Anyone have a good explanation of "native" vs "expanded" iso?

[samthefish]

When Canon describes the 1Dx as having "100 to 51200 native ISO (expandable to ISO 50-204800)" what does that really mean?  Does "native" mean setting the sensitivity of the sensor itself and "expandable" mean setting it after the signal comes off of the sensor?  I've googled around and haven't seen a great explanation.

I'm just curious where the amplification of the signal occurs and if you would ever be better off underexposing a raw image and then upping the "gain" in post processing.

SamTheFish

[Dave92F1]

As far as I can tell, "native" just means that Canon thinks the resulting noise is acceptable, and "expanded" means it's not really acceptable but you might want to use it.

I don't think there's really any technical difference - it's just that when somebody points at a really noisy image, Canon wants to be able to say "oh, but that that was at an expanded iso".  It's their way of washing their hands about the quality, while still allowing the photographer to crank up the gain really high.

I don't think there would be much (if any) difference between upping the gain in-camera ('expanded' iso) and doing it in post-processing.

[Mt Spokane Photography]

My understanding is that native ISO is handled on sensor with the amplifiers for each pixel.  Expanded is done in digic, and, like you said, you can up the exposure in raw processing and get the same thing.

Not so for jpeg, it would be better done in camera than reprocessing a jpeg image.

[neuroanatomist]

As far as I can tell, "native" just means that Canon thinks the resulting noise is acceptable, and "expanded" means it's not really acceptable but you might want to use it.

I don't think there's really any technical difference - it's just that when somebody points at a really noisy image, Canon wants to be able to say "oh, but that that was at an expanded iso".  It's their way of washing their hands about the quality, while still allowing the photographer to crank up the gain really high.

ISO 50 isn't 'acceptable'?  But on the high end, you're right - expanded is Canon's way of saying use if you dare/are desperate.

I don't think there would be much (if any) difference between upping the gain in-camera ('expanded' iso) and doing it in post-processing.

A sensor does have a 'native' signal that corresponds to some low-ish ISO (perhaps 100, per 160, etc.)  Between ISO 100 and some higher but well short of max nonexpanded ISO (in the neighborhood of ISO 800), there is an analog amplifier that applies gain (and gain of <1 if necessary) before the signal is sent to the analog-to-digital converter.  ISO increases beyond that are digital, applied after the ADC.  So, in theory there would be a difference between going from ISO 200 to 800 in-camera vs. +2 EV in post on an ISO 200 shot, but no difference between ISO 1600 to 3200 in-camera vs. +1 EV in post.  How much difference does that make in real world shots? Probably not too much...

[dr croubie]

It's their way of washing their hands about the quality, while still allowing the photographer to crank up the gain really high.

Or in other words, it's buying a car limited to 110km/h (the highest speed limit in my state), with a big red button on the dashboard saying "push this to go up to 200km/h", then technically it's your own fault as to what happens...

On my 7D i've got the full range up to 12800, but when in AutoISO it only goes up to 3200 anyway, so to get to 12800 I have to first set the custom function, *then* use manual ISO and select that high anyway. I set the custom function to allow 12,800 on day one, and never changed it (and never used 12800, except for preliminary framing/timing of pinhole exposures (which end up getting taken at iso100 for a few minutes)

[olderdog]

Sure, camera sensors all have a native resolution. It's fairly low, usually the first iso number you can use. On most cameras, the native number is 100. I think the Canon 5D Mark II sensor is 200. There is some variation.

If you move on either side of that native number, you're dealing in expanded ISO. The sensor sees the light the same way, but the image processing compensates for the higher or lower ISO including noise which is the inevitable result.  That noise is no longer as much a problem between improved processors and improvements to the sensors.

You may remember the 5D had a standard low ISO of 100 and it could be dropped to 50. It actually made very sweet images. The newer version goes the other way.

Some people take expanded ISO to mean only that part beyond the standerd ISO range, i.e. the 5d2  can drop to 100 and go up to 12800 or 25,600 (unless i've jumbled the numbers after 12 and 25).

[neuroanatomist]

Sure, camera sensors all have a native resolution. It's fairly low, usually the first iso number you can use. On most cameras, the native number is 100. ... the 5d2  can drop to 100

Based on some tests (read about, not my own) with video mode, and accepting the assumption that the noise should be lowest at the 'native' ISO, it may be ISO 160.  Even multiples of that ISO had lower noise than their 1/3-stop adjacent neighbors, too.  Also, the L expanded ISO on the 5DII is 50.

