My D700 has an ISO range from 200-6400 with a boost to 25,600. I've searched the web trying to learn what the IQ effects are within that range vs. in the boosted ISOs.
Your best bet is simply to make some test shots and see. That'll tell you how your camera performs.
One thing I've learned is that there doesn't seem to be a commonly accepted definition of base ISO (200 in this case)
The base ISO is set by the manufacturer so as not to clip highlights at the calculated auto-exposure. Bear in mind that the definition of "highlights" varies, with different headroom provided on different camera models.
I know that noise (sensor gain) will increase as ISO increases
This is not correct. There are two kinds of noise of significance. The most important on a large-sensor camera is shot noise. This is a function of the quantum physics of light, and has nothing to do with ISO. The less light, the more shot noise. Of course, the less light, the higher the ISO you'll be using, but it's not the ISO setting that increases the noise, it's the reduced amount of light captured by the sensor. If you're using auto-exposure, selecting a higher ISO setting will cause the AE system to capture less light, so in that regard a higher ISO setting results in more noise; but it's not the sensor, it's the auto-exposure system — you'd get the same result from setting a negative Exposure Compensation.
The other significant kind of noise is read noise. On most sensors, read noise decreases with higher ISO settings. On some recent camera models, especially those with Sony Exmor sensors, the read noise is constant at all ISO settings. On some camera models, it decreases with higher ISO to a certain point, then is constant.
Why even specify an ISO range?
The ISO setting is used by the auto-exposure system to calculate the exposure. In M mode, it's used by the metering system to determine how much light results in a centered meter.
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