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I've seen in the CHDK page, that you can change the ISO values, and put lower ones. Now lets suppouse I want to shoot a waterfall in day, and also I want a long exposure time (10 sec. i.e.).

I know I can use ND filters, but what about using a very low ISO (like 15)? Is there a limit for the lowest ISO in each camera?

In theory, the light sensibility would be very low, so the shutter time will need to be longer. Don't name aprerture sizes, because I can only have f11 as minimum.

If all of this is true, the best of all is the reduction of noise!

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In my case, the lowest ISO available with the original firmware is 80. When I change it to a lower one with CHDK loaded, the photos are just the same as if they have been taken with ISO 80. Don't know why! –  tomm89 Oct 20 '10 at 17:08
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3 Answers

up vote 8 down vote accepted

I don't really know much about CHDK, outside of that it lets you program P&S cameras. When it comes to ISO, though, the story is rather tricky. It may be possible to program ISO down to 15 with CHDK, but it may not really do you any good. There are "real" ISO settings, and there are "artificial" ISO settings. A real ISO setting would be something like ISO 100, which is usually the base ISO for many cameras (some have a base ISO of 200). This is an analog setting, adjusting the ISO to any of the "real" native settings will usually work by changing the analog readout of the sensor.

In contrast, an artificial ISO setting is either achieved by making other "behind the scenes" changes to other settings on the camera, or by "digitally enhancing" the nearest real ISO setting. In the case of "behind the scenes" ISO, rather than actually changing the ISO when choosing, say, ISO 50, the camera may actually reduce the exposure time at ISO 100 instead. In the case of "digitally enhancing" ISO, the camera may use ISO 100 when you choose ISO 50, then apply a digital filter to the resulting image to make it appear as though it was shot at ISO 50.

Generally speaking, using artificial ISO settings is undesirable. You generally don't really know what is going on behind the scenes, and if the camera is changing settings to make it appear as though you are really using ISO 50, it may adversely affect your image, preventing you from capturing the scene you really wanted to capture. Most of the time, it is best to use "real" ISO settings that change the analog readout from the sensor. If you need to lengthen your exposure times beyond what the native base ISO setting allows (usually ISO 100, sometimes ISO 200), then filtration is usually the best option.

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You also risk highlight clipping with these fake low ISOs. –  Paul McMillan Oct 20 '10 at 6:35
    
Yep, essentially fake ISO settings overexpose (at the base ISO) and use a tone curve to bring the image down to the correct exposure, clipping highlights. Generally you don't really use IQ other than that, since there is more precision in the higher bits anyway (hence the movement to ETTR). If anything, the shadows may have less noise due to allowing more light in, unless their processing is really bad. –  Eruditass Oct 20 '10 at 18:35
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This doesn't add anything new beyond jrista's answer, but to summarize: probably what is happening when you select those ISOs is that the camera overexposes and then divides the numbers by 2 (or whatever). So you lose IQ fairly rapidly.

I would suggest looking up on CHDK website exactly how it achieves those low ISOs. I'm certainly curious, so do report back if you can! :)

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It is possible, but there is a limit to the lowest ISO your camera supports based on the sensitivity of the sensor.

Every 1/2 ISO level requires 2x the light. So ISO 15 would roughly be 2.5 stops darker compared to ISO 100.

So while lower ISO would help (in theory), lower-iso + an ND filter would give you a lot more options to work with.

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