplifier

Now if the microphone gives those maximum 2v from a 96dB sound pressure, screaming into the microphone at 102dB won't increase the maximum output. It will just distort the sound, by clipping. It doesn't matter if you turn the amplifier down, the distortion (clipping) is still there, since it's present at the source, the input of the amplifier. And there's nothing you can do about it except get another microphone with a lower base sensitivity (lower base ISO).

Actually it's quite easy to test - point a camera at something with graded highlight, take one shot at base ISO and one shot with exactly the same shutter speed and aperture at the faked ISO. Try to pull in the highlights in the raw file. Is there any difference?
If there's no difference, you could just as well have shot at base ISO with -1Ev compensation. You're just adjusting if you want to clip the highlight or the shadows, you gain absolutely nothing. It's a 1:1 trade-off.

 

[TrumpetPower!]

50iso is no 50 iso, it is equivalent to 100 iso over exposed 1 stop, what you gain is for example slower exposure time which can be handy , running water etc but at the same time that the sensor is richer exposed you lose 1 stop of DR and that is in the highest levels= earlier clipping

Thank you for the explanation. To me, when I re examine the photos based on what you saying, it seems like ONLY the whites are getting overexposed, NOT the entire frame. Could I conclude that not the entire frame but only the whites get over exposed?

sanj, I would strongly recommend ignoring everything Mikael is writing in this thread. He's very, very worng.

If you shoot with the same aperture and shutter at both ISO 50 and ISO 100, you will get the exact same raw file. When you open it in Lightroom (or wherever), it's starting with the same raw data, but it sees the ISO metatags are different and so, before doing anything else to the image data, it divides all the numbers in the ISO 50 shot by 2.

When you set ISO 50 on the camera, it tells the meter to change by one stop, so the common workflow would be to expose the ISO 50 shot one stop brighter. When you then open it, Lightroom first divides all the values by 2, which gives you the appearance of a file that looks properly exposed. But it wasn't actually exposed at ISO 50, which doesn't actually exist in the camera; it was really exposed at ISO 100. And, just as if you had actually exposed it at ISO 100 and applied one stop of darker exposure compensation in Lightroom, you'd discover you had lost one stop of highlights, you always lose one stop of highlights with ISO 50.

Cheers,

b&

 

[TexasBadger]

Mikael you are missing the point. Changing film speed is similar to changing the signal amplification +/- in the digital world. The analogy is that signal amplification is the digital equivalent of changing the crystal size in the films emulsion. What I am demonstrating here is that the physics of film and digital hasn't changed. Just think of the light sensitive crystals in the film emulsion as the analog equivalent of light sensitive pixels in the sensor.

The digital sensor is the equivalent of film. The amplification of the digital signal is the equivalent of the silver crystal size in the film's emulsion. Shooting at ISO 50 is the equivalent of pulling film. Shooting at high ISO is the equivalent of pushing film.

 

[TrumpetPower!]

TheSuede, thanks for the details and corrections on the workings of the electronics!

b&

 

[TheSuede]

sanj, I would strongly recommend ignoring everything Mikael is writing in this thread. He's very, very worng.

If you shoot with the same aperture and shutter at both ISO 50 and ISO 100, you will get the exact same raw file. When you open it in Lightroom (or wherever), it's starting with the same raw data, but it sees the ISO metatags are different and so, before doing anything else to the image data, it divides all the numbers in the ISO 50 shot by 2.

When you set ISO 50 on the camera, it tells the meter to change by one stop, so the common workflow would be to expose the ISO 50 shot one stop brighter. When you then open it, Lightroom first divides all the values by 2, which gives you the appearance of a file that looks properly exposed. But it wasn't actually exposed at ISO 50, which doesn't actually exist in the camera; it was really exposed at ISO 100. And, just as if you had actually exposed it at ISO 100 and applied one stop of darker exposure compensation in Lightroom, you'd discover you had lost one stop of highlights, you always lose one stop of highlights with ISO 50.

Cheers,

b&

Well, isn't that exactly what he's saying?

If you let the camera chose exposure, it will overexpose the raw file, clip more highlights - if you set ISO50.

If you expose manually - same shutter speed, same aperture - the ISO50 and the ISO100 raw files will be almost exactly identical. I don't see where your statements clash in any way.

The main difference is in in-camera jpg, since the in-camera jpg engine usually never uses all the raw data all the way up to blown white. I think the in-camera jpg actually gains something when going to ISO50 from ISO100, but that comes from a different development setting - not better base data.

 

[Loswr]

Neuro is wrong about 50iso not you.

