ISO 100, OMD-EM1II and firmware 3.0?

Started 2 months ago | Questions thread
spike29 Senior Member • Posts: 1,698
Re: Basics of digital camera ISO

bobn2 wrote:

spike29 wrote:

bobn2 wrote:


That's a common problem in this discussion, although also an irrelevance. When you realise what the photographic process is, and what ISO actually does, you realise that whilst where the sensor starts and stops is of academic interest, it's irrelevant to the discussion. ISO says what lightness should result from a given exposure.

This is one of the clearest , most understandable short definitions i read about this subject .🙂

Simplified but accurate enough.

Thanks for the feedback. I'll remember it and use that text again

I will explain why i find this to the point, most of the time ISO value is connected to "gain" of voltage or current or digital gain in threads and webpages which brings your mind to a "amplifier". Which is a trap of misconceptions and assumptions. (I did) most internet searches don't help to untangle the confusion either, on the contrary it tighten the knots even more.

That's why this simple text don't distract you and don't sent you in a maze of half truth's. and keep you on track for the next information.

So as far is i remember all the basics:

While ISO isn't a Gain/Amplifier but more a (wanted) result of the several gains (to get the signal in range of correct processingvalues for readout and conversions) along the road of capturing photons towards a image represented in a rawfile. This in order to get the image correct(lightnes) represented on screen or paper. By getting a underexposed sensor/pixel , low amount of photons on a given shuttertime, and by setting a ISO value you define the lightnes you want to correct that (under)exposure in the camera to get a correct lightness in your jpeg/tiff. (edit: Correct?)

new question:

What if you alter a rawfile of a 1600iso image and change the stored value of 1600iso in the exif in lets say 200iso? (lets say this is "base iso") drops the lightnes of the image in a rawdeveloperapplication 3stops down? so it looks like a -3EV Exposurecorrection?

(highlights stayed clipped so those are gone and black is black so you scrunch the DR of the tonecurve. )

and this altered exif shows the "real image" what's captured by the sensor showing the real lightnes it would show on base ISO? (except the clipped data)

Or are some "gains" non reversible? (i think simplified that all gain before ADC is irreversible, after the ADC it's just "software calculation" and fully reversible. )

To determine the lightness in the output file, all the camera needs to be able to do is measure the exposure in each pixel. 'Gain' doesn't change the exposure in the pixel, but it can change how accurately it is measured.

Is this the reason that dynamic range changes when ISO raises? less accurately means less steps between black and white (full exposure)

So far as DR is concerned, the problem is that if you apply voltage gain before the ADC you boost the higher signals up above the ADC maximum, the larger exposures can't be digitised. Thus each stop of gain means a loss of a stop at the high end. Once that loss stops lowering the noise floor, there is no point applying any more voltage gain. There's another question of just why manufacturers continue unnecessarily clipping the raw file, which we can have if you want.

People have difficulty with this concept because they tend to call both the input and output 'exposure', in which case they think of 'gain' as giving a bigger 'exposure' from a smaller one, but that is not what is happening. Exposure and lightness are different things, which is why two words are needed.

Extended ISO at the high end i can think of: Just add measurement's at the lowest part of the sensor output voltages until noise overgrows mesurable signal and higher iso values are redicules because you can't separate random noise from the actual latent image capturing enough to get a reasonable clear image at the end anymore.

The real problem is that the noise is in the signal, which is why photography is somewhat different from common electronic applications. The SNR goes as the square root of the number of photons captured. The problem with very small exposures is just not many photons.

But at the low end below base iso. Why don't use that as base iso?

See upthread. The thing,about th Lo settings is you can meter as normal, get higher exposures, thus more photons, thus less noise.

Got it. metering is for the determination of the shuttertime to get a proper exposure and shuttertime increases when lowering iso so more photons are able to reach the sensor/pixel with in the shuttertime.

More then enough light to "fill" pixels at base iso so when extend to a lower iso, you get some longer ss and or smaller diafragma at the cost of what?

Whether pixels are 'full' or not isn't important. It's the absolute number of photons that matters.

(Sorry to bump in but this is interesting, most things i read is that extended iso at the low end is extended dynamic range , so less overexposed pixels.)

Bumping in is what a public forum is for

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