# Why 'more or less stopped using ISO'?

Started Feb 19, 2014 | Questions thread
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 Like? 1
 Re: Maybe this will help. In reply to crames, Feb 23, 2014

crames wrote:

bobn2 wrote:

crames wrote:

bobn2 wrote:

crames wrote:

bobn2 wrote:

crames wrote:

bobn2 wrote:

How does brightness figure into this? That is, CIE brightness, or are you using another definition?

I don't see how the connection between luminance and brightness would cause there to be "no such thing as ISO."

You can calculate a brightness for an sRGB value of 118, since the viewing conditions are defined and you have enough info to plug into a CIECAM02 calculator.

What would be the brightness for an sRGB value of 118, in candela per square metre?

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Bob

CIECAM02 brightness correlate Q of the 8 bit sRGB value (118,118,118), Q=102.28.

Candelas per square meter are the units for luminance, not the unit-less brightness, so your question doesn't make sense.

You're obviously fishing around for something, without explaining how brightness, that is CIE brightness as discussed in this sub-thread, has anything to do with the existence or non-existence of "ISO."

Apparently "'brightness' is dependent on 'absolute luminance of the stimulus' ".

While you mull things over,

Sorry, you have the mulling the wrong way round. It's you that needs to be mulling. Apparently 'brightness' is dependent on 'absolute luminance of the stimulus' and yet we find that 'brightness' has nothing to do with luminance, so mull that one over.

"And yet we find that 'brightness' has nothing to do with luminance..."

I really don't think "we" have found that to be the case at all. Certainly nothing I have posted agrees with that.

Where in the world did you get that from?

'Candelas per square meter are the units for luminance, not the unit-less brightness'. That was what you wrote, was it not?

Brightness, a perception, does not have the units of luminance. You think that means that brightness has nothing to do with luminance?

If brightness has 'something to do with luminance' then the units of brightness (whatever they are) must be derivable from the units of luminance (and possibly other units too, if there are other things that are something to do with brightness).

This is something that you obviously just made up.

It's a basic principle of physics and metrology. If you have units which measure related phenomena, it must be possible to define that relationship.

Here's a hint, the Wikipedia article on 'luminance' says it quite nicely:

Brightness, the subjective impression of luminance.

and the one on 'Brightness' says

Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light

It seems that you're missing an important distinction, and are confusing the measurable physical stimulus: luminance, with the psychological impression of the stimulus: brightness. The luminance can be measured, while the psychological impression is determined using psychophysical methods.

The 'Psychophysical methods' if they end up with a metric that is at all normative, have to boil that 'psychological impression' down to some constants, conversion factors and formulae so that these values can be determined from measurable physical phenomena. That was what CIE is all about.

It's the same with L* (CIELAB lightness), you input measured tristimulus values for the color of interest along with the tristimulus values of a reference white, what you get out is a mapping to a uniform lightness scale between 0 and 100 in L* lightness units. Can you guess whether the other dimensions of CIELAB, a*, b*, Chroma, and hue have physical units?

They are all derivable from physical units. Otherwise there would be no way of engineering devices to CIE standards.

That would be like saying the perception of loudness has nothing to do with sound intensity.

So, what do you think is the sound power output of a pair of iPod earphones? Can that give the same perception of loudness as a 100W amplifier and loudspeaker?

The point is, what is the unit for Loudness (Sones (N) or Phons (L))? Again it's perceptual.

here's a question for you, Bob:

I take a picture at f/16 1/100 ISO 100. Now I want to take another shot where the final image is twice as bright. What ISO setting should I use?

I wouldn't be using f/16, 1/100 in the first place, so it's a hypothetical. But I wouldn't be using 'ISO' or exposure to set the brightness, whatever.

Good for you. However I believe you have stated in various threads that ISO is a brightness control, so I was just trying to establish what might be the relationship between the two.

That is easy, you'll find it in the ISO standard:

Yes, you showed this before. I keep looking at this but nowhere do I see the word "brightness" mentioned.

What do you think 461/1000 x Omax represents?

That particular value gives an L* lightness of 50 in the sRGB space, so I would say it's a target lightness.

Think also that the standard demands that the white balance be set so R,G and B channels yield the same value. What now is the relationship between 'lightness' and 'brightness'.

Lets see, there's definitely a luminance component of exposure,

For sure, the standard even tells you what it is:

but nowhere do I see a mapping from a physical luminance component to a perceptual brightness scale. Did you leave something out in your cut & paste?

As I said, what would you think 461/1000 x Omax represents?

As I said, it appears to map a target luminance to lightness L*= 50, perceptual middle gray. Still no sign of brightness anywhere!

See above. In any case, it makes not difference to the fundamental discussion, that 'gain' or 'amplification' is not a part of 'ISO', whether it's 'lightness' or 'brightness'.

You didn't need me to tell you that. Sorry, I thought you were asking for advice. I was wondering why you'd ever choose f/16, 1/100, it seems a pretty unlikely combination to me.

"Sunny 16," rule, but really irrelevant for the ISO->brightness question that you've managed to avoid.

I haven't avoided it, you've just put the blinkers on. Why would you use a rule that gives such a silly choice of f number and shutter speed? And if using it ended up with you having the image a stop too dark, why would you go on using it and adjust the ISO? Why wouldn't you find some more sensible method of exposure management?

Now you're just jerking my chain.

No, I'm not. Really just returning this long and mostly futile sub-thread to the topic.

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Bob

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