No, you are wrong about me being wrong. You often seem completely unable to distinguish between changing aperture and/or shutter speed, which change the actual amount of light reaching the sensor, and changing ISO, which changes only what the sensor does with the light that falls on it, but has no effect on the amount of that light. For someone who professes to understand the principles of photography, that's a key point - one that seems beyond your comprehension.

There's a reason ISO 50 needs to be unlocked to be used. It's not actually ISO 50. It's ISO 100 being reduced in camera to ISO 50. The same as shooting ISO 100 and then in post bringing the exposure down a stop. I would never shoot with it.

Exactly. There's no real benefit to ISO 50, except perhaps convenience if you're shooting in Av mode and want a stop slower shutter speed. But if a highlight would be blown at ISO 100 with a given aperture/shutter combo, it'll be just as blown at ISO 50.

Neuro, my tests indicate differently to this. Iso 50 highlights are clearly more blown. I can never contradict you, so please tell where I am going wrong here..

You're not contradicting me at all, and we are both correct. What I stated was, 1) there is no benefit to ISO 50 and, 2) if a highlight is already blown at ISO 100, using ISO 50 won't help. Both of those are true (despite Mikael's seeming inability to comprehend them). What you're showing simply goes further - not only can ISO 50 not save highlights that would be blown at ISO 100, it can actually cause highlights to blow that would not have blown at ISO 100.

My point was in reference to using ISO 50 and ISO 100 at the same aperture/shutter speed - in that case, there's no difference in the highlights in the RAW image files, and ISO 50 would not blow any more highlights (the meter would show a stop of underexposure). In your example, you are talking about changing aperture or shutter speed when going from ISO 100 to ISO 50 to maintain a metered exposure. It's that change in aperture/shutter that's causing the blown highlights, not setting ISO 50, per se.

 

[TrumpetPower!]

Mikael, let's try to make this even more simple.

Consider that you've got four cameras and one tripod set up in a studio. All four cameras are 5DIIIs, and all four have 50mm f/1.4 lenses attached. The tripod is fixed and the lighting (continuous, not flash, just to keep things simple), is held constant. All we're doing is swapping out cameras, focussing on the same spot, and releasing the shutter.

All four cameras are set in manual mode to record to raw (no JPEG), with the following settings:

Camera A: 1/100s @ f/8 @ ISO 50
Camera B: 1/100s @ f/8 @ ISO 100
Camera C: 1/100s @ f/8 @ ISO 125
Camera D: 1/100s @ f/8 @ ISO 200 + HTP

If you then examine each of the four raw files, you will find that the only difference is in the metadata.

Now, we shoot as follows:

Camera A: 1/200s @ f/8 @ ISO 50
Camera B: 1/100s @ f/8 @ ISO 100
Camera C: 1/80s @ f/8 @ ISO 160
Camera D: 1/50s @ f/8 @ ISO 200 + HTP

What we now discover is that each raw file is different.

Let's assume that there was a gray card in the scene. And let's pick a completely arbitrary number of 100 to indicate the average pixel value of that gray card in the 1/100s @ f/8 @ ISO 100 exposure.

What we discover is the following values for that gray card in each of the raw files:

Camera A: 1/100s @ f/8 @ ISO 50: 100
Camera B: 1/100s @ f/8 @ ISO 100: 100
Camera C: 1/100s @ f/8 @ ISO 125: 100
Camera D: 1/100s @ f/8 @ ISO 200 + HTP: 100

Camera A: 1/200s @ f/8 @ ISO 50: 50
Camera B: 1/100s @ f/8 @ ISO 100: 100
Camera C: 1/80s @ f/8 @ ISO 160: 125
Camera D: 1/50s @ f/8 @ ISO 200 + HTP: 200

Now, we're going to try another experiment, shooting the last series, but all at ISO 100. This is what we get:

Camera A: 1/200s @ f/8 @ ISO 100: 50
Camera B: 1/100s @ f/8 @ ISO 100: 100
Camera C: 1/80s @ f/8 @ ISO 100: 125
Camera D: 1/50s @ f/8 @ ISO 100 + HTP: 200

Finally, we're going to do one last pair of experiments with a different set of ISO values. Here're the results:

Camera A: 1/100s @ f/8 @ ISO 100: 100
Camera B: 1/100s @ f/8 @ ISO 200 (no HTP): 200
Camera C: 1/100s @ f/8 @ ISO 400: 400
Camera D: 1/100s @ f/8 @ ISO 800: 800

Camera A: 1/100s @ f/8 @ ISO 100: 100
Camera B: 1/200s @ f/8 @ ISO 200: 100
Camera C: 1/400s @ f/8 @ ISO 400: 100
Camera D: 1/800s @ f/8 @ ISO 800: 100

If that doesn't clear it up, nothing will....

