Those "in-between" ISOs...

[Daniel Browning]

The short answer is: don't use tweeners.

Read on for the long answer.

I'm curious if those 1/3 increment ISOs are good to use, or should be avoided.

It depends on how picky you are. If you want maximum dynamic range, you'll avoid at least half of them.

In various photo mags I see ISO 500 or 320 etc. but more often I see 100, 200, 800, etc.

Not surprising. Few people are aware of what the optimal settings are and the reason why they are optimal.

Is it true that camera CPUs arrive at those 1/3 ISOs by interpolation?

In some cameras, yes. This includes the 5D2, xxD (50D, 40D, 30D), xxxD (500D, 450D, 400D), and 1000D.

In other cameras, the tweener ISO settings are achieved through a separate analog amplifier. That includes the 5D1, xD (1D, 1D2, 1D3), and xDs (1Ds, 1Ds2, and 1Ds3).

That would mean they're not "native" to the capture system,

Correct.

but does that make them (by definition) inferior?

Yes. Digital manipulation of the raw file is always inferior because it always increases quantization error and sometimes also clips highlights by 1/3 stop. It could have been implemented as metadata ISO and would have avoided posterization completely while still offering the same benefit to the user.

If the cameras were engineered with the correct bit depth (e.g. 12 bits for the 5D2 instead of 14), then the quantization error from tweener ISO settings might actually show up in the extreme shadows as posterization. In this case the two mistakes cancel each other out (14 bits is enough to hide posterization).

I've used my 40D at various ISOs and don't see anything unexpected at the 1/3 ISOs...

The difference is not visible to most people when they use typical tone curves and default raw conversion parameters. That's because most defaults are tuned for very little dynamic range: just 5-7 stops between highlights and shadows. If you shoot scenes that have a little more contrast, such as 9 to 11 stops, the difference is more noticable. That is because the difference is in read noise dominated tones.

I'd like to learn the "best practice" ways to shoot - is avoiding in-between ISOs part of it?

On some of cameras with analog tweeners (1D3, 1Ds3), some of the settings have slightly less read noise, but generally the loss in highlight headroom is not worth it. In all other situations the tweeners are either worse or the same.

I should mention that tweeners can sometimes help broken raw converters. Canon's DPP, for example, doesn't use the correct white point for many of their own cameras. They set it too low, ignoring plenty of good data in the highlights completely. In that case, the -1/3 ISO settings (160, 320, 640, etc.) will move the actual white point down to the same spot where DPP "thinks" the white point is, so you get back that 1/3 stop of highlights. Of course, other raw converters don't have this bug, so they see the highlights either way. I don't suggest forming your shooting strategy around bugs in raw converters. Better to just live with the 1/3 loss (which is very minor) or switch to a better converter.

The +1/3 digital tweeners (ISO 125, 250, 500, etc.) are the ones that have negative effects. Compared to ISO 100 with a fixed exposure, ISO 125 clips 1/3 stop of highlights with no improvement in read noise. Compared to ISO 200 with fixed exposure, ISO 125 has 1/3 stop *more* highlight headroom, but it also has nearly double the read noise. That means ISO 200 actually has more dynamic range and less shadow noise than ISO 125.

That's not to say they that tweeners should never be used. For JPEG shooters (not raw), it's important to get the image brightness just so, and sometimes ISO is easier to change than exposure. Video, too, benefits from tweeners for the same reason.

...instead of jumping to 1600, I like to use 1000 for soem indoor shooting that doesnt quite need to be 1600, so i guess you coudl say I use 1/3 ISO's more so at higher ends.

I would suggest ISO 1250 instead, because it has noticably less noise than ISO 1000 for the same exposure.

in RAW bumping up the Exposure by 1/3 takes it to 125 and 2/3 would take it to 160.

Yep. That's exactly how 125/250/500 work in the camera, too. The other tweeners (160, 320, 640) are slightly different: they start with the next-highest ISO and subtract 1/3.

The tweener ISOs on a 40D at 160, 320, 640, 1250 are better than the default 100,200,400,800,1600.

I wouldn't put it that way. They yeild the same result as shooting ISO 200 +1/3 EC. You lose 1/3 stop highlights, but gain less noise. They're really the same, not better. (In a fixed exposure, ISO 160 has lower read noise in absolute ADU, but the SNR is the same, so it's not better that way.)

ISO 160 is as good as it gets on a 40D.

ISO 100 has 1/3 stop more dynamic range and 1/3 stop higher SNR in the shadows than ISO 160.

To attain maximum dynamic range, tweeners should be avoided until they are implemented the correct way, as metadata.