Started Jan 16, 2013 | Discussions thread
Mark H
Mark H Veteran Member • Posts: 3,491
Re: 'Extended' low ISO lowers clipping point...

Steen Bay wrote:

Mark H wrote:

Steen Bay wrote:
.... And it doesn't really matter, it's not a bad thing if ISO 100 turns out to be an extended ISO. It'll just mean that ISO 100 and 200 will have the same saturation capacity (can 'handle' the same amount of light/exposure without clipping),...

No, that's wrong.

The main consequence/cost of a so called 'extended' lower ISO is that you actually lose highlight range - because the sensor is getting a higher exposure, so it saturates at a lower point in that exposure.

Only relative to the cameras metering. I'm talking absolute exposure (amount of photons collected).

That is a flawed/bogus argument.

If you were going to override the metering at 'extended' ISO-100 by -1EV just in order to achieve the same 'absolute exposure', same highlight range/saturation point, as that of ISO-200 (as you refer to above) - then you should just use ISO-200 in the first place.

Where ISO-100 was an 'extended ISO' it will simply have fewer/twice as coarse quantisation/bit level steps as that of the ISO-200.

The only purpose of 'extending' ISO in this way, is for the convenience of being able to meter and exposure for the lower ISO exposure value, and achieve a correctly exposed image straight from the camera (albeit with greater probability of blown highlights).

...and it's quite likely that the X20 will have at least the same saturation capacity at ISO 100/200 as the X10 has at ISO 100. It's just a matter of how Fuji has chosen to calibrate the X20, a matter of how much highlight headroom the camera has above metered middle gray.

It's really impossible to say. There is no obvious reason for any particular sensor to have an inherently high, or low, 'base ISO'.

It's still better though, to have a low minimum ISO - for reasons such as lower noise (lower 'photon shot-noise') and better ability to cope with high light levels, and ability to use creative effects such as for example, wide aperture/DoF and motion blur without resorting to ND filters.

To that end, having ISO-200 as a lowest setting can be a problem - it's preferable to have a lower minimum like ISO-100 (or even lower).

What really matters is what the 'measured' base ISO is (on DxO). That tells us how much (absolute) exposure the sensor can handle.

That is true, at base ISO, and as far as it matters (and so far as we might trust DxO) - and for the X20, it's as yet an unknown.

It's the absolute exposure that determines the noise.

And in most cases, for most people, that 'absolute exposure' depends on the metering, which depends on ISO value used.

The cameras metering will suggest different exposures at different ISO, but I'm not forced to follow what the camera suggests, and if I'm aware of how much highlight headroom the camera has above metered middle gray at different ISO, then I can always optimize the exposure (by e.g. using ETTR) and get the lowest noise that's possible in the situation.

You may work that way (be honest, how often do you really?) - but the vast majority of people don't.

Most people simply want to get the exposure right 'straight out of camera', without having to custom post-process each exposure.

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