[funkboy]

I can't remember where I read it as it was a while ago, but my understanding is that (as others have said here) "expanded" iso is simple linear amplification that could be just as easily done in the RAW converter.  But a lot of things in the new 1Dx are very different from the way they were done in the past, so who knows...

[smclaren]

The native ISO for my 5D mark ii, is 160, 320, 640 etc. if I was to set a expanded ISO of 200, the camera uses the native 160 and draws the rest from the exposure compensation.

[LuckyRosco]

After exhaustive test shooting for the calibration of my 7D for my Sekonic lightmeter, it seems that my camera's native ISO is multiples of 200. I base this off of the dynamic range that is given for each of the various ISO. With 200 being the best and the multiples being within the 0.1 - 0.3 range of it. ISO 100 was actually 5.4 compared to ISO 200 of 6.3 . And field testing of my camera between ISO 100 and 200 seem to verify the results.

[motorhead]

Like Neuroanatomist I have read that noise levels are best at certain ISO "bands" and worse between. My 30D was supposed to be best at 160, 320, 640 etc. while newer bodies were said to be best at 100, 200 etc.

I must admit this information is dated! Whether that is still the case I have no way of telling. But I see my new (well, new to me) 5D2 has a low expanded 50ISO and I imagine there is no noise advantage to be gained by using it but that it is there just in case its required. Personally I am never going to be exploring the high limits as I have never shot higher than 1600ISO in my life, rarely going past 400ISO - and I'm 64 now!

[Jettatore]

Any data what range one should stay in between on a 7D and also for a 5D Mark II?  If I can get the same results in post than I can by going higher than a certain number then I would really like to know what that number is.  And if there is a determined optimal number to work with in general, I'd like to know what that number is as well for those cameras.  Thank you in advance.

Even better would be some method to figuring this out for any camera.

[Picsfor]

The Canon guy on the 1DX information videos covers the definition of native and expanded ISO.

By looking at a group of images with software, they reach a conclusion as to how 'clean' an image is, with regards to noise. If they can consistently achieve those clean results with noise levels that are acceptable low - then it is called native. If it is not, then it is called expanded - even if quite a few people may consider it as acceptable.

http://cpn.canon-europe.com/content/education/technical/eos_1d_x_cmos_sensor_explained.do

Click on above link to get the full blurb off Canon man...

[dougkerr]

Hi, s,

When Canon describes the 1Dx as having "100 to 51200 native ISO (expandable to ISO 50-204800)" what does that really mean?  Does "native" mean setting the sensitivity of the sensor itself and "expandable" mean setting it after the signal comes off of the sensor?  I've googled around and haven't seen a great explanation.

In the sensor systems we are most familiar with, the electrical outputs of the individual photodetectors are amplified (on an analog basis) and then given to an analog-to-digital converter (ADC) to be digitized. Both amplifiers and ADCs are "multiplexed"; that is, there is not normally one per photodetector. There are many schemes for this multiplexing.

The basic mechanism for changing the ISO sensitivity of the system is to change the gain of the analog amplifiers, normally among a fixed set of predetermined values (controlled by a digital command to all the amplifiers).

The resulting repertoire of sensitivities are often described as the "native" sensitivities of the system.

However, ISO sensitivities greater than that given for by the greatest provided amplifier gain - as well as sensitivities between those given by established amplifier gain values, or lower than those, - are provided by digital scaling of the digital values out of the ADC.

Sometimes the former of these - or all of these - are described as "expanded" sensitivities.

This technical article from 2006 gives some insight into this concept of sensor operation:

http://dougkerr.net/Pumpkin/#CMOS-APS

Best regards,

Doug

[neuroanatomist]

Any data what range one should stay in between on a 7D and also for a 5D Mark II?  if there is a determined optimal number to work with in general, I'd like to know what that number is as well for those cameras.  Even better would be some method to figuring this out for any camera.

Noise is subjective.  Some people think there's too much in an ISO 800 image from a 5DII.  Others think ISO 6400 on a 7D is usable.  The method to figure out an acceptable range is for you to take a bunch of pictures with different ISOs in different lighting conditions, then decide for yourself what is an acceptable range. 

On newer camera models (60D, 600D, 1D X), you can then do something about that - you can set auto ISO to the range that you personally find acceptable, rather than Canon's idea of what's acceptable.