Please explain expanded ISO

[Badger]

Just got the 6D and love it however, I have never really understood expanded ISO. The fact that the word expanded is used to describe the feature implies to me that its not real. So, my question is, if I switch from ISO 100 to 50, what is happening in the camera? There isn't less noise in the image is there? Is it just 100 with half the light allowed?
I would also like to understand what is happening at the other end of the scale. Not that I would ever, but what is going on at ISO 102,800?

 

[Loswr]

Native ISO is analog gain applied before analog-to-digital conversion. Expanded ISO is digital gain (negative gain, in the case of ISO 50), applied after ADC.

 

[phixional ninja]

Native ISO is analog gain applied before analog-to-digital conversion. Expanded ISO is digital gain (negative gain, in the case of ISO 50), applied after ADC.

So if expanded ISO is just digital, is using ISO 50 when shooting RAW doing anything that can't be done in post processing with an overexposed ISO 100 shot? My 7D doesn't do downward expansion, so I can't play around with it myself... Say I took a shot at ISO 100 that had blown highlights, would ISO 50 with the same aperture and shutter speed save those highlights in a way that couldn't be done during RAW conversion? Thanks.

 

[Badger]

Thanks Ninja, I guess that is ultimately my question too. Is anything happening in the expanded zones that can't be done in post processing and if not, is it better done in the camera by expanding ISO, or done in Lightroom (for example)?

 

[And-Rew]

I was once given this answer, by a former Canon employee:

Expanded ISO are settings that Canon are not prepared to put their name to as 'always usable' settings - but include because the setting can produce an image where none would have been possible on other settings. Furthermore, such is the quality of the image gained with an expanded setting (noise/ grain) that Canon do not wish to advertise it as a working setting.

I think that's how it was explained. Essentially, they are declaring the range of the ADC as considered acceptable but provide the additional settings even though Canon do not consider them to produce images of an acceptable quality.

 

[Sporgon]

I was once given this answer, by a former Canon employee:

Expanded ISO are settings that Canon are not prepared to put their name to as 'always usable' settings - but include because the setting can produce an image where none would have been possible on other settings. Furthermore, such is the quality of the image gained with an expanded setting (noise/ grain) that Canon do not wish to advertise it as a working setting.

I think that's how it was explained. Essentially, they are declaring the range of the ADC as considered acceptable but provide the additional settings even though Canon do not consider them to produce images of an acceptable quality.

That doesn't really explain expanding ISO downwards, unless it actually reduced quality of the data. Does expanding down have any quality implication - better shadow recovery perhaps.

I thought it was, but this could be my imagination

 

[docholliday]

Usually, the ISO L (50) has less dynamic range than the native of 100. Pretty much there just in case you absolutely need something slower than 100 (for example, less DOF in bright light). Think of the ISO H ones as over-amplified versions of the highest "actual" ISOs...it'll get you an image, not the best - but actually getting a shot vs. not.

 

[Badger]

I have always known the high ISO would produce a poorer image but assumed 50 would be as good as 100 not worse. I have never used 50, but had always thought I might with enough light to get the absolute best picture posible. It sounds like 100 would be my best option.
Thanks for the feed back!

 

[CharlieB]

I was once given this answer, by a former Canon employee:

Expanded ISO are settings that Canon are not prepared to put their name .....

I think I know who said that!

 

[rj79in]

Do shots at ISO 50 suffer any significant degradation as compared to ISO 100?

 

[Loswr]

Do shots at ISO 50 suffer any significant degradation as compared to ISO 100?

No, but there' no improvement, either. Not in DR, not in noise.

Basically, there's no real advantage to using ISO 50. If your highlights are blown at ISO 100, they're going to be blown at ISO 50, too. So whether you shoot with the longer shutter speed / wider aperture at ISO 50, or shoot at ISO 100 and pull it down a stop in post, there's no difference. But note - you need to overexpose at ISO 100 for that to be the case - if you expose properly at ISO 100 when you needed that shallower DoF or slower shutter, then you didn't get it. In that sense, setting ISO 50 in-camera is useful, as it will show you a 'proper' exposure a the aperture/shutter you choose.

 

[Don Haines]

I don't know how this fits in with the Canon circuitry, but amplifiers are never 100% linear. As you increase the gain of the amplifier you are increasing the gain of the signal and the noise. At some point, the gain will become non-linear and the noise will get amplified much more than the signal.... say you turn up the gain by 2 stops... the signal might only go up one stop but the noise goes up by two, what refer to as clipping or saturation.... I would guess, (and I really mean guess because I really don't know) that this would be the extended range at the top of the ISO tree.

 

[RustyTheGeek]

Don, I'm with you on your assumption. However, since you sound like a fellow audio nut, keep in mind the myriad amplifier designs and the subtle differences how they introduce various "flavors" of distortion into the signal. (Tube vs Darlington vs MOSFET, etc.) This variation in distortion effect has only become more varied in the last 10 years or so with the introduction of the newer digital amplifier designs. The newer digital amp designs are revolutionary in the power supply section in that they can deliver more power to the amp stages more efficiently and waste less power in heat dissapation.

OK, I know this has nothing to do with cameras neccesarily, but I agree that some of the concepts about gain, noise, distortion and efficiency sound similar, just in different ways. While an audio amplifier is using energy to create speaker motion, a camera sensor is turning light energy into an amplified electron signal and converting that information into a digital data stream. In the process, heat is generated by the amplifiers in the sensor. But I believe that's where the similarity ends if you can call it a similarity. The data signal is indeed analog and is amplified but I suspect (because I'm making most of this up as I type) that most of the voltage feedback and bias techniques for mitigating audio distortion don't apply with this type of analog signal. Not sure.

Here is a fascinating read on Canon's Europe website about all the technology in these cameras. Use the "Browse" link to look at a ton of cool info. Don't stay up too late reading it because that is what might happen if it's as interesting to you as it is to me.

http://cpn.canon-europe.com/content/education/infobank/capturing_the_image/photo_sensors.do

 

[bycostello]

lots of grain....

 

[RLPhoto]

Horrible 1 and Horrible 2 are indeed, pretty horrible. :|