"Expanded" ISO range?

[jasonrandolph]

So, what does expanded ISO range really mean?  I know that the expanded higher ISOs, noise is going to be a significant issue, but what about the expanded lower ISOs, such as ISO 100 on my D300 or ISO 50 on the Canon 5DMkII?  Is there any image quality to be gained, like lower noise, by going low?  Is there any image degradation (i.e., signal-noise ratio falling)?  I almost always try and shoot at the "base" ISO of 200 on the D300, but there are times where I want to use the ISO 100.  But if I will be sacrificing quality, forget it.

[Daniel Browning]

So, what does expanded ISO range really mean?

It's an arbitrary distinction made by the manufacturer.

I know that the expanded higher ISOs, noise is going to be a significant issue,

In addition, all the ISO above 1600 throw away highlights for raw shooters with no benefit to read noise. You can get the same read noise and another 1-4 stops of highlight headroom by just using negative EC or manual exposure. The Canon 10D correctly used metadata for such ISO settings, but they and other manufacturers have failed to get this right ever since.

but what about the expanded lower ISOs, such as ISO 100 on my D300 or ISO 50 on the Canon 5DMkII?  Is there any image quality to be gained, like lower noise, by going low?

Yes: you get the same results by shooting ISO 200 with +1 EC. The increasing absolute exposure reduces photon shot noise and read noise, but the cost is one stop less highlight headroom. For most photographers and situations, the latter is more important than the former.

[jasonrandolph]

It's an arbitrary distinction made by the manufacturer.



In addition, all the ISO above 1600 throw away highlights for raw shooters with no benefit to read noise. You can get the same read noise and another 1-4 stops of highlight headroom by just using negative EC or manual exposure. The Canon 10D correctly used metadata for such ISO settings, but they and other manufacturers have failed to get this right ever since.



Yes: you get the same results by shooting ISO 200 with +1 EC. The increasing absolute exposure reduces photon shot noise and read noise, but the cost is one stop less highlight headroom. For most photographers and situations, the latter is more important than the former.

Thanks Daniel.  That was a very thorough reply!!

[dwdallam]

I've been doing some experimenting with ISO 3200, which is "expanded" on the 1DS3. If you have enough light to shoot and get a good exposure at 3200, then you can get quite a nice file with some post processing. The bigger you print the more noticeable the artifacts will be, although I've printed 12x18 at ISO 3200 and had them come out as clean as ISO 800 for the same type of file.

If I'm out of fStops and still can't get a decent shutter speed, I'll go to 3200. The difference between 125th and 250th is great when shooting freehand with a heavy Canon 200mm IS L. That means if I can shoot at 1600 and drop one stop with EC I'll use it. However, with the 1DS3 I've noticed that underexposing at ISO 1600 can create noise really fast.

Depending on your camera, I'd say use it if you need it, but then again, for landscape you would never use it, at least I can't think of a situation where you would need to use such a high ISO setting. You'd just use a longer shutter speed on a tripod.

[Daniel Browning]

However, with the 1DS3 I've noticed that underexposing at ISO 1600 can create noise really fast.

I would suggest comparing two shots: one with ISO 1600 and -1 EC, the other with ISO 3200 and no EC. I think you will find that after you add +1 EC in raw processing, they both will have the same noise level, but the ISO 1600 will have one more stop of highlights. In Tungsten, I find that the red channel blows very quickly on faces, so I appreciate the extra highlight headroom.

[Panopeeper]

So, what does expanded ISO range really mean?

There is no uniform answer, it depends on the camera. Let's restrict this to DSLRs, as MFDBs exhibit very different characteristics, and P&S? Well, who knows that.

As the first step, one needs to separate between low and high ISO extentions.

If the wells (the electron collecting buckets) get saturated, then any further exposure is either lost (the good case) or there will be a "spill over" to other wells; see the discussions about the D3/D700 banding. Thus reducing the ISO lower than the "native ISO" is plain overexposure.

Thus shooting with the 5D2 @ ISO 50 will certainly yield very low noise even in extremely underexposed areas, but the dynamic range will be less than the maximum, due to saturation of the wells, which can not be countered any way except by lowering the exposure.

The "expansion ISO" at the high end is a completely different issue.

First, let's see what "real ISO gain" means, in terms of digital photography, as opposed to the discussion in terms of electronics. You meter the exposure and make a shot @ ISO 100, with, let's say f/2.8, 1/125s. However, the 1/125s is too long, you need to reduce it to prevent motion blur. You have the choice between underexposing by one stop at 1/250s and shooting with ISO 200 at 1/250s.  Then you make a shot @ ISO 200, with half the exposure.

What difference do you expect between

a. the original, higher exposure at ISO 100,

b. the plain underexposure with increasing the intensity in post processing,

c. the lower exposure with increased ISO?

Obviously, the highlights should not look differently, but how will the shadows look like?

With the plain underexposure one loses exactly one stop; that's obvious. But how much does the ISO increase help?

Example: the Canon 40D.

Increasing from ISO 100 to 200 is almost painless, i.e. the one stop lower exposure is almost unnoticable, it makes perhaps 1/6 EV.

From ISO 200 to 400: the loss compated to the higher exposure is about 0.6 EV; viewd from the other side: the gain is about 0.4 EV, compared to the plain underexposure.

From ISO 400 to 800: the gain is about 0.25 EV

From ISO 800 to 1600: the gain is perhaps 0.15 EV.

And now the next stage, from ISO 1600 to 3200: the gain would be negligable. Thus Canon decided, that there is no point to complicate the hardware with further increasing the nominal gain, when it does not yield any better result. Instead, the firmware "pumps up" the ISO 1600 values by plain multiplication, as if there were an ISO 3200 hardware setting, but the effective gain is nothing. In fact, there is a huge loss: doubling the pixel values means, that one stop from the highlights is cut off (assumed there is something in the highlights). Though this is the case with each ISO increase, in this case there is nothing in return.

Well, that is the ISO expansion.

[dwdallam]

I would suggest comparing two shots: one with ISO 1600 and -1 EC, the other with ISO 3200 and no EC. I think you will find that after you add +1 EC in raw processing, they both will have the same noise level, but the ISO 1600 will have one more stop of highlights. In Tungsten, I find that the red channel blows very quickly on faces, so I appreciate the extra highlight headroom.

That's really good information. Thanks.