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p.1 #7 · p.1 #7 · Understanding 'Native' ISO and resulting non-native ISO's | |
snapsy wrote:
The native ISO of a sensor is that which has the highest light saturation capacity when metered at middle gray, which also yields the highest dynamic range offered by that sensor. The fake "low" ISO setting on cameras is produced by exposing 1 stop above metered gray and then digitally reducing the exposure by 1 stop to produce the same output brightness, albeit with less dynamic range. ISOs above native ISO are handled either via analog amplification, digital multiplication, or a combination of the two.
For traditional CMOS sensors which have high sensor electronic read-out noise (all Canon sensors and Nikon D3/D700/D3s sensors), analog amplification yields lower effective read noise at higher ISOs than it does at native ISOs because the amplification increases the relative signal to the constant read-out noise. This doesn't mean the resulting image at High ISO will be less noisy, since most of the noise in the image is the result of photon shot noise, which is a function of only exposure, and typically when you double the ISO you halve the exposure, which means 1/2 the amount of light reaching the sensor. On most entry level/prosumer cameras there is only a single amp which only handles full ISO increments (200, 400, 800, etc..), and so the intermediate levels are achieved by over/under exposing by the fractional stop difference and then adjusting digitally, thus the intermediate ISO levels will not be optimal. Pro-level cameras typically have two amps, one for the full ISO increments and another for the 1/3 stop increments.
For newer CMOS sensors which have low sensor electronic read-out noise (Sony EXMOR sensors), there is not much to be gained via analog amplification vs digital since the relative read-out noise is so low to begin with. Still, it has been measured that the D800 uses analog gain up to around ISO 1000 and digital gain thereafter....Show more →
Yeah that pretty much covers it. Other than to give a few more details about the +1/3 and +2/3rds intermediates on the non-1 series cameras:
the +1/3's underexposing and then boosting digitally means that they give up DR and tend to have, at lower ISOs, less DR than both the primary ISO below AND above, digitally pulling up from the noisy near blacks brings up more shadow banding muck and noise. The +2/3rds simply take the primary ISO 1/3 higher and over-expose it by 1/3 and then shift 1/3 stop digitally and you get the same DR as from the primary ISO just above it and more than from the ISO 1/3 below it, shot as metered you get 1/3 stop better SNR since it exposes 1/3 stop longer, of course you could do the same thing by say shooting ISO 200 with +1/3 EC compared to ISO160 with 0 EC (likewise ISO160 EC -1/3 gives you ISO200 EC 0, the RAWs shift the numbers so it's not bit for bit the same file, but you basically get the exact same content either way) it is sort of like a 1/3 stop STP (shadow tone protection) in a sense if you just leave metering at EC 0.
The +1/3 ISOs aren't so hot, the +2/3 ISOs don't do anything for you per se but they don't hurt either. If you shoot them all with EC 0 and auto exposure the +2/3 ones give you images with a 1/3 stop better SNR and cleaner shadows but 1/3 less highlights than the shots you take at the primary ISO just above it. If you use M mode they are basically the exact same, if you use ISO200 and then flip to ISO160 you'd just then increase shutter speed 1/3 stop.
At some point all of the ISOs get treated digitally. For all the earlier Canon stuff the last time they used analog gain was ISO1600. SOmewhere around 7D or 5D2 era they started using analog gain up to ISO3200 I believe.
Once you get above ISO1600 and certainly ISO3200 the whole 1/3 thing mentioned above doesn't really apply any more for any of the cameras since it is mostly or all straight up digital raising it 1/3 step each time all the way up.
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