Cheers,

b&

 

[rs]

This is the best entertainment I've had in ages ;D

 

[rpt]

This is the best entertainment I've had in ages ;D

LOL!

 

[Loswr]

i have been writing about shutter speed/ f-stops read back

You have been writing about HTP affecting the amount of light that hits the sensor, read back.

If you spew enough drivel throw enough darts, some are bound to hit the target.

 

[Meh]

Well, isn't that exactly what he's saying?

He's been saying many different things regarding ISO50 and HTP, some correct and some incorrect. He's been particularly wrong about HTP. It turned into an argument due to some rather rude and insulting comments.

 

[TheSuede]

"Even" I misuse the word 'exposure' very often. I guess this stems from have to many discussions with people that have no real interest in (or meaningful use of) the correct usage.

Try having a conversation with a reviewer or even a seasoned photographer - or a surveillance camera client - and do on all matters use the word 'exposure' correctly. It will confuse them to the brink of insanity

The only photographers that I know that actually grasp the meaning of the word 'exposure' are people old enough (or interested enough) to have experimented with and compared results with different film types and development push/pull schemes on several different base formats (135, MF, large format). They know the importance of the separation of exposure and brightness.

The only other two groups I know of that use it correctly are astrophotographers and people with some kind of electroptics degree (and even the people in the camera base module development department often get befuddled when they discuss the resulting images rather than the process in itself).

The important part where you're talking straight past each other seems to be this:
If you let the camera chose exposure, it will overexpose in ISO50
In manual exposure mode it makes no or very little difference between ISO50 and ISO100
None of you seem to disagree on this, so I fail to see the problem.

 

[sanj]

Here is the dumbest question in the thread, but since I am the one that made the test, could I please ask what is HTP?

 

[TrumpetPower!]

The important part where you're talking straight past each other seems to be this:
If you let the camera chose exposure, it will overexpose in ISO50
In manual exposure mode it makes no or very little difference between ISO50 and ISO100
None of you seem to disagree on this, so I fail to see the problem.

TheSuede, your two-line summary is correct (if, naturally, a bit oversimplified), but that's not what Mikael has been writing. And when neuroanatomist and I have been writing that, Mikael has been arguing that we're worng.

Cheers,

b&

 

[Loswr]

My point was in reference to using ISO 50 and ISO 100 at the same aperture/shutter speed - in that case, there's no difference in the highlights in the RAW image files, and ISO 50 would not blow any more highlights (the meter would show a stop of underexposure). In your example, you are talking about changing aperture or shutter speed when going from ISO 100 to ISO 50 to maintain a metered exposure. It's that change in aperture/shutter that's causing the blown highlights, not setting ISO 50, per se.

@sanj - the reason this is relevant, at least to me, is that many times (usually, in fact), I choose aperture and shutter speed for a reason. Aperture is chosen for the desired DoF so I can include/exclude what I want to be sharp in the image, shutter speed is chosen to stop or show motion, as desired. If those values are dependent variables (fixed for the shot, ISO becomes the independent variable. In that case, there is no benefit to ISO 50, which was my point. Of course, given the constraints of my personal range of acceptable ISO values (which is pretty broad on the 1D X!), I still cannot achieve the exposure I want, then I decide where to compromise.

But the key point is that ISO 50 offers no benefit, and can in some cases result in a detriment - therefore, I see no reason to use it.

 

[agierke]

i would like to now see Neoro and Mikael do a rendition of "Who's on First".

 

[TrumpetPower!]

Here is the dumbest question in the thread, but since I am the one that made the test, could I please ask what is HTP?

Not dumb at all. I've tried to expand the acronym at appropriate places, but obviously not everywhere it's appropriate.

HTP stands for Highlight Tone Priority. Rather than ISO 100 being the lowest selectable ISO, ISO 200 becomes the lowest selectable ISO. The camera actually exposes at ISO 100 but applies a tone curve to brighten the image. This has the end result of recording more highlight detail, but there's no such thing as a free lunch, so you get nosier shadows at the same time. If you're shooting raw, you can achieve the exact same result by shooting at ISO 100, underexposing by a stop, and brightening the image by a stop in post-production (with, of course, highlight compression to avoid digital clipping).

Cheers,

b&

 

[Loswr]

Here is the dumbest question in the thread, but since I am the one that made the test, could I please ask what is HTP?

LOL. Earlier, it was suggested that it means 'Half The Photons' - that seems to be Mikael's definition, anyway.

Really, it's Highlight Tone Priority, which is a way to preserve one stop of highlights in an image (at the cost of an extra stop of noise in the shadows).