Is analog gain really actually power-of-two only?

Question

Context

I'm using a Nikon D5200, which features extra ISO values between powers of two, that the Nikon D60 did not offer.

• D60 offers 100, 200, 400, 800, 1600 (1-stop interval).
• D5200 offers two more values (1/3-stop interval) between each of the power-of-two that D60 provided: 100 / 125 / 160 / 200 / 250 / 320 / 400 / 500 / 640 / 800 / 1000 / 1250 / 1600 / 2000 / 2500 / 3200 / 4000 / 5000 / 6400 / 12800 / 25600

(I'm a seasoned StackExchange user and reasonably advanced amateur photographer. To avoid any misunderstanding one might want to see my profile).

ISO usually mean analog gain

As already documented, most of the time the difference between a photograph taken at e.g. ISO 200 and another at ISO 400, all other things being equal, is that an analog amplification gain is applied at the analog-to-digital conversion step. Sources: NX101 -- Capture NX - Understanding Nikon's NEF and How is ISO implemented in digital cameras? - Photography Stack Exchange

Are all ISO values actual distinct analog gains ?

Recently a friend told me that only power-of-two ISOs are actually implemented with the analog gain, and that other values (like 250, 320) are actually digital post-processing and thus less interesting form a noise and dynamic range point-of-view. For example "shooting at 640 loses some information". There's a similar claim in learning - What is something you learned recently about your gear that you wish you discovered earlier? - Photography Stack Exchange. It reads:

E.g. In-between stops (640 ISO for example) is a software push above ISO 400.

If only power-of-two ISOs were actual analog gains...

If ISO 640 was a software push above ISO 400, it would mean that ISO 640 does not gain any noise advantage over ISO 400, only possibly (depending on scene) losing by post-processing some highlights that would have been available at ISO 400. Or what else?

It could also be a software reduce from ISO 800, just like ISO 50 in Nikon D800 and D800E was a software reduce from ISO100, couldn't it ? (source: 24 Things You Need to Know About the New Nikon D810).

In that latter case, the noise at ISO 640 would be as good as in ISO 800 (all things being equal, better than ISO 400 as explained on What is "ISO" on a digital camera? - Photography Stack Exchange) but reduction could just (depending on scene) not reach as much highlights as an actual ISO 800 (analog clipping). Or what else ?

Perhaps the actual processing is only "hinting" the post-processing (base curve, dynamic lighting and the like), producing a picture that actually has the overall exposure requested at 1/3-stop granularity, without actual hard loss. In that case, the RAWs actual exposure might jump only by power-of-two ISOs and not 1/3 steps. Such hypotheses could be corroborated by taking RAWs and JPEGs at all ISOs and see how they behave.

Or perhaps this is all nitpicking?

Question, summarized

• Is it true that analog gains are only power-of-two? In Nikon cameras? In others?
• Should one should prefer shooting with power-of-two ISOs only? Or only in certain cases (JPEG shooting, high dynamic range scenes, ...)?

Answer 1

It all depends on the specific camera model and the design of the sensor and the firmware installed.

Many Nikon cameras with sensors made by Sony amplify the base signals by 1/3 stop intervals. Other Nikon cameras don't. Even some Sony sensors don't.

As a long time Canon shooter, I can more fully explain how it works with practically all of their sensors produced over the past decade or so.

Most Canon DSLRs set the analog gain at even ISO stops (e.g. ISO 100, 400, 800, etc.).

The +1/3 stops (e.g. ISO 125, 250, 500, etc.) use the nearest even ISO stop and underexpose by 1/3 stop (if you are using an automatic exposure mode such as Av priority or Tv priority), then push exposure 1/3 stop when converting to JPEG. This tends to increase noise in the shadows (because when you underexpose and then push exposure you also amplify the noise) and also reduces dynamic range by 1/3 stop as anything within 1/3 stop of saturated when the photo is taken is blown out when increased 1/3 stop.

The -1/3 stops (e.g. ISO 160, 320, 640, etc.) overexpose by 1/3 stop (if you are using an automatic exposure mode such as Av priority or Tv priority) and then pull the exposure when converting to JPEG. This is pretty much the equivalent of exposing to the right by 1/3 stop: You get slightly better performance in terms of noise in the shadows at the expense of slightly less dynamic range in the highlights.

Even if you are shooting in Manual Exposure Mode and select both the Tv and Av yourself, the camera will include instructions in the RAW file to increase/decrease exposure by 1/3 stop when the RAW file is converted. The exposure meter in the viewfinder when you take the photo will also reflect the 1/3 stop difference. If the meter shows proper exposure for, say, ISO 200, f/5.6, and 1/100 seconds it will show -1/3 stops underexposure for ISO 160, f/5.6, and 1/100 seconds when metering the exact same scene.

For the practical implications of using the +1/3 and -1/3 stops with such a camera, as well as some links to some very interesting test data, please see this answer to Is it really better to shoot at full-stop ISOs?

Answer 2

"This is all nitpicking?" I'd pretty much say "yes", particularly for recent Nikon cameras, all of which use Sony sensors which have the nice property of being "ISO invariant" - it doesn't matter if you turn up the gain in camera or do it in post-process as you get essentially the same result.

Canon sensors, and the sensors used in older Nikons, don't have the same ISO invariance, so this is potentially more interesting there - but still far less important in the real world than just getting your shot right in the first place. Photographs are not made in lab tests.