It bothers me a bit when I have to hunt through menus to find the 'enable expanded ISO settings' setting in my 5D mkIII.

I'd like to know why the low ISO's are disabled however and if there is any technical reason for it?


Simply because it gives a non-trival drop in image quality and manufacturers do not want people to use it without knowing that. By enabling it explicitly, you are accepting to use an Expanded ISO.

See my answer here for why it is not part of the normal ISO range. Neither side of expanded ISOs in itself stress more the sensor, they both tigger extra processing from the camera though the one for higher sensitivities is most likely more resource intensive since it has much more noise to clean up, at least when shooting JPEG. Some cameras do not even allow RAW for expanded sensitivities.

  • I thought only increasing the ISO reduced the image quality. Therefore i assumed that reducing the ISO would improve it... This is not the case? – NULLZ Feb 28 '13 at 0:47
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    @D3C4FF - No because the lower ISO is not native. Higher ISO show more noise but ISO sensitivity below the base has lower bit-depth. – Itai Feb 28 '13 at 1:24
  • Correct me if i'm wrong but there are film speeds that are less than 100 ISO. Fuji Velvia 50 for example? Does this also apply to that or is this a specific thing to digital? – NULLZ Feb 28 '13 at 2:27
  • this link has answered my question: digitaljournalist.org/issue0604/westfall.html Some customers claim that they see less noise at ISO 50 than ISO 100 on a 5D, but in my experience both settings are about equally noiseless. There is about a stop less dynamic range in the highlights at ISO 50, which is the reason why this setting is normally locked out. It's still a very usable setting for most professional photos, as long as you are not trying to record an extreme range of contrasts. I find ISO 50 useful for studio strobe shots, since it provides more flexibility in aperture selection. – NULLZ Feb 28 '13 at 3:29
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    @D3C4FF: Reducing the strobe power by one stop would have the same effect on aperture without reducing the dynamic range of your camera. You would be better off at ISO 100 and using less power from the strobes in my opinion. Reducing the bit depth also reduces tonal graduation and smoothness from one shade to another. – Michael C Feb 28 '13 at 9:20

Because just as the high ISO expansion is a fake one, the iso 50 is, too. It is like shooting ISO 100 and in photoshop dividing the values by 2, so you lose dynamic range (it doesn't allow you do get more o the highlights into the 14bit range). and the high iso expansion from X to 2*X is like shooting at iso X and multiplying everything, incl noise, by 2.

  • ISO values can be "fake"? Please elaborate. – dpollitt Feb 28 '13 at 1:51
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    The "Real" ISO are based on gain before readout. The "fake" expanded ones are done in post by the camera, which means you could just do it yourself in lightroom. – Michael Nielsen Feb 28 '13 at 7:24

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