Low ISO - What does it really mean?

[Longimanus]

Should I select ISO 200 or Low ISO, what do I gain by using Low?

[Ralph Harwood]

Re: Low ISO - What does it really mean?

Hi there Longimanus!

There is no improvement in IQ or dynamic range by choosing low - the only real advantage is that it can be used to reduce shutter speeds if you want to use wide aperture lenses in bright light - for motion blur purposes. I only tend to use is with my ND filters to give really long exposure times.

Cheers,

Ralph.

[pdk42]

Re: Low ISO - What does it really mean?

Technically, ISO Low is achieved by over-exposing by 1 stop and then pulling it back after. As well as using a lower shutter speed, it also leads to lower noise and a slightly reduced DR (risk of blown highlights). Personally I think it's best avoided.

[Ricoh]

Re: Low ISO - What does it really mean?

Pulling back after what. After JPEG conversion, or does it do the same to RAW?

It's clear I don't understand it.

[pdk42]

Re: Low ISO - What does it really mean?

Pulling back after what. After JPEG conversion, or does it do the same to RAW?

It's clear I don't understand it.

Yes, sorry - I mean pulled-back in post-processing after exposure. If you're using in-camera jpgs, then it's the camera that does it. Otherwise, there's a marker put in the raw file so that OV3/LR etc can do the same thing.

[Ricoh]

Re: Low ISO - What does it really mean?

Concentrating on RAW, does LR apply on import, or is under user control?

[Zuiko]

Re: Low ISO - What does it really mean?

Pulling back after what. After JPEG conversion, or does it do the same to RAW?

It's clear I don't understand it.

The exposure adjustment within the camera to achieve Low ISO applies equally to JPEG and raw files, just as it does when adjusting for ISO settings higher than the base ISO setting. There can only be one true ISO setting for the base sensitivity of a sensor (200 on recent Olympus cameras) and anything higher or lower has to be adjusted electronically. It's a bit like using ISO 200 film, which you can push or pull and adjust the development times accordingly. Naturally we are more accustomed to pushing the ISO and with modern sensors almost take for granted the ability to use ISO 3200 or even higher and still get an acceptable image. However, using the ISO 3200 setting is still under-exposing the image by 4 stops and this has to be recovered electronically (within the camera) by amplifying the signal from the sensor and this is what causes noise. Imagine the effect of pushing ISO 200 film to 3200!

The reverse applies when using Low (100) ISO and although it can actually reduce noise during very long exposures it also reduces the dynamic range. Therefore it is best avoided unless there is a specific need, such as preventing over-expousre when using a fast prime wide open on a sunny day and the camera does not have a fast enough shutter speed to cope.

[Zuiko]

Re: Low ISO - What does it really mean?

Concentrating on RAW, does LR apply on import, or is under user control?

Yes, it applies automatically upon import and is effectively the same as adjusting it in camera. There is no user control, although you can of coure apply further adjustments yourself in pp.

[Ricoh]

Re: Low ISO - What does it really mean?

Thanks for the clarification John. It's clear I didn't understand, but now I do, I won't be using low ISO unless required, for the reason given.

[pdk42]

Re: Low ISO - What does it really mean?

The exposure adjustment within the camera to achieve Low ISO applies equally to JPEG and raw files, just as it does when adjusting for ISO settings higher than the base ISO setting. There can only be one true ISO setting for the base sensitivity of a sensor (200 on recent Olympus cameras) and anything higher or lower has to be adjusted electronically. It's a bit like using ISO 200 film, which you can push or pull and adjust the development times accordingly. Naturally we are more accustomed to pushing the ISO and with modern sensors almost take for granted the ability to use ISO 3200 or even higher and still get an acceptable image. However, using the ISO 3200 setting is still under-exposing the image by 4 stops and this has to be recovered electronically (within the camera) by amplifying the signal from the sensor and this is what causes noise. Imagine the effect of pushing ISO 200 film to 3200!

The reverse applies when using Low (100) ISO and although it can actually reduce noise during very long exposures it also reduces the dynamic range. Therefore it is best avoided unless there is a specific need, such as preventing over-expousre when using a fast prime wide open on a sunny day and the camera does not have a fast enough shutter speed to cope.

I agree John. Sensors work by the incoming photons causing electrons to be emitted in the sensels; these electrons accumulate as charge in a "charge-well" during exposure. After exposure, the accumulated charge is read off, amplified (electronically) to create a voltage and then this voltage is converted to a number using an ADC (analogue to digital converter). There may well then be digital amplification too (i.e. making numbers bigger by maths!).

There will be a given inbound flux of photons which will fill the charge well and this in effect sets the "base" ISO. Anything less than this light level will result in partially-full wells necessitating amplification to gain an image at a normal luminance. Partially full wells and amplification = noise.

In the case of lowering the ISO though, we have a problem in that we'll have exceeded the inbound photon count resulting in the charge well overflowing. This is "blown highlights" of course and there is nothing that can be done to recover it.

[Zuiko]

Re: Low ISO - What does it really mean?

Thanks for a more detailed explanation of what actually happens within the camera, Paul. My knowledge is rudimentary to say the least and is limited to basic principles.

[Longimanus]

Re: Low ISO - What does it really mean?

Thanks for all the clarification, I wont be using this in normal circumstances then

[pdk42]

Re: Low ISO - What does it really mean?

There's a tool called rawdigger that shows the numeric values from the raw file. It's an interesting exercise to examine two "identical" shots taken at ISO 100 and 200.

[StephenL]

Re: Low ISO - What does it really mean?

Those are the explanations of the theory behind it, which are fine, but ... I used it on numerous occasions and I found in practice, on real photos without pixel-peeping or checking numerical data, it actually resulted in smoother-looking files with no noticeable reduction in dynamic range. But then, I seldom take photos of graphs and charts. Ignorance is bliss!

[pdk42]

Re: Low ISO - What does it really mean?

So long as you don't overexpose Stephen, that's true. And in fact, the Oly sensors are pretty good at recovering highlights so you're really just exercising that feature!