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I have a Pentax K-x. I've had it set on "auto ISO up to 6400" for a while now, and I'm starting to wonder if this is making my images worse. Honestly I don't trust myself to tell.

Does a high ISO generally result in smaller dynamic range and less accurate color?

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Look at the DxOmark website where they review the K-x: http://www.dxomark.com/index.php/en.../Cameras/Camera-Sensor-Database/Pentax/Kx

In the measurements tab, you can see a graph of how the dynamic range appears limited when using ISO 6400. It appears to be limited to a range of 8.5EV. The color ranges also suffers.

I find the site to be a good tool to compare what different cameras may have at similar settings as technology gets better.

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  • Yes! And if you read their detailed explanation about Dynamic Range: Dynamic range falls rapidly with higher ISO settings, as any analog or digital amplification performed will increase the noise in the darker areas, making it harder to distinguish between fine levels of contrast - though @MattGrum might still be right in that with a lot of light the camera will not have to exploit amplification/tricks even with higher ISO. – Stefano May 15 '12 at 11:37
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Raising the ISO value only ever increases noise indirectly, due to the camera changing one of the other exposure parameters in order to compensate for the ISO setting you've chosen. What causes noise is not capturing enough light.

This is very important, as it means you can use auto-ISO in manual mode without any fear of increasing noise. Using the highest ISO setting the doesn't blow the highlights actually reduces noise compared to underexposing and having to boost exposure in post.

Using auto-ISO in Program, aperture priority or shutter priority can cause the camera to use a shutter-speed or aperture that will let less light in therefore increasing noise (and reducing dynamic range / tonal range).

However it's important to realise there are tradeoffs to be made, had the camera not increased shutter speed (and therefore captured less light, leading to more noise) as a result of using a higher ISO, then the image may well have suffered from motion blur.

Some of the images you captured in auto-ISO mode may have had more noise than if you had fixed the ISO at a certain value, but they would probably have suffered in other ways. These features were all implemented to make your photos better. If raising ISO actually generated noise, then everyone would shoot ISO100 all the time.

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    I really, really like using autoISO in manual. When you're working in changing light, but nothing drastically changes (like strong backlighting where you may need to compensate) - it allows me to set my exposure creatively, not worry about minor changes in the light, and then just focus on composition and working with the model. – rfusca May 15 '12 at 13:42
  • That's because you don't have TAv mode ;) Otherwise it is kind of counter-intuitive to do that and most non-Nikon DSLRs wont let you do this. – Itai May 16 '12 at 0:51
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Shooting at ISO 100 is very difficult due to several competeing compromises. you need to be in very bright light, or use wide open apertures, or use some kind of stabilization due to low shutter speeds. wide open apertures are not a feature of most zooms, even 2.8 zooms are a bit slow. Wide apertures result in extremely shallow depth of field and also some lens loss of sharpness. This could result in missed Auto focus shots. So although we would all prefer to shoot at low SIO, it is sometimes not possible. And yes, increasing ISO results in higher noise levels, and loss of available dynamic range.

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  • Can you explain a little more about the last sentence? That's the question here, after all. (We have other questions about why someone might need to use higher ISO at all.) – mattdm Apr 23 '13 at 14:55

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