Exposure basics, lesson two point one (& ISO)

Started Mar 19, 2013 | Discussions thread
richarddd Veteran Member • Posts: 3,087

By the way, probably better to refer to "ETTR via higher ISO" as "BTTR" (brightness to the right) or "GTTR" (gain to the right), as the exposure is unaffected.

I like it! Not just the Read noise; also analog gain noise?

The read noise is simply all the sources of noise from the sensor and supporting hardware lumped into a single figure. Thus, Total Noise = Photon Noise + Read Noise, where "+" represents the quadrature sum: P + R = sqrt (P² + R²).

So "ETTR up to ISO 400"? 800?

ETTR is always best when possible, as it increases the actual exposure which lowers the photon noise, and this has a more dramatic effect on lowering the overall noise than using a higher ISO to lower the read noise.


And you're right, "Exposing to the right" is a bit of silly term, but I guess it works as a teaching aid. The whole point as far as I've understood it is getting the least noise and thereby the most details, less fuzziness, clearer colours and whatever other advantages that brings us.

I've advocated earlier in these threads that if you find you have headroom after getting the maximum practical exposure without clipping significant highlights, you should turn up the ISO to reduce noise. Now, you've pointed out that after ISO 800 the read noise pretty much evens out, so it starts having less of a practical effect. No argument that the effect diminishes, my question is, if it's still there, isn't it worth pursuing? Even if it only drops from 2.8 to 2.6 from ISO 800 to ISO 1600 (sensorgen.info), isn't that still something... or is something else going on that I'm unaware of?

The best way to answer these questions is to take a series of pictures and see what happens. Does going from 800 to 1600 help?

For a more extreme example: Looking at the sensorgen.info, the DR of the different ISO levels on the E-M5 it goes from 11.1 at ISO 800 to 6.5 at ISO 25600 (5 more clicks on the ISO dial). Am I right in interpreting this as if the maximum exposure available to us is only enough to saturate ISO 25600, it still gives us more DR to turn it to this than to keep the dial at 800?

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